Matrix Infrared Spectra and Electronic Structure Calculations of Linear

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Matrix Infrared Spectra and Electronic Structure Calculations of Linear Alkaline Earth Metal Di-Isocyanides CNMNC, Ionic (NC)M(NC) Bowties, and Ionic (MNC) Rings 2

Lester Andrews, Han-Gook Cho, Wenjie Yu, and Xuefeng Wang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b01286 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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The Journal of Physical Chemistry

Matrix Infrared Spectra and Electronic Structure Calculations of Linear Alkaline Earth Metal Di-isocyanides CNMNC, Ionic (NC)M(NC) Bowties, and Ionic (MNC)2 Rings Lester Andrews,*,a Han-Gook Cho,b Wenjie Yu,c and Xuefeng Wangc aDepartment

of Chemistry, University of Virginia, Charlottesville, Virginia 22904

bDepartment

of Chemistry, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon,

22012, South Korea, cSchool of Chemical Science and Engineering, Tongji University, Shanghai 200092, P. R. China

ABSTRACT: Laser-ablated Group 2 metal atoms exhibit different reactivity with (CN)2 in excess argon and neon during condensation at 4 K. UV irradiation (220-290 nm) is required to activate Be to produce the linear CNBeNC di-isocyanide molecule with a strong antisymmetric C-N stretching band at 2104.3 cm-1 and a C-N-Be-N-C stretching mode at 1265.7 cm-1. The diisocyanide appears at lower frequency and exhibits more nitrogen and less carbon isotopic shift than the cyanide counterpart, which is confirmed by B3LYP isotopic frequency calculations. Two weak bands were observed for the cyanide NCBeCN, and three absorptions were found for the mixed ligand CNBeCN molecule, which would be difficult to synthesize and put into a bottle. Mg reacts with (CN)2 to form the CNMgNC counterpart at 2085.8 cm-1 on annealing to 25 K. Absorptions for the Ca(NC)2, Sr(NC)2 and Ba(NC)2 molecules at 2060.8, 2048.1 and 2045.9 cm-1 increase on sample annealing with these more reactive heavier alkaline earth metal atoms, which have calculated twisted bowtie side bound (CN) structures of C2 symmetry with shorter computed M—N than M—C distances. NBO calculations for the latter molecules reveal natural charges of +1.54, 1.64, 1.71 e on the metal centers and – 0.77, 0.82 and 0.855 e on the corresponding CN subunits, respectively, with a doubly occupied sp-sp σ and two p-p π bonds on each (CN), which supports an ionic model for bonding in these molecules. A weaker band at 2056.6 cm-1 behaves nearly the same as the Ca bowtie band in the spectrum on 25 K annealing 1 ACS Paragon Plus Environment

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and photolysis, and it is assigned to the 6 kJ/mol higher energy linear CNCaNC isomer. Additional similar sharp absorptions for a new Ca, Sr, and Ba reaction product at 2036.3, 2040.6 and 2036.0 cm-1 increase on annealing at the expense of adjacent broader bands: B3LYP and NBO calculations validate their assignment to ionic hexagonal C2h (MNC)2 rings from the reaction of two M atoms with (CN)2.

INTRODUCTION The Group 2 metals produce solid cyanides, M(CN)2, in the reaction of the metal hydroxides, M(OH)2, with HCN,1 but the molecule formed in the reaction of magnesium vapor and cyanogen gas in a dc discharge with argon is MgNC.2 Earlier CCSD calculations found the Group 2 metal products to be isocyanides, MNC, which are more stable than the corresponding MCN molecules by 29 to 50 kJ/mol.3 An ab initio theoretical study of the MgNC/MgCN isomerization in the ground electronic state (2∑+) has been performed, and the ground vibrational state of 24MgCN was found to be 7.8 kJ/mol above that of 24MgNC.4 Nevertheless the metastable MgCN isomer was also observed in the above glow discharge system.2,5 The detection of MgCN in the outer shell of the late type star IRC + 10216 implies an abundance ratio of about 22/1 for MgNC/MgCN.6 Laser ablated Group 2 metal atom reactions with HCN in excess argon in this laboratory gave all of the MNC molecules plus a weak band about a hundred wavenumbers higher than BeNC for the BeCN isomer.7,8 An absorption slightly higher than MgNC and attributed to a matrix site of MgNC in the earlier work8 will be reassigned here to Mg(NC)2. This work was done with the cyanogen reagent in order to investigate the anticipated diisocyanide reaction products for the Group 2 metals, and to find out if Be could also produce the dicyanide as well. We also searched for additional new molecules that might be formed in these reactive systems and found an ionic hexagonal ring dimer for (CaNC)2, (SrNC)2 and (BaNC)2. 2 ACS Paragon Plus Environment

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The alkaline earth metal dicyanides have been investigated at the electron-correlated MP2 level of theory, and the Be and Mg salts were identified as the linear CNBeNC and NCMgCN molecules, but the heavier metal bearing molecules preferred twisted bridged structures of C2 symmetry.9 These side bound (NC) structures for the more electropositive of the Group 2 metals follow earlier infrared and Raman matrix isolation investigations of lithium superoxide as an isosceles triangular ionic (Li)+(O2)− structure, and rotational spectra for KCN molecules gave a T-shaped side-bound structure as well.10-13 We have investigated the aluminum counterpart and found the di-isocyanide to be bent (115°, C2v) and 3.8 kcal/mol more stable (lower energy) than the dicyanide isomer at the B3LYP/ aug-cc-pVTZ level of theory.14 A similar calculation performed here for comparison with the Zr analog found a bent CNZrNC molecule. Beryllium atoms were first reacted with O2 in this lab as a test of principle, and OBeO and noble gas-BeO complexes were formed.15,16 Analogous experiments have been done reacting ablated Group 2 metal atoms and H2O, H2 and O2 mixtures, and H2O2 in argon to produce the metal dihydroxides.17,18 Calculations at the B3LYP and MP2 levels found linear OBeO and HOBeOH with opposite bent Be-O-H angles, likewise for Mg with B3LYP but all linear for HOMgOH and HOCaOH with MP2, and increasingly bent structures at the metal centers using B3LYP for the dihydroxides of Ca, Sr, and Ba.18 Beryllium atom reactions with F2, ClF and Cl2 were recently investigated, and the major products were dihalide molecules.19 The beryllium dimer itself has been characterized as a weakly bonded van der Waals molecule with a bond dissociation energy of 10 kJ/mol.20,21 Recently the electronic structure of Be2 complexes with radical ligands (L:Be-Be:L) have been calculated using CCSDT)/cc-pVTZ theory, and substantial increases in the Be--Be bond strength have been predicted.20 In fact the di-isocyanide

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was the strongest complex computed with a Be-Be distance of 2.05 Å the same we obtain here. Clearly such diberyllium complexes are of considerable interest. Here follows an investigation of laser-ablated Group 2 metal atom reactions with cyanogen in excess argon or neon and calculations at the B3LYP and CCSD(T) /aug-cc-pVTZ levels of theory for covalent linear metal di-isocyanide, ionic (CN)M(CN), and ionic hexagonal (MNC)2 ring molecules.

EXPERIMENTAL AND COMPUTATIONAL METHODS Experiments. Group 2 atoms were laser-ablated from the solid metal and reacted with cyanogen synthesized in this laboratory.23-25 The Ca, Sr and Ba solids (Johnson Matthey, 99%) were stored under mineral oil: 5x10x10 mm pieces were sawed off and placed in hexane to remove the oil. These pieces were epoxy glued to ¼”-20 nuts, the surface to be ablated was sanded to remove oxide, and the target was placed under vacuum. Cyanogen at 0.1% to 3% in excess argon or neon (Spectra Gases) was condensed along with ablated and excited metal atoms for 1 h onto a 4 K CsI window cooled by a closed-cycle refrigerator (Sumitomo).7,8,15,16,23-26 The Nd:YAG laser (Continuum Minilite II, 1064 nm, 10 Hz repetition rate, 10 ns pulse width, 10-20 mJ/pulse) was focused onto the rotating (one rpm) metal target. The emission plume from the laser focal point on the metal target extended to the cold window and served as a qualitative diagnostic for the experiment. Generally the laser is operated at 30-40% of full power, but this beam is reflected by two mirrors and passes through a glass window and lens on the way to the metal target. After sample deposition for about 60 min infrared spectra were recorded at 0.5 cm-1 resolution using a Nicolet iS50 spectrometer with a liquid nitrogen cooled Hg-Cd-Te range A detector (4000-620 cm-1). Next, samples were annealed (warmed and cooled back to 4 K) using 4 ACS Paragon Plus Environment

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resistance heat (temperature measured by gold/cobalt vs. chromel thermocouple), and selected samples were irradiated for 10 min periods by a mercury arc street lamp (175 W, output range 220580 nm with the globe removed). A Pyrex glass filter (>290 nm) was used in several experiments to help determine the effective range of the irradiating light. Computations. The initial structures and harmonic vibrational frequencies of expected product molecules were calculated at the density functional theory level using the B3LYP hybrid functional with the aug-cc-pVTZ basis set for C, N, Be, Mg and Ca and the SDD pseudopotential and basis for Ca, Sr and Ba (10, 28 and 46 electron cores and 10 valence electrons) in the Gaussian 09 package.27-33 Orbital occupancies were determined using NBO6 for the natural bond orbital (NBO) population analysis at the B3LYP level.33 Such harmonic DFT calculations predict vibrational frequencies with reasonable accuracy (typically 2-5% higher than observed in the argon matrix) for metal compounds.14,18,26,34 Geometries were fully relaxed during optimization, and the optimized geometries were confirmed by vibrational analysis. Separate calculations with the CCSD(T) method35 were employed for some of the smaller molecules to supplement the B3LYP results. The zero-point energy is included in the calculation of binding energy for metal bearing molecules. RESULTS AND DISCUSSION Matrix infrared spectra will be illustrated and products will be identified for several Group 2 metal atom and cyanogen reaction products, and their calculated structures and bonding will be described.



Infrared Spectra. Beryllium. Figure 1 compares two sets of infrared spectra from experiments using 0.5% (12CN)2 and 1% (13CN)2 in argon for reaction with laser ablated beryllium in the N-C and C-N-Be-N-C stretching regions. The weak band at 2127.4 cm-1 is due to (N12C13CN) present in natural abundance for the (12CN)2 sample and as a residual in the (13CN)2 isotopic sample (prepared from K12CN, 99%). There are very weak product absorptions in the initial deposited sample shown with the 1% reagent sample Figure 1(h) at 2067.8 2064.0, 2062.8 and 2058.9 cm-1, and irradiation with Pyrex filtered mercury arc at > 290 nm increased them several fold, but using the full 220-580 nm mercury arc (Figure 1 (c, j) increased the bands at 2104.3, 2100.2, 2096.0 and at 2067.8, 2064.0, 2058.9 cm-1 markedly). This full arc irradiation showed that there are two partially resolved bands at 2099 and 2063 cm-1. Next annealing to 25 K sharpened and increased the 2104.3 and 2100.2 bands, and to 30 K decreased the major 2067.8 band and the 2062.8 feature. A second full arc irradiation, Figure 1(f), again increased the 2104.3 band. The lower region shows two matrix site split bands at 1265.7, 1264.3 and at 1259.5, 1258.7 cm-1, which track on annealing and irradiation with the two major bands described first. An * denotes the NCNC photoisomer of (CN)2,36 which is much more intense in the samples produced by photolysis from the Ca and Ba ablation plumes. Figure 2 features spectra using (12,13CN)2, which is our abbreviation for the statistical mixture made from equimolar K12CN and K13CN (ie. 25% (12CN)2, 50% 12CN13CN) and 25% (13CN)2) where the precursor bands exhibit a very strong 1/2/1 intensity triplet like that shown in Figure 3 for the (C14,15N)2 sample. The first scan at the bottom is taken from the 3% (CN)2 experiment and the top (13CN)2 spectrum came from Figure 1(l): these two book-end pure isotopic spectra were added to show which absorptions from the (12,13CN)2 sample are due to the pure (12,12 or 13,13) and the mixed (12,13) isotopic species. No absorptions are observed in the 6 ACS Paragon Plus Environment

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deposition and 25 K annealing scans (Figure 2 (b, c)), which tells us that reactions of Be and (CN)2 are not spontaneous, but reagent association occurs so that marked growth of product absorptions follows 220-580 nm irradiation, Figure 2(d). The highest band in each region is the same band observed in Figure 1 for the 12C and 13C reagents, but the second bands at 2101.2 and 2063.0 cm-1 are slightly (0.9 and 1.0 cm-1) different from the second bands in Figure 1. As in Figure 1, annealing to 30 K increases the second band which is at 2100.2 and 2064.0 cm-1 on the side of the second band first produced in Figure 2. Further annealing to 35 K decreases the first band in each region and the new bands that appeared on the previous 30 K annealing. Spectra in the lower C-N-Be-N-C stretching region containing a triplet of doublets for the major new band at 1265.7 cm-1 are also illustrated in Figure 2. (Figure S1 focuses on this important region). The middle five spectra are from the 1% (12,13CN)2 experiment: The strong doublet in the top left corner at 1265.7, 1264.3 cm-1 and the lower doublet at 1199.4, 1198.0 cm-1 increase markedly on full arc irradiation as does the first band in the N-C region at 2104.3 cm-1, and the next doublets at 1259.5, 1258.7 cm-1 and at 1192.7,1192.2 cm-1 increase substantially on annealing to 30 K as does the second band at 2100.2 cm-1 in the upper region, but the former bands are destroyed by further annealing to 35 K and about 40% of the 2100.2 cm-1 band remains. (These small < 2 cm-1separations are due to matrix site splittings.) The 13C counterparts from the (13CN)2 experiment shown in Figure 1 are associated by inspection in Figure 3, and the frequencies and 12/13 isotopic frequency ratios are listed in Table 1. Perhaps most important are the three scans in the middle from the statistical (12,13CN)2 sample. In the upper region the first doublets from the top and bottom sets of spectra are observed, and a new doublet at 1262.0, 1260.4 cm-1 with twice the intensity from the 12CN13CN precursor is observed between the higher and lower doublets. This triplet of doublets demonstrates that two equivalent carbon 7 ACS Paragon Plus Environment


atoms are involved in the vibration and molecule that contributes the 1265.7, 1264.3 and 1258.1, 1256.6 cm-1 absorptions even though the carbon atom is only slightly involved in this largely antisymmetric C-N- Be-N-C stretching mode. No intermediate component was found for the lower 1199.4, 1198.0 and the 1194.7, 1193.0 bands nor the 1192.7, 1192.2 and the 1188.2, 1187.3 bands, which shows that these bands involve the motion of a single carbon atom. An expanded scale photograph of the lower N-Be-N stretching region with higher signal to noise ratio is shown in Supporting Information as Figure S1. An experiment was done with (C14,15N) statistical cyanogen sample made from a mixture of KC14N and KC15N (ie. 25% (C14N)2, 50% C14NC15N), and 25% (C15N)2) at 1.5% total (CN)2. The spectrum in the lower region (not shown) gave on photolysis a partially resolved triplet at 1265.1 cm-1, which is the average of the all-14N precursor product peaks, the strongest peak at 1264.3 and a lower peak at 1263.4 cm-1. This tells us that our product molecule has two equivalent N atoms although the total 14-15 N shift for this mode is only 1.7 cm-1, because the Be atom does most of the motion in this mode. A similar band appeared at 1199.4, 1198.0 cm-1. Annealing to 30 K gave a new doublet at 1258.8, 1258.2 cm-1 and an associated band at 1194.2 cm-1. The N-C stretching region for this experiment is illustrated at the top in Figure 3. No absorption was observed on sample deposition or 25 K annealing but > 290 nm irradiation produced very weak bands which increased markedly upon > 220 nm irradiation as described above. This tells us that the resonance absorption of atomic Be in solid argon observed strongly at 236 nm by Weltner, et al.37 is responsible for the Be reactions observed here. Strong IR absorptions were observed in the (C14,15N)2 sample at 2104.3, 2101.0, 2068.2 and 2064.7 cm-1, and annealing to 30 K decreased the first two and increased a shoulder band at 2100.1 cm-1 while decreasing the 15N counterpart band at 2068.2 (14/15 ratio 1.01745) and the shoulder absorption 8 ACS Paragon Plus Environment

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at 2063.8 cm-1 (Figure 3 (m)). Next annealing to 35 K reduced the shoulder at 2100.1 and the peak at 2064.7 cm-1, and another > 220 irradiation increased both band systems and weaker absorptions at 2095.8 and 2060.3 cm-1 (14/15 ratio 1.01723). These bands will be identified in a later section. Investigation was done with a neon matrix and the (13CN)2 sample, and the spectra in blue color are compared in Figure 3 with a similar argon matrix experiment. The strong (13CN)2 absorption in solid argon at 2105.9 cm-1 is blue shifted to 2115.4 cm-1 in solid neon, which is the typical difference in absorption for a small molecule in these two matrixes.38 A weak absorption was observed at 2063.9 cm-1 on sample deposition, which increased more than an order of magnitude on UV irradiation while a weak band appeared at 2070.1 cm-1. Annealing the neon matrix to 8 K reduced the 2063.9 cm-1absorption and increased the 2070.1 cm-1 band several fold, and final annealing to 10 K continued the latter changes in the spectrum. The strongest product absorption increasing on uv irradiation in neon at 2063.9 cm-1 corresponds to the strongest absorption increasing at 2067.8 cm-1 in solid argon although there is a small (3.9 cm-1) red shift in neon, which is uncommon. That leaves the broad weaker band at 2070.1 cm-1 in neon, which increased on final annealing (Figure 3(c, d)), to correspond with the broad argon matrix band produced at 2064.0 cm-1 on uv irradiation, which increased on 30 K annealing (Figure 3(g, h)). Finally, two sets of spectra from the highest and lowest (CN)2 concentrations employed here, namely 3 and 0.1%, are compared in Figure 4. The 2104.3 cm-1 band is dominant in both spectra, and other absorptions above and below this feature were relatively stronger at the higher concentration. The major bands at 2104.3 and 2100.2 cm-1 are observed as weak bands on 9 ACS Paragon Plus Environment


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sample deposition in the more concentrated sample. This means that UV irradiation from the ablation plume activated some laser ablated Be atoms. Annealing to 25 K sharpened, but did not increase, these bands. The spectra again show that UV irradiation increases the 2104.3 band most of all, and annealing reduces it and the 2100.2 band sharpens. The weak band at 2089.1 cm-1 on deposition also increases on UV irradiation to a sharp band at 2089.8 cm-1, and it is close to the 2088.7 BeNC band from the HCN experiment, which was produced here first by UV radiation in the ablation plume during deposition.8 Table 1. Frequencies (cm-1) Observed from the Products of Reactions of Laser-Ablated Beryllium and Magnesium with Cyanogen Isotopes Trapped in Solid Argon at 4 K. (12CN)2 2209.5 2204.6,2201.9 2154.0 2112.2 2107.4 2104.3 2100.2 2098.8 2096.0 2089.8 2089.1 1265.7, 1264.3 1259.5, 1258.7 1199.4, 1198.0 1192.7, 1192.2 1114.6

(13CN)2 2159.7 2155.6 2109.4

12/13 freq. ratio 1.02306 1.02273 1.02114

(12C15N)2

2073.3 2067.8 a 2064.0b 2062.8 2058.9 2051.9

1.01645 1.01765 1.01754 1.01745 1.01802 1.01847

2070.5 2068.2 2064.7

1258.1, 1256.6 1251.9, 1251.2 1194.7, 1193.0 1188.2, 1187.3 1113.4

1.00600,1.00613 1.00607,1.00599 1.00393,1.00419 1.00378,1.00413 1.00108

1263.4 1258.2 1194.2 1191.1

2085.8d 2083.3 2081.2 2076.5 2044.3

2045.2 2042.7 2041.0 2035.6 2002.1

1.01985 1.01988 1.01970 1.02066 1.02113

2052.0e 2049.7 2047.2 2043.2 --

2175.4 2121.2

2060.3 2053.9


14/15 freq. ratio |assignment | NCBeCN 1.01343 | CNBeCN 1.01546 | (CN)2 | aggregate 1.01782 | aggregate 1.01745 |CNBeNC, Be(NC)2 1.01719 | CNBeCN | CNBeNC site 1.01733 | (CNBeBeNC )c 1.01748 | BeNC | BeNC site 1.00182 |CNBeNC, Be(NC)2 1.00103 | Be(NC)2 site 1.00435 | NCBeNC 1.00134 | NCBeNC | NCBeCN 1.01647 |CNMgNC, Mg(NC)2 1.01639 | Mg(NC)2 site 1.01656 | (CNMgMgNC) c 1.01635 | MgNC -| CN

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aNeon

matrix counterpart at 2063.9 cm-1. bNeon matrix counterpart at 2070.1 cm-1. cTenative

assignment. dMg-26 shift to 2085.6 cm-1. Neon matrix counterpart at 2081.2 cm-1. eSplit bands at 2087.0 and 2053.1 cm-1 with 14NC/15NC mixture. Also increasing on annealing in the 3% (CN)2 spectra were sharp satellite bands at 2155.4, 2156.8 and 2158.4 cm-1 above the very strong precursor band at 2154.0 cm-1. The first deposition scan has the first of these as a shoulder and a weak second peak. Annealing to 25 K increases and resolves the first two, and annealing to 30 K brings in the highest of the three bands. Next annealing to 35 K increases all of them slightly, and final annealing to 40 K increases only the highest peak. The strong isolated (CN)2 band decreases during these annealing cycles. Notice that none of these satellite peaks appear in the top spectra using 0.1% sample, which did not give product absorptions until the 220-580 nm photolysis, and they were much weaker. Annealing to 30 K had the same effect as before decreasing the major 2104.4 cm-1 band in favor of the 2100.2 peak. A second 220-580 nm photolysis again increased the major band, and gave a sharp, weak 2089.1 cm-1 band. Final annealing to 35 K decreased the major band in favor of the broad 2112 feature. These observations show that the NCCN precursor molecule diffuses and aggregates on annealing, and we can identify the higher satellite bands as NCCN clusters, and the highest of these is probably the largest cluster without any information on the number of monomers or the structures of these clusters. This information is important for the identification of the extra bands above the major 2104.3 band, which we assign first to the antisymmetric C-N stretching mode for the linear CNBeNC molecule converged with both the B3LYP and CCSD(T) methods. The latter method predicts this mode just 4.9 cm-1 higher than the observed value, which is a lucky break because



anharmonicity and matrix shift have not been considered. Our B3LYP calculation of 2158.8 cm-1 is a reasonable 54.4 cm-1 or 2.6 % higher than the argon matrix value. Product assignment. Beryllium. The strong 2104.3 cm-1 band is assigned to the Be(NC)2 or more explicitly the CNBeNC molecule for several reasons. First it is the dominant band in the spectrum particularly after 220-580 nm irradiation, which is shown in Figure 4 using 3 and 0.1% reagent concentrations. With a linear CNBeNC structure this antisymmetric mode has the highest intensity calculated here (1102 km/mol). This peak decreases on annealing to 30 K in both sets of spectra, while the lower band at 2100.2 cm-1 increases, suggesting that the lower band might be due to a higher energy version of the former band. Our quantum chemical energy, structure and frequency calculations confirm this assignment: B3LYP and CCSD(T) calculations predict the linear CNBeNC di-isocyanide structure is the global minimum energy structure with NCBeCN 38 kJ/mol higher in energy at the B3LYP level and the aug-cc-pVTZ basis sets and 26 kJ/mol higher using CCSD(T). The hybrid DFT functional predicts the harmonic antisymmetric C-N stretching mode for Be(NC)2 at 2158.8 cm-1, which is 54.4 cm-1 or 2.6% higher than observed, and CCSD(T) finds this mode at 2109.3 cm-1, which is just 4.9 cm-1 higher than our observed band. These calculated frequencies are listed in Table 2. The B3LYP computed isotopic harmonic frequency ratios for 12/13 (1.01782) and 14/15 (1.01768) are very slightly higher than the observed values owing to anharmonicity (Table 1), and the CCSD(T) method gives almost the same ratios 12/13 (1.01746) and 14/15 (1.01776). In addition the dicyanide isomer is further ruled out for this assignment because its B3LYP antisymmetric BeCN mode is predicted at 2310.5 cm-1, a total of 152 cm-1 or 7.2 %, higher than our observed band, and the computed isotopic frequency ratios for 12/13 (1.02325) and 14/15 (1.01369) are quite different from the observed values. Notice that the carbon bound cyanide isomer has the 12 ACS Paragon Plus Environment

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much higher computed 12/13 ratio indicating more carbon movement, and the nitrogen bound isocyanide has the higher 14/15 ratio showing more nitrogen movement in the antisymmetric CN stretching mode. The above 12/13 isotopic frequency ratios can be compared with the 1.01848 value for BeNC and the 1.02113 value computed for diatomic CN from argon matrix frequencies where the carbon contributions to each normal mode are different.7 However we must note that the antisymmetric BeC-N stretching mode for NCBeCN has about one fifth of the intensity (216 km/mol) of the di-isocyanide, and we will consider this higher signature region of the product spectrum below. The 1265.7, 1264.3 cm-1 doublet for matrix site splitting (Figure 1,2,S1, The frequencies in Table 1 can help the reader to identify the absorption peaks in Figure S1) tracks precisely with the sharp 2104.3 cm-1 band upon UV photolysis and annealing so this doublet is also assigned to the CNBeNC molecule. Mixed isotopic spectra for the lower frequency CN-Be-NC stretching region demonstrate clearly that this CNBeNC molecule contains two equivalent C and N atoms. What about the strong 2104.3 cm-1 band? Our calculated frequencies for the mixed isotopic molecules C15NBe14NC and 13CNBeN12C show that the C-N stretching modes couple only very slightly such that the two C-N stretching modes of the single 15N substituted isotope differ by only 0.1 cm-1 from the two active modes for the 14,14 and 15,15 isotopes. Likewise the single 13C

substituted isotope frequencies differ by 0.1 and 1.5 cm-1 from the two active modes for the

12,12 and 13,13 isotope values. These differences are hidden in the 1.5 cm-1 bandwidths for the mixed isotopic molecules. Unfortunately the observed 14,15 mixed isotopic band is not as well resolved as the 12,13 case (Figure S1).The strong antisymmetric CN-Be-NC stretching mode is computed as 1302.5 cm-1 (Table 2) that is 36.8 cm-1 or 2.9% higher than observed, which is excellent for the B3LYP density functional. The CCSD(T) method yields a still lower value of 13 ACS Paragon Plus Environment


1284.2 cm-1 for this 1265.7 cm-1 mode. The B3LYP calculated 12/13 isotopic frequency ratio 1.00634 is very close to the observed values (Table 1), and the observed 14/15 ratio1.00113 is near the 1.00085 B3LYP value. This vibration is a mostly Be motion between the two NC groups hence very small C and N isotopic shifts. In Table 3 we compare the CCSD(T) computed energies for the Group 2 metal atom and cyanogen reactions. The Be insertion and rearrangement reaction, which requires initiation by UV light (in the 2s → 2p excitation for Be),37 is the most exothermic insertion reaction at 367 kJ/mol for the formation of CNBeNC. The 1194.4, 1198.0 cm-1 and weaker 1192.7, 1192.2 cm-1 matrix split doublets appear to behave on annealing and photolysis like the above bands assigned to CNBeNC, but they are not in the region of any frequencies for that molecule (Table 2), and they have different isotopic frequency ratios (Table 1), so we must identify another product molecule. The three bands in Figure 1 at 2204.6, 2100.2 and 1199.4 cm-1 group together on annealing and photolysis (Figures 1, 2, S1, S2, S3, S4): in all probability they arise from the same new molecule. The middle band at 2100.2 is just 4.2 cm-1 lower than the very strong band at 2104.3 assigned to the antisymmetric N-C stretching mode for the primary insertion reaction product CNBeNC. This is clearly the region for BeN-C stretching modes as also supported by the observation of BeNC nearby7 at 2089.8 cm-1. Next the weaker band at 2204.6 cm-1 is 104.4 higher than the 2100.2 band, which as described above, is in the region for a cyanide Be-C-N stretching mode so this new molecule contains Be-C as well as Be-N bonds. This important region is illustrated using expanded scales in Figure S2. The BeCN molecule has been observed at 2183.1 cm-1 with a 1.02272 carbon 12/13 isotopic frequency ratio.7 Figure S3 shows that the 2204.6 band forms a clear doublet with


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2175.4 using (14,15CN)2 , which provides the signature for a single N atom motion with C and a lower 14/15 ratio of 1.01343. In another experiment with (13CN)2 the 13C component was observed at 2155.6 cm-1 (Figure S4), and a larger 12/13 isotopic frequency ratio of 1.02273 is obtained. Notice that the 1199.4, 1198.0 matrix split doublet has its 13C counterpart at 1192.7, 1192.2 and there is no intermediate component so this lower vibration also involves a single C atom. The simple related molecule for this product is CNBeCN, which is a 12 kJ/mol higher energy primary reaction product isomer, Table 3, reaction (3). This molecule has a Be core and vibrationally uncoupled Be-CN and Be-NC groups. B3LYP frequency calculations confirm this assignment: the computed isotopic frequencies are listed near the bottom of Table 2. First, the strong BeN-C stretching mode is predicted as 2153.4 or 5.4 cm-1 lower than the value for CNBeNC, and we observed a difference of 4.2 cm-1 for these bands. The lowest frequency mode at 1199.4 is predicted as 1230.8, which is 31.4 or 2.6% higher than observed. The highest frequency motion at 2204.6 cm-1 is computed at 2304.7 cm-1, 100.1 or 4.5 % higher than observed. All of these are in excellent agreement with the B3LYP calculations. The lack of coupling between the -CN and -NC ligands with nonequivalent N and C atoms is revealed by the calculations with single isotopic substitution on the other side of the terminal -N-C stretching mode calculated at 2153.4 for the natural isotopes: this terminal -N-C ligand vibration for N13CBeN-C is calculated as 2153.3, and for 15NCBeN-C the vibration is also predicted as 2153.3 cm-1. Spectra from the (12,13CN)2 reaction give an extra splitting at 2101.1 and at 2062.9, which are separations of 0.9 and 1.1 from the mixed isotopic bands. This provides extra support for two nonequivalent ligands in this product.



The dicyanide NCBeCN isomer (26 kJ/mol higher energy than CNBeNC, Table 3) antisymmetric -C-N stretching mode is computed as 2310.5 which is 5.8 cm-1 higher than the analogous value for the above mixed isomer at 2304.7 cm-1. Expanded scale spectra from the 3 % (CN)2 experiment (Figure S2) also reveal a weak band at 2209.5 which is produced by the same UV photolysis that made the 2204.6 band for CNBeCN and the 2104.3 absorption for CNBeNC. Annealing to 30 K decreased both bands and produced a 2201.9 satellite for the stronger 2204.6 feature. The new band at 2209.5 is 4.9 higher and in excellent agreement with the 5.8 difference in the two B3LYP calculations. The computed value for this N-CBeC-N stretching mode at 2310.5 is 101.0 or 4.6 % higher than the observed value. The N-13CBe13C-N counterpart is observed at 2159.7 with a relatively large 12/13 isotopic frequency ratio of 1.02306, which is characteristic of the cyanide. The computed frequencies have a 1.02325 ratio. Recall that the di-isocyanide had a distinctly smaller 1.01765 ratio for less carbon and more nitrogen motion in the strong antisymmetric C-N stretching mode. Another slightly stronger band computed at 1137.4 is observed at 1114.6 cm-1 (not shown) for the NC-Be stretching mode. This computed band is 22.8 cm-1or 2.0% higher than the observed band. Its observed and computed 13C shifts are both 1.2 cm-1. The weak 2209.5 and 1114.6 cm-1 absorptions and their 13C

counterparts are assigned to the higher energy beryllium dicyanide molecule formed in

primary reaction (4), Table 3. The BeCN molecule is observed at 2183.1 cm-1 two orders of magnitude weaker than the very strong BeNC band at 2088.7 cm-1 using the HCN reaction.7 bands were measured for (BeO)2 at 1131.5 and 866.6 cm-1 at 0.3 cm-1 higher than in the HCN experiments.7,16 The next band in the BeN-C stretching region is at 2096.0 cm-1. This band like the others increases markedly upon 220-580 nm irradiation and decreases steadily on the final annealing 16 ACS Paragon Plus Environment

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cycles. Calculations predict the next absorbing molecule to be CNBeBeNC at 9.9 cm-1 lower than CNBeNC and the 2096.0 cm-1 is 8.3 cm-1 lower. The CNBeBeNC molecule is the dimer of BeNC which absorbs 6-7 cm-1 lower in solid argon,7 but in this experiment with low BeNC yield it would be made by reaction (7) the insertion of a second Be atom into the CNBeNC molecule. The calculation (Table 2) predicts a Be-NC stretching mode in the mid 900 cm-1 region about 1/3 as intense, and our detector noise prevents its detection. Without this lower band our identification of the novel CNBeBeNC molecule is tentative.

Table 2. Calculated Frequencies, Intensities and Bond Lengths for Possible Beryllium Cyanogen Reaction Products Using the B3LYP and CCSD(T) Methods with the aug-ccpVTZ Basis Sets. (Since CCSD(T) does not compute intensities, we match modes with the proper B3LYP counterparts, thus the highest frequency is the antisymmetric stretching mode for CNBeNC and the second frequency is the antisymmetric stretching mode for CNBeBeNC.) BeNC: B3LYP: r(Be-N) = 1.526 Å , r(N-C) = 1.176 Å BeNC: 2151.7 cm-1 (407 km/mol), 967.4 (151), 187.8 (1) BeN13C: 2111.9 (407), 960.3 (145), 186.2 (1) Be15NC: 2114.8 (385), 964.4 (152), 184.1 (1) BeCN: r(Be-C) = 1.667, r(C-N) = 1.155 BeCN: 2290.4 (72), 823.1 (131), 255.6 (5) Be13CN: 2238.7 (65), 820.4 (131), 249.0 (4) BeC15N: 2258.5 (74), 817.9 (129), 253.9 (4) CNBeNC: B3LYP: r(N-C) = 1.1754, r(Be-N) = 1.5155 17 ACS Paragon Plus Environment


CNBeNC: 2158.8 (1102), 2158.5 (0), 1302.5 (461), 522.0 (0), 359.4 (85), 359.4 (85), 180.1 (0), 180.1 (0), 102.9 (28), 102.9 (28). 13CNBeN13C:

2121.0 (1110), 2118.1 (0), 1294.3 (438), 511.0 (0).

C15NBe15NC: 2121.3 (1048), 2121.1 (0), 1301.4 (466), 513.2 (0). 13CNBeNC:

2158.7 (571), 2119.6 (535), 1298.4 (449), 516.5 (0).

C15NBeNC: 2158.7 (554), 2121.2 (520), 1301.9 (463), 517.6 (0). CNBeNC: CCSD(T): r(N-C) = 1.1864, r(Be-N) = 1.5275 CNBeNC: 2109.3, 2103.5, 1284.2, 513.4, 338.0, 338.0, 161.2, 161.2, 96.8, 96.6. 13CNBeN13C:

2073.1, 2064.3, 1275.7, 502.5.

C15NBe15NC: 2072.5, 2066.8, 1283.3, 504.8. 13CNBeNC:

2072.5, 2066.8, 1283.3, 504.8.

C15NBeNC: 2106.6, 2069.5, 1283.8, 509.1. B3LYP : CN-Be-Be-NC: r(N-C) = 1.1754, r(Be-N) = 1.5237, r(Be-Be) = 2.0402. CN-Be-Be-NC: 2159.9 (0), 2148.9 (989), 1170.4 (0), 972.1 (366), 366.1 (0), 366.1 (0), 302.3 (0), 236.0 (10), 236.0 (10), 137.8 (0), 137.8 (0), 70.1 (28), 70.1 (28). 13CN-Be-Be-N13C:

2120.6 (0), 2109.1 (987), 1165.1 (0), 965.0 (353).

C15N-Be-Be-15NC: 2122.6 (0), 2112.0 (934), 1169.3 (0), 969.1 (368). 13CN-Be-Be-NC:

2155.2 (360), 2114.1 (628), 1167.8 (0), 968.5 (359).

C15N-Be-Be-NC: 2155.2 (360), 2116.5 (601), 1169.9 (0), 970.6 (367).


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CCSD(T) CNBe-BeNC: R(N-C) = 1.1864, r(Be-N) = 1.5378, r(Be-Be) = 2.0507. CN-Be-Be-NC: 2104.5, 2094.4, 1158.8, 954.1, 356.8, 355.4, 302.0, 222.1, 212.7, 124.7, 88.7, 68.8. 13CN-Be-Be-N13C:

2066.5, 2055.9, 1153.5, 947.0.

C15N-Be-Be-15NC: 2068.0, 2058.3, 1157.8, 951.3. 13CN-Be-Be-NC:

2100.1, 2060.5, 1156.2, 950.5.

C15N-Be-Be-NC: 2100.1, 2062.4, 1158.3, 952.7.

CNBeBeCN B3LYP/aug-cc-pVTZ CNBeBeCN: 2293.0 (82), 2153.8 (502), 1131.4 (25), 886.0 (308), 386.9 (0), 386.9 (0), 293.3 (0), 267.6 (5), 267.6 (5), 147.6 (1), 147.6 (1), 69.6 (31), 69.6 (31). 13CNBeBe13CN:

2241.2 (73), 2114.2 (501), 1127.1 (21), 882.3 (305), 383.3 (0), 383.3 (0).

C15NBeBeC15N: 2261.3 (85), 2116.7 (474), 1129.6 (26), 881.5 (305), 385.9 (0), 385.9 (0). 13CNBeBeCN:

2293.0 (84), 2114.2 (499), 1127.3 (21), 883.8 (306), 386.9 (0), 386.9 (0).

C15NBeBeCN: 2293.0 (84), 2116.7 (473), 1130.3 (25), 884.9 (309), 386.3 (0), 386.3 (0). CNBeBe13CN: 2241.2 (70), 2153.8 (505), 1131.2 (25), 884.4 (307), 383.4 (0), 383.4 (0).



CNBeBeC15N: 2261.3 (82), 2153.8 (505), 1130.7 (27), 882.6 (303), 386.5 (0), 386.5 (0).

NCBeBeCN B3LYP/aug-cc-pVTZ:NCBeBeCN: 2294.1 (0), 2291.4 (180), 1058.8 (0), 831.5 (304), 404.2 (0), 404.2 (0), 303.0 (3), 303.0 (3), 285.1 (0), 164.5 (0), 164.5 (0), 68.3 (36), 68.3 (36). N13CBeBe13CN: 2242.2 (0), 2239.7 (164), 1058.0 (0), 828.8 (303), 399.2 (0), 399.2 (0). 15NCBeBeC15N:

2262.5 (0), 2259.6 (186), 1055.7 (0), 826.2 (299), 403.7 (0), 403.7 (0).

N13CBeBeCN: 2292.8 (86), 2240.9 (86), 1058.4 (0), 830.2 (303), 401.8 (0), 401.8 (0). 15NCBeBeCN:

2292.8 (82), 2261.0 (101), 1057.3 (0), 828.8 (301), 403.9 (0), 403.9 (0).

Ions. CNBeNC anion is bent (C2v), but the cation is linear. Frequencies of the cation: CNBeNC+: 2239.1 (0), 2098.1 (2835), 1143.0 (915), 492.5 (0), 308.7 (98), 306.7 (89), 211.4 (0), 211.4 (0), 94.9 (9). 13CNBeN13C+:

2196.1 (0), 2054.9 (2807), 1139.3 (884), 482.4 (0).

C15NBe15NC+: 2201.2 (0), 2065.1 (2677), 1140.1 (925), 484.0 (0). 13CNBeNC+:

2220.8 (64), 2073.3 (2757), 1141.2 (900), 487.4 (0).

C15NBeNC+: 2222.4 (53), 2079.4 (2702), 1141.5 (920), 488.3 (0). NBeNC− : 2204.4 (67), 2203.3 (121), 896.0 (46), 556.0 (8), 404.7 (0), 331.1 (13), 298.8 (81), 279.2 (0), 109.9 (2). 20 ACS Paragon Plus Environment

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13CNBeN13C:

2155.6 (68), 2154.6 (117), 894.2 (45), 551.0 (8), 393.7 (0).

C15NBe15NC: 2173.0 (61), 2171.3 (116), 892.1 (45), 552.8 (8), 400.4 (0). 13CNBeNC:

2203.8 (93), 2155.1 (93), 895.1 (45), 553.5 (8), 399.5 (0).

C15NBeNC: 2203.9 (92), 2172.1 (89), 894.0 (45), 554.4 (8), 402.5 (0). NCBeNC B3LYP/aug-cc-pVTZ NCBeNC: 2304.7 (119), 2153.4 (541), 1230.8 (409), 179.6 (1), 383.9 (62), 383.9 (62), 211.3 (10), 211.3 (10), 105.7 (34), 105.7 (34). N13CBeN13C: 2252.5 (114), 2114.5 (541), 1225.6 (399), 470.7 (1), 379.0 (63), 379.0 (63). 15NCBe15NC:

2273.3 (122), 2116.0 (513), 1227.7 (409), 470.6 (1), 382.1 (62), 382.1 (62).

N13CBeNC: 2252.6 (121), 2153.3 (531), 1230.5 (409), 475.3 (1), 379.1 (63), 379.1 (63). 15NCBeNC:

2273.4 (129), 2153.3 (534), 1228.6 (406), 474.4 (1), 383.4 (62), 383.4 (62).

NCBeN13C: 2304.7 (116), 2114.6 (547), 1225.9 (398), 474.9 (1), 383.9 (62), 383.9 (62). NCBe15NC: 2304.7 (114), 2116.1 (518), 1229.9 (412), 475.7 (1), 382.7 (61), 382.7 (61). 15N13CBe15N13C:

2220.2 (117), 2076.1 (513), 1222.8 (398), 462.2 (1), 377.1 (63), 377.1 (63).

NCBeCN B3LYP/aug-cc-pVTZ



NCBeCN: 2310.5 (216), 2295.2 (0), 1137.4 (377), 447.5 (0), 402.4 (53), 402.4 (53), 248.6 (0), 248.6 (0), 110.3 (47), 110.3 (47). N13CBe13CN: 2258.0 (199), 2243.2 (0), 1136.2 (378), 439.9 (0), 395.2 (53), 395.2 (53). 15NCBeC15N:

2279.6 (222), 2263.2 (0), 1131.9 (370), 438.5 (0), 401.7 (54), 401.7 (54).

NCBe13CN: 2304.0 (137), 2249.6 (71), 1136.8 (377), 443.7 (0), 398.9 (53), 398.9 (53). NCBeC15N: 2304.8 (157), 2269.5 (62), 1134.6 (374), 443.0 (0), 402.0 (53), 402.0 (53).

NCBeCN CCSD(T)/aug-cc-pVTZ NCBeCN: 2218.6 (0), 2202.5 (0), 1123.3 (0), 441.4 (0), 385.3 (0), 385.3 (0). N13CBe13CN: 2167.9 (0), 2152.3 (0), 1122.3 (0), 433.9 (0), 378.6 (0), 378.5 (0). 15NCBeC15N:

2189.4 (0), 2172.0 (0), 1117.6 (0), 432.4 (0), 384.7 (0), 384.7 (0).

NCBe13CN: 2211.8 (0), 2158.9 (0), 1122.8 (0), 437.6 (0), 382.0 (0), 382.0 (0). NCBeC15N: 2212.7 (0), 2178.5 (0), 1120.5 (0), 436.9 (0), 385.0 (0), 385.0 (0). Group 2 Metal comparisons. As we move down the periodic table to the heavier Group

2 metals it is useful to compare the C-N stretching frequencies for different metal reaction products on a common frequency scale under nearly the same reaction conditions. The spectra in Figure 5 were recorded from 2% (CN)2 samples (except 3% for Be) after 25 K annealing, 220-


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580 nm irradiation, and annealing to 30 K, which maximized most of M(CN)2 the product absorptions. The bands marked d will be assigned here to the antisymmetric C-N stretching modes of the metal di-isocyanides M(NC)2 although the structures of the Be and Mg products are linear while those for the more electropositive Ca, Sr, and Ba atoms have twisted bowtie sidebound C2 structures. The Be reaction required UV excitation, but the heavier metal atoms reacted during annealing without photolysis, except for Ba, which is probably too big and heavy to diffuse much in our cold argon matrix although UV excitation substantially increased the intensity of the major C-N stretching band for each product. These antisymmetric –N-C stretching modes pretty much follow the C-N stretching bands reported previously for the MNC molecules where the M(NC)2 bands have higher frequencies through Ca then switch over and become lower for Sr(NC)2 and Ba(NC)2 than for SrNC and BaNC, respectively.8 The sharp weak bands at 2089.1 cm-1 (see the 25 K scan in Figure 4) and 2076.5 cm-1 in the Be and Mg spectra are close enough to the stronger bands at 2088.7 and 2076.1 cm-1 for BeNC and MgNC in our earlier experiments using the HCN reagent7,8 to make those assignments here. The broader band at 2058.9 cm-1 is reasonable for CaNC as it is near the sharper and stronger band assigned at 2058.4 cm-1 to CaNC.8 The 2051.2 cm-1 band in the Sr experiment increases on annealing and probably covers the 2052.3 cm-1 SrNC band from HCN experiments,8. Finally, the sharp, weak band at 2049.9 cm-1 in the Ba scan is probably more accurate for BaNC than the broader band measured before8 as 2048.9 cm-1. We will now see how the decrease in ionization energy in the heavier Group 2 metals affects the M(NC)2 and (MCN)2 product structures.



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Table 3. Reaction Energies Calculated Using CCSD(T) and aug-cc-pVTZ Basis Sets for C, N, Be and Mg, cc-pVTZ for Ca, and SDD Pseudopotentials and Basis Sets for Sr and Ba. Reactions (19), (20) and (21) Calculated with B3LYP. Reactions

Energies

Be + CN → BeNC

ΔE = -383 kJ/mol

(1)

Be + NCCN → CNBeNC

ΔE = -367 kJ/mol

(2)

Be + NCCN → NCBeNC

ΔE = -355 kJ/mol

(3)

Be + NCCN → NCBeCN

ΔE = -341 kJ/mol

(4)

CN + BeNC → CNBeNC

ΔE = - 556 kJ/mol

(5)

2 BeNC → CNBe-BeNC

ΔE = -322 kJ/mol

(6)

Be + CNBeNC → CNBeBeNC

ΔE = -148 kJ/mol

(7)

CNBeBeNC → C-(BeN)2-C

ΔE = 241 kJ/mol

(8)

Mg + CN → MgNC

ΔE = -322 kJ/mol

(9)

Mg + NCCN → CNMgNC

ΔE = -212 kJ/mol

(10)

CNMgNC → NCMgCN

ΔE = 8 kJ/mol

(11)

CNMgNC → CN + MgNC

ΔE = 462 kJ/mol

(12)

2 MgNC → CNMg-MgNC

ΔE = -197 kJ/mol

(13)

Mg + CNMgNC → CNMgMgNC

ΔE = -57 kJ/mol

(14)

CNMgMgNC → C-(MgN)2-C

ΔE = 46 kJ/mol

(15)

Ca + NCCN →(CN)Ca(CN)

ΔE = -316 kJ/mol

(16)

Sr + NCCN → (CN)Sr(CN)

ΔE = -269 kJ/mol

(17)

Ba + NCCN → (CN)Ba(CN)

ΔE = -354 kJ/mol

(18)

__________________________________________________________________________________ Ca + Ca(CN)2 → (Ca(CN)2

ΔE = -150 kJ/mol

(19)

Sr + Sr(CN)2 →

(SrCN)2

ΔE = -153 kJ/mol

(20)

Ba + Ba(CN)2 → (BaCN)2

ΔE = -171 kJ/mol

(21)


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_________________________________________________________________________________________________

Infrared Spectra and Product Assignments. Magnesium. Figure 6 compares spectra from Mg experiments using natural isotopic (CN)2 (middle set) and (13CN)2 (top set) in argon, and the frequencies are given in Table 1. The deposition scan in Figure 6 (e) shows a weak, broad band at 2081.2 cm-1, and 25 K annealing reduced this band and produced sharp new bands at 2085.8 and 2076.5 cm-1. Next uv irradiation (220-580 nm) produced a strong, broad absorption with resolved peaks at 2085.8, 2083.3 and 2081.2 cm-1. As described above for Be, annealing to 25 K and to 30 K after UV photolysis increased the major absorption now at 2085.8 cm-1 and a minor band at 2081.2 cm-1. The major band remained on 35 K and decreased slightly on 40 K annealing, and the 2081.2 cm-1 band increased slightly (not shown). Also detected were weak bands at 2044.5 and 2002.1 cm-1 due to 12CN and 13CN which decreased on annealing in agreement with previous work using HCN as the reagent.8 The major absorption agreed with a previous weak absorption observed at 2085.7 cm-1 in experiments using HCN as a source of cyanide, and assigned there to a matrix site of MgNC, which absorbed strongly at 2076.1 cm-1.8 These absorption frequencies are given in Table 1, and now the strongest band is assigned to Mg(NC)2 based on agreement with our DFT and CCSD(T) calculations of harmonic frequencies, which predict the strong antisymmetric N-C stretching mode at 2141.6 and 2083.3 cm-1, respectively. The B3LYP value is 55.8 cm-1 or 2.6% too high and excellent agreement for the B3LYP functional, and the CCSD(T) result is 2.5 cm-1 lower than observed. A similar experiment using 26Mg metal (96% enriched) gave sharper absorptions here at 2085.6 cm-1 (full width at half maximum (0.9 cm-1), and at 2080.7 cm-1. Hence the stronger absorption at 2085.8 cm-1 using natural isotopic Mg (80 % 24Mg) exhibits a 0.2 cm-1shift with 26Mg. This shift agrees 25 ACS Paragon Plus Environment


with that computed for Mg(NC)2 from the frequencies calculated for each Mg isotopic molecule, which are given in Table S1 (Supporting Information). It is important to note that the major 2085.8 cm-1 Mg(NC)2 band is produced upon annealing to 25 K before UV irradiation (Figure 6 (f)) but the analogous spectrum for Be, Figure 4 (h) does not reveal any of the major Be(NC)2 product until irradiation. We conclude from these observations that Mg is more reactive with (CN)2 than Be. In addition the isotopic shifts for the 13C and 15N substituted isocyanide product are presented here as isotopic frequency ratios: computed as 1.02005 for the 12/13 frequency ratio and 1.01681 for the 14/15 ratio from the B3LYP frequencies (Table S1 in SI). These ratios are slightly higher than the observed values of 1.01985 and 1.01647 given in Table 1 for the diisocyanide Mg(NC)2 because of anharmonicity in the observed measurements not accounted for in the harmonic B3LYP method. Similar calculations find the Mg(CN)2 cyanide antisymmetric C-N stretching frequency to be 117 cm-1 higher and 10% as strong and to have different computed 1.02258 and 1.01462 isotopic frequency ratios because the carbon bound isomer supports more carbon and less nitrogen participation in the antisymmetric C-N stretching mode for Mg(CN)2. Hence the higher position of the calculated frequency and the distinctly different isotopic frequency ratios (Table 1) rule out assignment to the dicyanide and confirm assignment to the di-isocyanide. Interestingly, our B3LYP calculations predict the dicyanide to be 11.7 kJ/mol higher in energy than the di-isocyanide, and the CCSD(T) method finds the dicyanide to be 7.6 kJ/mol higher in energy. The previous computational work by Kapp and Schleyer using the MP2 method and standard 6-31+G* basis sets found the magnesium dicyanide to be 0.00139 a.u. (3.6 kJ/mol) 26 ACS Paragon Plus Environment

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lower energy than the di-isocyanide.9 Our CCSD(T)/aug-cc-pVTZ calculation is surely more accurate than their MP2 calculation, and the matrix isolation experiment clearly trapped the lower frequency CNMgNC isomer with the higher 15N shift and lower 13C shift as compared to those calculated for the unobserved cyanide isomer NCMgCN. In addition we observed MgNC isotopic molecules here and with the HCN reaction without observation of the higher energy MgCN isomer.8 Figure 7 shows the product of reaction with statistical (C14,15N)2. Although the strong cyanogen isotopic triplet indicating two equivalent N atoms in the precursor is not shown here, it appeared like that in Figure 3. The spectra using (C14,15N)2 revealed broad absorptions at 2080 and 2048 cm-1 and sharp bands marked * for mixed isotopic NCNC on sample deposition. Next annealing to 25 K produced sharp, weak bands at 2085.8 and 2052.0 cm-1 and increased absorptions at 2076.5 (Mg14NC) and 2043.1 cm-1(Mg15NC). Then > 220 nm irradiation (Fig.7) increased the original broad bands markedly. As before, annealing to 30 K produced strong, sharp absorptions at 2085.8 cm-1, this time with a resolved shoulder at 2087.0 cm-1 and at 2052.0 cm-1 with a shoulder at 2053.1 cm-1. These shoulders are due to the mixed isotopic molecule containing both 14N and 15N, and they define splittings from the pure 14N and 15N bands of 1.2 and 1.1 cm-1, which are in satisfactory agreement with the computed differences between the appropriate frequencies in Table S1(Supporting Information). Peaks are identified in Figure 7 for the three isotopic cyanogen reaction products Mg(NC)2, Mg(14NC)(15NC) and Mg(15NC)2. Photolysis also produced sharp weak bands at 2081.2 and 2047.2 cm-1, and these peaks remained on 30 K and increased slightly on 35 K annealing, which opens the possibility of a higher order product species. Reaction (11) shows that the addition of another Mg atom to



Mg(NC)2 is exothermic by 58 kJ/mol to form the CNMgMgNC molecule. Frequency calculations compared in Table S1) predict that the strong antisymmetric -N-C stretching mode for CNMgMgNC is 1.3 cm-1 lower (B3LYP) and 3.2 cm-1 lower (CCSD(T), which are in acceptable agreement with the 4.6 cm-1 difference between the observed frequencies for CNMgNC and CNMgMgNC. Without observation of the lower antisymmetric stretching mode, which is below the limit of our detector, we must make this a tentative assignment. The blue set of spectra was recorded in a neon matrix (blue was used to discriminate between the neon and argon matrix spectra in the same figure), and a weak 2081.2 cm-1 band was observed on deposition, which decreased on annealing to 8 K. UV irradiation increased this sharp band and a broad blue shoulder absorption, but annealing to 10 K reduced the sharp band. This sharp band is surely due to the same Mg(NC)2 species as the stronger band at 2085.8 cm-1 in solid argon although the red shift (4.7 cm-1) in neon is less common than a blue shift.36

Infrared Spectra and Product Assignments. Calcium. Figure 8 illustrates IR spectra for products of Ca reacting with (CN)2 and with (C14,15N)2 (top set). The strong band at 2054.2 cm-1 on sample deposition is due to the CNCN photoisomer (marked*) of our reagent, and it is produced more efficiently with calcium and barium resonance radiation in the emission plume than with light from the other metal plumes (see Figure 5 where the * band is larger for Ca and Ba than for the lighter Group 2 metals). The sharp, weak band at 1997.1 cm-1 is due to another precursor isomer, CNNC.36 The spectrum for the Ca reaction with (13CN)2 is shown in Supporting Information as Figure S5, and the new product absorptions are listed in Table 4. The


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spectra for Sr and Ba reactions in Figures 9, S8, and S9 show band profiles much like the Ca spectra. In contrast to Be and Mg, the Ca deposition provided stronger metal reaction product bands, which are located at 2060.8, 2058.9, 2056.8, and 2036.3, 2032.1 cm-1. The second of the first three weak bands on the left of CNCN is close to the 2058.4 cm-1 band assigned previously to CaNC.8 The first three bands increased on annealing to 25 K but decreased on UV irradiation, and annealing to 30 K increased the first band at 2060.8 cm-1 substantially, decreased the second band assigned here to CaNC, and increased the third 2056.6 cm-1 peak which falls 4.2 cm-1 below the strong 2060.8 cm-1 band. Following the BeNC, CNBeNC, MgNC and CNMgNC reaction product absorptions and their behaviors on annealing, the strongest absorption at 2060.8 cm-1, just above CaNC here at 2058.9 cm-1, is assigned to the most stable isomer of Ca(NC)2. However, the structure changes as the metal becomes more electropositive,9-13 and the isosceles triangular structures of the alkali metal superoxide molecules weigh in here.10-12 Following the above linear Be and Mg di-isocyanides, linear CNCaNC was found to have a degenerate negative N-Ca-N bending frequency at 47 cm-1, but it is 39 kJ/mol more stable than the analogous linear cyanide NCCaCN. However, a seagull structure with bent C-N-Ca bonds is 0.2 kJ/mol lower in energy than the linear molecule, and continuing this geometric change, a twisted bowtie structure is 3.4 kJ/mol lower in energy than the linear di-isocyanide and has all real frequencies. Previous MP2/6-31+G* calculations also found this C2 bowtie structure to be the global minimum for the three heaviest Group 2 metals, and to be 0.00707 a.u. (18.3 kJ/mol) lower in energy for Ca(NC)2 than for CNCaNC.9 Our calculations using CCSD(T) with aug-ccpVTZ for C and N and SDD for Ca find the bowtie structure to be 6.4 kJ/mol lower in energy than the linear molecule and to have all real frequencies. Thus, we assign the strong dominant 29 ACS Paragon Plus Environment


band at 2060.8 cm-1 to the antisymmetric C-N stretching mode for the twisted bowtie lowest energy isomer Ca(NC)2, which can be written as (CN)Ca(CN) to denote side bound ligands that are neither cyanide or isocyanide. The third band at 2056.6 cm-1 behaves nearly the same as the bowtie band through the spectrum after 25 K annealing and photolysis Figure 8 (d), but it does not increase as much as the bowtie band on 30 K annealing. The bowtie band still increases on 35 K annealing, but the 2056.6 band now decreases. This is precisely the behavior we expect for a higher energy isomer of the bowtie, namely the linear CNCaNC molecule. Our B3LYP calculations predict the strong antisymmetric N-C stretching mode of the linear isomer to fall 3 cm-1 below the bowtie, and the 2056.6 band is 4.2 cm-1 lower and in excellent agreement. The clear increases for this band on annealing to 25 and to 30 K in solid argon show that we are really dealing with a super argon complex of linear CNCaNC, which should be able to overcome any effect of the 47 cm-1 imaginary bending mode calculated for the isolated molecule. The two CN ligands and their atomic positions are equivalent in the twisted bowtie structure, and there is no distinction between cyanide and isocyanide since the Ca—N and Ca— C bond lengths on each side of the Ca center are equal for this structure (Ca—N=2.279 and Ca— C=2.588 Å). Note that the Ca—N bond is about 0.3 Ằ shorter. This bowtie molecule is labeled as Ca(NC)2 in Figure 8, and its structure is shown in Figure 10 along with the Sr and Ba analogs. The bowtie structure with two sideways (CN) subgroups has two different computed C-N stretching modes, stronger antisymmetric and weaker symmetric frequencies (SI) which are nearly the same, 2110.0 and 2110.7 cm-1, owing to very little coupling through the heavier Ca atom between them. This small difference cannot be resolved within the 0.8 cm-1 bandwidth.


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We can make a comparison of the 12/13 and 14/15 isotopic frequency ratios from the calculated frequencies for the twisted bowtie and the slightly higher energy linear molecule. The observed ratios given in Table 4, 1.02050 and 1.01602, respectively, are in good agreement with both ratios for the computed linear molecule (1.02078 and 1.01620) and bowtie (1.02135 and 1.01579) so this does not provide a basis for choice of structure for the carrier of the strongest band. Neither do the computed antisymmetric C--N stretching frequencies for each structure 2107.2 (linear) and 2110.0 cm-1 (bowtie), respectively, which are really within computational error of each other. Thus we are left with the computed energies for each structure where the bowtie is lower energy and the dominant product absorption assigned above. The 12/13 ratio for the minor linear isomer product also fits the calculated values. Now back to Figure 8 where sample deposition also gives new bands at 2036.3, and 2032.1 cm-1. Full arc photolysis produced more absorption at 2032.1 and at 2053.6 cm-1 on the lower side of the * band and reduced the Ca(NC)2 band. Next annealing to 30 K increased the 2060.8, 2056.6 and 2036.3 cm-1 bands and decreased the CaNC absorption. Annealing to 35 K increased the 2060.8 and 2036.3 cm-1 bands even more, but 40 K allowed for a slight decrease in each. The annealing behavior of the lower frequency bands suggests that they are due to a higher order species. Accordingly we did calculations of the dimer species linear CNCaCaNC, which is formed in the 44.5 kJ/mol exothermic reaction of Ca with CNCaNC. The linear dimer CNCaCaNC is predicted to absorb 2 cm-1 above the bowtie, but we have no distinct absorption in this region. However, the 2036.3 cm-1 band is near the frequencies given below for the analogous Sr and Ba reaction products (Figures 9, S8 and S9) so we likely have a common structure for this



lower frequency band in the Ca, Sr and Ba reactions. More importantly the calculated isotopic frequencies give the 12/13 and 14/15 frequency ratios [1.02168 and 1.01548], which are in excellent agreement with the observed values (Table 4). We will examine the ionic bonding in this unique six-membered ring in a later section of this paper. The upper group of absorptions in Figure 8 comes from the Ca reaction with (C14,15N)2, and the original absorption pattern for 14N repeats itself for 15N. However, there is no shift nor splitting in the strong Ca(NC)2 bands. In particular our isotopic frequency calculations show only 0.3 cm-1 difference between the mono- and di-substituted (ie. the 14,15 and 15,15) species, and this is within the experimental bandwidth (0.7 cm-1). As in Figure 5, there are four bands labeled * for CNCN with two nonequivalent N atoms. The isotopic counterparts including 15N

for these absorptions are listed in Table 4. The blue colored scan at the bottom of Figure 8 was recorded using a neon matrix. The

strongest bands at 2067.2 and 2035.2 cm-1 may be compared with the argon matrix values in Table 4. Annealing to 8 K (not shown) reveals a larger decrease in the upper band than in the lower band so it is likely that these two bands do not belong to the same product species. The strong Ca(NC)2 band at 2067.2 cm-1 is blue shifted by 6.4 cm-1 from the probable 2060.8 counterpart in the argon matrix. These bands are in the range expected for a metal species,26 but the strongest absorption for (CaNC)2 is red shifted by 1.1 cm-1 in solid neon from the sharp 2036.3 counterpart in solid argon. Note that the more aggregated ring species exhibits a stronger absorption in the more slowly condensed neon matrix. It is not clear why the linear CNBeNC and CNMgNC molecules interact more strongly and attractively with the neon matrix environment to produce red shifts in their strongest modes (-3.9 and -4.7 cm-1) than does the


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different Ca(NC)2 bowtie structure, which exhibits a repulsive blue shift (+ 6.4 cm-1) in solid neon as compared to solid argon. Table 4. Frequencies (cm-1) Observed from the Products of Reactions of Laser-Ablated Ca, Sr, and Ba with (CN)2 Trapped in Solid Argon at 4 K. (12CN)2

(13CN)2

12/13 freq ratio

(12C15N)2

14/15 freq ratio

assignments

2060.8 2058.9 2056.6 2054.2 2044.3 2036.3 2032.1

2019.4 covered by * 2015.3 2017.7 2002.1 1994.3 1990.0

1.02050

2028.3 2026.7 2021.3 2018.2 --2005.3 2001.4

1.01602 1.01589 1.01746 1.01784

Ca(CN)2 CaNC CNCaNC *CNCN CN (refs7,8) (CaNC)2 (CaNC)2

2051.2 2048.1a 2046.4 2040.6 2038.6 2035.4

2009.2 2006.4 2004.5 1998.4 1996.4 1993.2

1.02090 1.02078 1.02080 1.02112 1.02114 1.02117

1.02049 1.01809 1.02108 1.02106 1.02111

--2016.2 covered 2009.5 2007.5

1.01546 1.01534 1

1.01582 1.01548 1.01549

NCCN-Sr(NC)2 Sr(NC)2 Sr(NC)2 site (SrNC)2 (SrNC)2 site (SrNC)2 site

2054.2 2017.7 1.01809 2018.2 1.01784 *CNCN 2049.9 2008.4 1.0245 --BaNC b 2045.9 2004.2 1.02081 2014.1 1.01579 Ba(NC)2 2039.5 1997.7 1.02092 --(Ba(NC)2 site c 2036.0 1994.1 1.02101 2004.6 1.01557 (BaNC)2 2031.8 1990.0 1.02101 --(BaNC)2 site 2030.6 1988.7 1.02107 2000.2 1.01529 (BaNC)2 site aSplit bands at 2049.9 and 2017.9 cm-1 with 12, 13. bSplit bands at 2048.2 and under 2046.0* with 14, 15. cSplit bands at 2037.4, 2005.8 cm-1 with 14, 15. The asterisk * is used to represent the precursor isomer CNCN. ______________________________________________________________________________ Infrared Spectra and Product Assignments. Strontium. Complete discussions of Strontium and Barium spectra and assignments are given at the end of Supporting Information. Now let us examine the Figure 9 spectra for Sr, which look a lot like the above Ca spectra. Notice the new 33 ACS Paragon Plus Environment


absorptions at 2048.1, 2040.6 and 2035.4 cm-1 after sample deposition, and that the first two absorptions, and a new one at 2051.2 cm-1 increase on annealing to 25 K. The first band at 2048.1 cm-1 is dominant; however, UV photolysis decreases this band and increases two bands at 2051.2 and 2040.6 cm-1. Annealing after photolysis increased the prominent 2048.1 cm-1 band almost threefold. Notice also that the Sr product shifts from the Ca and (CN)2 product spectra in Figure 8 are different: the major band at 2048.1 cm-1 is red shifted 12.7 cm-1 from the Ca(NC)2 counterpart, while the favored band at 2040.6 cm-1 is 4.3 cm-1 higher than the Ca counterpart at 2036.3 cm-1 for the higher order species. The strong, sharp 2048.1 cm-1 band is just 4.2 cm-1 lower than SrNC as observed in the earlier reaction with HCN.8 The observed 12/13 and 14/15 isotopic frequency ratios 1.02078 and 1.01582 are slightly lower than the computed isotopic frequency ratios of 1.02138 and 1.01574 for the twisted (CN)Sr(CN) bowtie structure. The calculated strongest harmonic antisymmetric C-N stretching frequency at 2116.1 cm-1 is 3.2% higher than the observed value, which is in the range of other B3LYP computed values in this work. A set of spectra is reported in Figure S6 for Sr and the statistical (14,15NC)2 reagent. Sample deposition produced weak bands (not shown) like in Figure 10. An extra band is observed at 2049.9 cm-1 just 1.8 cm-1 higher than the stronger main peak, and a similar peak is observed in the lower region at 2017.9 cm-1 just 1.7 cm-1 higher than the main band and 0.3 cm-1 below the 2018.2 cm-1 peak for C15NC15N. The ratio of these two new bands for Sr(14NC)(15NC) is 1.01586 and slightly higher than the above 14/15 ratio for the main bands. Assignment of the main bands to the bowtie structure for Sr(NC)2 is made on the basis of energy calculations as the calculated frequencies are nearly the same for all plausible 34 ACS Paragon Plus Environment

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structures. The bowtie is 10.7 kJ/mol lower in energy than the linear CNSrNC molecule at the B3LYP/ aug-cc-pVTZ/SDD level and 25 kJ/mol lower with the MP2 method.9 It is interesting to point out that the splittings in the main bands in Figures S8 and S6 for the single and double isotopic substitution are computed to be 2.0 cm-1, and the measured separations for these two peaks given above are 1.9 cm-1 in agreement within experimental error (± 0.1 cm-1) . Our calculations for the reaction energies of two Group 2 metal atoms with (CN)2 show that the hexagonal (SrNC)2 ring molecule is almost 22 kJ/mol lower in energy than the linear dimer, and the strong antisymmetric C-N stretching mode at 2040.6 cm-1 is predicted to be 5.5 cm-1 higher than that for the Ca analog, which is close to the 4.3 cm-1 experimental difference. The calculated value for the (SrNC)2 species is 40.5 cm-1 or 1.9% higher than observed, which is almost the same relationship given above for the Ca analog. The calculated 12/13 and 14/15 isotopic frequency ratios [1.02125 and 1.01591] for the strong 2040.6 cm-1 band are almost the same as the observed values (Table 4) and the values given for the Ca counterpart above. Infrared Spectra and Product Assignments. Barium. The spectrum for Ba and (CN)2 reaction products using 2% (CN)2 follows those for Ca and Sr in appearance with several noteworthy comparisons (Figure S9 (f to l), top). The major insertion product bowtie M(NC)2 frequency decreases steadily from 2060.8 cm-1 for Ca(NC)2 with increasing metal size (Figure 5) to 2045.9 cm-1 for Ba(NC)2. The lower pattern of sharp and broad bands where the sharp band increases on annealing at the expense of the broad band repeats but following a frequency increase from Ca to Sr these sharp bands decrease from Sr to Ba even though the B3LYP calculations for the ring species predict an increase of 3.3 cm-1. The computed frequency error with the SDD pseudopotential is more than this difference. The barium bowtie molecule is



predicted to absorb strongly for the antisymmetric C-N stretching mode at 2109.1 cm-1, (Table S2), which is 63.2 cm-1 or 3.0% higher than our observed band with about the same difference as found for the analogous Sr and Ca bearing hexagonal ring structures. The broad band at 2032 cm-1 gave way to resolved features at 2036.0, 2031.8 and 2030.6 cm-1 on annealing, and the sharp 2036.0 band became the strongest absorption observed in these experiments. Full arc irradiation (scan h) only increased the original broad band. Our B3LYP calculations predict the small ring C-(BaN)2-C to have its strongest absorption 25 cm-1 lower than the Ba bowtie and the very strong band is ten inverse centimeters below the observed bowtie absorption. The calculated 12/13 and 14/15 isotopic frequency ratios [1.02126 and 1.01584] for this strong band are almost the same as the observed values (Table 4) and the above values given for the Ca and Sr counterparts, but this molecule has two imaginary frequencies. However, the six membered ring triplet state also has almost the same computed isotopic frequency ratios [1.02161 and 1.01558], and all real frequencies (Table 5), and it is 50 kJ/mol more stable than the rhombus core species! The B3LYP computed antisymmetric C-N stretch at 2184.2 cm-1 for this hexagonal ring is 148 cm-1 or 7.3% higher than the observed value, pretty much the same as found for the analogous Sr and Ca species. It is expected that the more ionic ring molecules will have a larger difference from the calculated value than the less ionic bowties owing to the stronger interaction with the matrix. The agreement of vibrational characteristics (frequencies: 2191, 2188, 2184 cm-1 and observed isotopic frequency ratios (Table 4)) for the (MCN)2 six atom-rings for M = Ca, Sr, and Ba substantiates their identifications in these matrix isolation experiments. Figure S7 using 0.5% (CN)2 also shows the higher band to be stronger than the lower band after sample deposition, and the lower band to increase far more on annealing than the higher band. 36 ACS Paragon Plus Environment

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The above annealing information shows that the hexagonal rings (MNC)2 (see the schematic diagram below Table 5) are most likely made by the simple reaction of another metal atom with the bowtie [M + (CNM(NC)  (MNC)2.] There is clearly not enough MNC produced in these experiments to form much of its own dimer. Annealing to 25 and 30 K has little effect on the relative band intensities, but annealing to the higher 35 and 40 K temperatures increases the (BaNC)2 bands almost two and three fold before 45 K decreases the 2036.0 band in favor of the broad 2040 absorption that is surely an aggregate with the reagent (CN)2. This process is more dominant with Sr in Figure 9, but less so with Ca in Figure 8. The lower set of spectra in Figure 9 using the statistical mixed isotopic (C14,15N)2 sample shows two sets of bands for stretching modes involving C-14N and C-15N subunits. These annealing spectra show stepwise increases in the sharp bands labeled r (for ring) at the expense of the original broad bands. Notice a splitting of the 2036.0 r band at 2037.4 cm-1 and the 2004.6 r band at 2005.8 cm-1. These extra bands are due to reaction with the (C14NC15N) isotopic molecule to form (Ba14NCBa15NC), and the observed splittings from the pure isotopic counterparts (Ba14NC)2 and (Ba15NC)2 of 1.4 and 1.2 cm-1 are reproduced exactly by our isotopic frequency calculations (Table 5) for the ring (BaNC)2 ionic molecule. The (MNC)2 molecules are planar six-membered rings in the C2h structure triplet ground state, and are obviously more stable than C-(MN)-C, which has a rhombus core at the center. Attempts to optimize the structure of C-(CaN)-C with keyword “tight”, ended up with the hexagonal (CaCN)2 structure, which means that there is essentially no barrier between them. Table 5. (MCN)2 Ring Frequencies (cm-1) and Intensities (km/mol) Calculated with B3LYP and the aug-cc-pVTZ) Basis Sets for C and N and SDD for Ca, Sr, and Ba. ________________________________________________________________________ 37 ACS Paragon Plus Environment


Triplet planar C2h, six-membered ring represented as (CaNC)2 or Ca-(NC)2-Ca: Ca-(NC)2-Ca: 2191.4 (67), 2189.6 (0), 362.6 (197), 323.3 (0), 277.0 (0), 258.6 (32), 175.3 (0), 166.0 (30), 165.5 (2), 144.7 (0), 101.8 (0), 54.5 (0). Ca-(N13C)2-Ca: 2144.9 (71), 2143.0 (0), 360.0 (193). Ca-(15NC)2-Ca: 2158.0 (59), 2156.3 (0), 355.5 (193). Ca-(N13C)(NC)-Ca: 2190.5 (35), 2143.9 (34), 361.3 (195). Ca-(15NC)(NC)-Ca: 2190.5 (35), 2157.1 (28), 359.3 (194). Triplet planar C2h Sr-(NC)2-Sr, six-membered ring Sr-(NC)2-Sr: 2188.4 (70), 2186.7 (0), 318.3 (157), 282.2 (0), 232.7 (0), 211.1 (23), 156.0 (0), 147.4 (1), 142.6 (35), 124.9 (0), 75.0 (0), 39.1 (0). Sr-(N13C)2-Sr: 2142.0 (75), 2140.2 (0). Sr-(15NC)2-Sr: 2154.9 (62), 2153.3 (0). Sr-(N13C)(NC)-Sr: 2187.5 (36), 2141.1 (36). Triplet planar C2h (BaNC)2 six-membered ring Ba-(NC)2-Ba: 2184.2 (101), 2182.7 (0), 304.1 (123), 265.1 (0), 214.2 (0), 194.7 (13), 147.7 (1), 139.8 (0), 130.3 (36), 120.4 (0), 59.0 (0), 25.2 (1). Ba-(N13C)2-Ba: 2138.0 (105), 2136.3 (0). Ba-(15NC)2-Ba: 2150.7 (91), 2149.3 (0).


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Ba-(N13C)(NC)-Ba: 2183.4 (52), 2137.1 (51). Ba-(15NC)(NC)-Ba: 2183.4 (53), 2150.0 (43).

Schematic diagram for hexagonal (CaNC)2

Reactions in the matrixes.

The reactions given in Table 3 and their energies from

(CCSD(T)/aug-cc-pVTZ) calculations are important here. The primary reaction (2) is exothermic, but it needs to be activated by 220-290 nm mercury arc UV irradiation especially for Be where our UV lamp reaches the UV 2s to 2p absorption for Be in solid argon.37 The fact that reaction (11) is endothermic shows that these di-isocyanide reaction products are the stable isomer. It is interesting to ponder reactions (2,3,4) in Table 3, which according to our UV irradiation experiments require 2s to 2p excitation of Be and proceed through the following reaction, where [ N≡C--Be--C≡N ]* is relaxed and rearranged in the matrix, to produce the successively lower energy products of reactions (4,3,2) in Table 3. This suggests that the 39 ACS Paragon Plus Environment


isomerizations involved in these Be reactions proceed in an excited electronic state rather than the ground state proposed for MgCN/MgNC isomerization.4 Previous calculations have suggested that considerable fluxionality is to be expected in these molecules.9

Be* + :N≡C---C≡N: → [ :N≡C--Be--C≡N: ]* → [ :C≡N--Be--C≡N: ]* → [ :C≡N--Be--N≡C: ]

Infrared spectra in Figure 5 illustrate the series of product molecules containing the BeNC linkage and exhibiting the BeN-C vibrational mode with the analogous spectra in Figure S2 which shows the higher energy carbon bound isomers in their approximately 100 cm-1 higher BeC-N vibrational mode. The

13C

and

15N

isotopic shifts are compared through the 12/13 and 14/15

isotopic frequency ratios where the N bound BeN–C isomer has higher 14/15 and lower 12/13 ratios and the C bound to Be isomer has higher 12/13 and lower 14/15 ratios (see Table 1). In these cases the isotopic frequency ratios help to identify the product molecules. Magnesium probably reacts spontaneously to form the same initial structure :N≡C--Mg--C≡N:, but 220-290 nm mercury arc irradiation produces a much higher yield of the final CNMgNC product. The more electropositive Ca, Sr, and Ba atoms also react on annealing in the cold argon matrix, but again UV radiation markedly increases the yield of this ionic product, which has side bound CN ligands with a twisted bowtie C2 structure that can be written as (CN)M(CN). Sample deposition and UV photolysis also produce a broad band in the 2030 cm-1 region which sharpens on annealing. This product is formed by the addition of a second Ca, Sr, or Ba atom to the bowtie structure, and the lowest energy structures are the ionic triplet planar C2h hexagonal rings of nominally two M+ and two (CN)− anions (See the schematic diagram after Table 5). 40 ACS Paragon Plus Environment

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Structure and Bonding. Structures for the M(NC)2 molecules were calculated at the B3LYP and CCSD(T) levels with the aug-cc-PVTZ basis set for all atoms through Ca and SDD (10 valence electrons) for Ca, Sr and Ba. The latter is shown in Figure 10 for consistency within the heaviest three members of the alkaline earth metal family. The bonding in linear CNBeNC and CNMgNC is straightforward, and our Natural Population Analysis for these two linear molecules plus CNCaNC is summarized in Table 6. These metals are electropositive, and they form sp hybrid orbitals, which make sigma bonds with sp hybrid orbitals on the N’s. This is followed by forming a sigma bond and a pi bond to the C atoms yielding the simple Lewis structure :C=N— Be—N=C: The electropositive nature of Be allows an electron transfer from each Be—N single bond into the orthogonal pi orbital resulting in the Wiberg bond orders listed in Table 6. Magnesium is more electropositive, and as a result more electron density is transferred from the Mg—N bond into the orthogonal pi bond leaving a weaker Mg—N sigma bond and a stronger pi N=C bond. These molecular orbitals for mostly covalent bonds are shown in Figure 11. The antisymmetric N-C stretching mode for CNMgNC (2085.8 cm-1) is lower than that for CNBeNC (2104.3 cm-1), and this difference can be ascribed to the greater negative charge in the isocyanide group (Table 6). Following the side bound ionic structures for Li(O2) and K(CN),10-13 which optimize ionic bonding, the even more electropositive character of Ca compared to Mg leads to a still different structure,9 which we describe as a twisted bowtie that contains two side bound C-N subunits (Figures 10 and S10). NBO calculations find the natural charges Ca(+1.54), N(− 0.79 each), and C(+0.02 each). It is interesting that the first bonding orbital found by NBO was (2e) 63% N (46% s, 53% p) sp hybrid and 37% C (32% s, 67% p) sigma molecular orbital followed by a pair of (2e) 66% N (99% p), 34% C (99%) bonding pi orbitals. Essentially the same occupancy and 41 ACS Paragon Plus Environment


orbitals were observed for the other CN group. There were no bonding orbitals presented for the Ca—C or Ca—N bonds, but the above natural charges reveal that ionic bonding is mostly responsible for the intramolecular (CN)—Ca—(CN) attraction where we have a Ca cation (+1.54) attracting two side bound C≡N anionic units (−0.77) at distances of approximately 2.43 Å (average of Ca—N and Ca—C distances). Notice (Table 5) that the linear CNBeNC and CNMgNC molecules have decreasing covalent character, and that we find no covalent character for the linear CNCaNC molecule. Recall that our energy calculations show linear structures are the most stable for the first two Be and Mg bearing molecules, but with Ca ionic bonding prevails and maintains two side bonded (CN) subunits for the most stable isomer. Moving down the Periodic Table to Sr we expect and find increased ionicity for the bowtie structure: the natural charges are Sr(+1.64), N(-0.81 each), and C(-0.01 each). The first bonding orbital was again sp on the N (2e) 63% N (42% s, 57% p), 37% C (30% s, 70% p) and with a pair of identical (2e) 66% N (99% p), 34% C (99%) pi orbitals. Again the same occupancy and orbitals were observed for the other CN group, and no other orbitals were computed. Barium is even more electropositive: natural charges Ba (+1.71), N (-0.88 each), C(+0.03 each): the first CN molecular orbital is (2e) 63% N(43% s, 56% p, 1 % d), 37% C (30% s, 69% p) and two sets of identical (2e) 67% N (99% p ), 33% C(99% p) orbitals. In conclusion ionic bonding between Sr(+1.64) or Ba (+1.71) and two side bound (C≡N) − anionic species (−0.82) or (−0.855) is responsible for most of the bonding and stability of these Ca, Sr, and Ba bearing molecular products. So the increasing natural charges go with increased ionic character going to larger metal atoms in the bowtie family. Schleyer et al. noted that metal cation core polarization of the (N≡C:) − anionic subunits twists them out of plane and suggested that there is d-orbital participation in the small covalent contributions to the metal cation–ligand anion bonds.9, 39 42 ACS Paragon Plus Environment

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As a measure of the preference for ionic bonding with the more electropositive Group 2 metals, we compare the relative energies of the linear CNMNC and bowtie (CN)M(CN) structures. The Ca bowtie molecule is 3.4 kJ/mol more stable than linear CNCaNC, the Sr bowtie is 10.7 kJ/mol more stable than the linear CNSrNC molecule and the Ba bowtie is 16.4 kJ/mol more stable than linear CNBaNC using the B3LYP/aug-cc-pVTZ method. The previous MP2/631+G* calculations found the bowties more stable by 18.3, 24.9, and 28.9 kJ/mol, respectively.9 As has been described earlier, the bonding between two isolated Be atoms is weak, and it exists at the dispersive energy level.20-22 But when bonded to radicals such as CN some antibonding electron density is transferred to the ligand radicals leading to a stable Be—Be bond which is computed here as 2.040 Å. The same rationale applies to the Mg counterpart, which forms a longer 2.788 Å Mg–Mg bond for the CNMg–MgNC molecule. The singlet CNCa–CaNC molecule has a 3.744 Å Ca–Ca bond, but our calculations find that six-membered rings are the most stable configuration for this stoichiometry using the more electropositive Ca, Sr, and Ba metals (Figure 12). These stable rings are a collection of two almost -1 cyanide anions bound to almost +1 metal cations as illustrated in Figure 12 and characterized through intense antisymmetric (N≡C)− stretching modes,13C and 15N shifts, and B3LYP and CCSD(T) frequency and structure calculations even though NBO analysis does not characterize any M-(CN) bonds. Figure S11 illustrates all of the structures computed for the (CaNC)2 stoichiometry with their energies relative to two Ca atoms and one CN)2 molecule. The CNBeBeNC and CNMgMgNC molecules with metal-metal bonds are of considerable chemical interest.20,40 Had we been able to present a strong spectroscopic case for their identification here this would have required a separate paper.



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Table 6. Natural Population Analysis (NPA) Calculated Bond Type, Bond Orbitals, Natural Charges and Wiberg Bond Orders for CNBeNC, CNMgNC and CNCaNC using theB3LYP/aug-cc-pVTZ Level of Theory. bond

bond type

Be-N

σ bond

N≡C

σ bond

N≡C

π bond

N≡C

π bond

Mg-N

σ bond

N≡C

σ bond

N≡C

π bond

N≡C

π bond

Wiberg occupancy bond orbitals bond order CNBeNC Be-N: 1.98730 (11.2%) Be, s(50.0%) p(50.0%) 0.476 N≡C: (88.8%) N, s(48.4%) p(51.4%) 2.401 1.99821 (66.1%) N, s(52.2%) p(47.2%) (33.9%) C, s(33.7%) p(66.2%) 1.97006 (74.5%) N, s(0.0%) p(99.7%) (25.5%) C, s(0.0%) p(99.9%) 1.97006 (74.5%) N, s(0.0%) p(99.7%) (25.5%) C, s(0.0%) p(99.9%) CNMgNC Mg1.98918 (5.8%) Mg, s(49.9%) N:0.245 p(49.9%) N≡C: (94.2%) N, s(45.4%) 2.498 p(54.6%) 1.99927 (64.9%) N, s(55.6%) p(44.0%) (35.1%) C, s(33.6%) p(66.3%) 1.99047 (73.3%) N, s(0.0%) p(99.9%) (26.7%) C, s(0.0%) p(99.8%) 1.99047 (73.3%) N, s(0.0%) p(99.9%) (26.7%) C, s(0.0%) p(99.8%) CNCaNCa


natural charge Be: 1.413 N: -1.039 C: 0.333

Mg: 1.699 N: -1.101 C: 0.252

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N≡C

N≡C

N≡C

σ bond

1.99749

(64.5%) N, s(54.3%) Ca: 1.758 N≡C: p(45.2%) N: -1.089 2.528 (35.5%) C, s(33.5%) C: 0.210 p(66.4%) π 1.98684 (72.3%) N, s(0.0%) bond p(99.8%) (27.7%) C, s(0.0%) p(99.8%) π 1.98684 (72.3%) N, s(0.0%) bond p(99.8%) (27.7%) C, s(0.0%) p(99.8%) aCNCaNC has a degenerate imaginary 47 cm-1 bending mode.

Table 7. Energies (kJ/mol) for products of the reactions 2 M + NCCN a M

E[CN-M-M-NC]

E[C-(MN)2-C]

E[(MNC)2] b

Be

-577

-186

-358

Mg

-237

-70

-195

Ca

-355

-349

-417

Sr

-321

-342

-402

Ba -378 -418 -469 is the energy of a product in kJ/mol relative to the reactants (2 M + NCCN) calculated with B3LYP/aug-cc-pVTZ. cc-pVTZ is used for Ca and SDD for Sr and Ba. bHexagonal ring in its triplet ground state. aE

CONCLUSIONS Group 2 metal atoms react with (CN)2 in excess argon or neon during condensation at 4 K to form different structures going down the family group: irradiation (220-290 nm) is required to activate Be to produce the linear CNBeNC di-isocyanide molecule, but Mg reacts directly to ultimately form CNMgNC. The di-isocyanide appears at lower frequency and exhibits more nitrogen and less carbon isotopic shift than the cyanide counterpart, which is confirmed by 45 ACS Paragon Plus Environment


B3LYP isotopic frequency calculations. Lower yields of the higher energy mixed ligand CNBeCN and cyanide NCBeCN isomers have also been observed through three and two new stretching absorptions, respectively. The mixed isocyanide-cyanide isomer will be difficult to synthesize in bulk. The decrease in ionization energy takes over with the heavier Ca, Sr and Ba metal atoms and ionic (CN)M(NC) twisted bowtie structured molecules are formed. NBO calculations for the latter molecules show natural charges of +1.54, 1.64, 1.71 e on the metal centers and –0.77, 0.82 and 0.855 e on the CN subunits, respectively. A doubly occupied sp-sp σ and two p-p π bonds on each (CN) support an ionic model for the bonding in these molecules. A decrease in (C-N) stretching frequency follows the increase in ionic character. The reaction of a second metal atom with the first reaction product also follows the ionic character. Beryllium and magnesium favor linear CNBeBeNC and CNMgMgNC molecules whereas calcium, strontium and barium form hexagonal planar triplet (MNC)2 rings on the basis of energy (Table 7) with natural charges of +0.77 for the Ca species and negative charges of – 0.71 for the N’s and –0.054 for the C’s. The frequencies of the ring (C≡N)– stretching modes for the Ca, Sr, and Ba products are 24.5, 7.5 and 6.4 cm-1 lower, respectively, than their slightly less ionic (CN)M(NC) counterparts. Absorptions for a new Ca, Sr, and Ba product at 2036.3, 2040.6 and 2036.0 cm-1 increase on annealing at the expense of their M(NC)2 bands, and calculations support assignments to planar hexagonal C2h (MNC)2 rings containing two (C≡N)– triple bonds and mostly ionic bonding among these two cyanide anions and two 1+ metal cations based on NBO analysis.


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■ ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Tables of calculated isotopic vibrational frequencies for products using the B3LYP and CCSD(T) methods; NBO population analysis and NPA charges for Be, Mg, and Ca isocyanides. Calculated structures for M(NC)2 and (MCN)2 molecules. ■ AUTHOR INFORMATION Corresponding Author: Lester Andrews *E-mail: [email protected]. (L.A.) ORCID Han-Gook Cho: 0000-0003-0579-376X Xuefeng Wang: 0000--0001-6588-997X Lester Andrews: 0000-0001-6306-0340 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS We gratefully acknowledge support from the KISTI supercomputing center (KSC-2017-C10016) and the National Natural Science Foundation of China (No. 21873070).

REFERENCES

(1) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, Ch. 7, Pergammon Press, Oxford, 1984.

(2) Anderson, M. A.; Ziurys, L. M. The Millimeter-Wave Spectrum of 25MgNC and 26MgNC:

Bonding in Magnesium Isocyanides, Chem. Phys. Letts. 1994, 231, 164-

170.



(3) Bauschlicher, Jr., C. W.; Langhoff, S. R. An Ab Initio Study of BeCN, MgCN, CaCN and BaCN. Chem. Phys. Letts. 1985, 115, 124-129. (4) Bludsky´, O.; Spirko, V.; Odaka, T. E.; Jensen, P.; Hirano, T. A Theoretical Study of the MgNC/MgCN Isomerization in the Electronic Ground State. J. Mole. Struct. 2004, 695-696, 219-226. (5) Anderson, M. A.; Steimle, T. C.; Ziurys, L. C. The Millimeter and Submillimeter Rotational Spectrum of the MgCN Radical (X 2∑+). Astrophys. J. 1994, 429, L41L44. (6) Ziurys, L. C.; Apponi, A. J.; Guelin, M.; Cernicharo, J. Detection of MgCN in IRC + 10216: A New Metal-Bearing Free Radical. Astrophys. J. 1994, 445, L47-L50. (7) Lanzisera, D. V.; Andrews, L. Reactions of Laser-Ablated Beryllium Atoms with Hydrogen Cyanide in Excess Argon. FTIR Spectra and Quantum Chemical Calculations on BeCN, BeNC, HBeCN, and HBeNC, J. Am. Chem. Soc. 1997, 119, 6392-6398. (8) Lanzisera, D. V.; Andrews, L. Reactions of Laser-Ablated Mg, Ca, Sr, and Ba Atoms with Hydrogen Cyanide in Excess Argon. Matrix Infrared Spectra and Density Functional Theory Calculations on New Cyanide and Isocyanide Products. J. Phys. Chem. A 1997, 101, 9666-9672. (9) Kapp, J.; Schleyer, P. v. R. M(CN)2 Species (M = Be, Mg, Ca, Sr, Ba): Cyanides, Nitriles, or Neither? Inorg. Chem. 1996, 35, 2247-2252. (10) Andrews, L. Infrared Spectrum, Structure, Vibrational Potential Function and Bonding in the Lithium Superoxide Molecule LiO2, J. Chem. Phys. 1969, 50, 42884299. 48 ACS Paragon Plus Environment

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(11) Andrews, L.; Smardzewski, R. R. Argon Matrix Raman Spectrum of LiO2. Bonding in the M+O2− Molecules and the Ionic Model. J. Chem. Phys. 1973, 58, 2258-2261. (12) Wang, X. F.; Andrews, L. Infrared Spectra, Structure and Bonding in the LiO2, LiO2Li, LiO, and Li2O Molecules in Solid Neon. Mol. Phys. 2009, 107, 739-748.

(13) Torring. T.; Bekooy, J. P.; Meerts, W. L.; Hoeft, J.; Tiemann, E.; Dymanus, A. Rotational Spectrum and Structure of KCN. J. Chem. Phys. 1980, 73, 4875-4882. (14) Andrews, L.; Cho, H.-G.; Gong, Y. Reactions of Laser-Ablated Aluminum Atoms with Cyanogen: Matrix Infrared Spectra and Electronic Structure Calculations for Aluminum Isocyanides Al(NC)1,2,3 and their Dimers. J. Phys. Chem. A 2018, 122, 5342-5353. (15) Thompson, C. A. ; Andrews, L. Noble Gas Complexes with BeO: Infrared Spectra of NG-BeO (NG = Ar, Kr, Xe). J. Am. Chem. Soc. 1994, 116, 423-424. (16) Thompson, C. A. ; Andrews, L. Reactions of Laser Ablated Be Atoms with O2: Infrared Spectra of Beryllium Oxides in Solid Argon. J. Chem. Phys. 1994, 100, 8689- 8699. (17) Thompson, C. A.; Andrews, L. Reactions of Laser Ablated Be Atoms with H2O: Infrared Spectra and Density Functional Calculations of HOBeOH, HBeOH and HBeOBeH. J. Phys. Chem. 1996, 100, 12214-12221.

(18) Wang, X.-F.; Andrews, L. Infrared Spectra and Electronic Structure Calculations 49 ACS Paragon Plus Environment


for the Group 2 Metal M(OH)2 Dihydroxide Molecules. J. Phys. Chem. A, 2005, 109, 2782-2792. (19) Yu, W.; Andrews, L. Wang, X.-F. Infrared Spectroscopic and Electronic Structure Investigations of Beryllium Halide Molecules and Cations in Noble Gas Matrices. J. Phys. Chem. A, 2017, 121, 8843-8855. (20) Brea, O.; Corral, I., Super Strong Be-Be Bonds: Theoretical Insight into the Electronic Structure of Be—Be Complexes with Radical Ligands, J. Phys. Chem. A, 2018, 122, 2258-2265. (21) Merritt, J. M.; Bondybey, V. E.; Heaven, M. C. Beryllium Dimer Caught in the Act of Bonding. Science, 2009, 324, 1548-1551. (22) Bondybey, V. E.; English, J. H. Laser Vaporization of Beryllium: Gas Phase Spectrum and Molecular Potential of Be2. J. Chem. Phys.1984, 80, 568-570. (23) Gong, Y.; Andrews, L.; Liebov, B. K.; Fang, Z.; Garner, E. B., III; Dixon, D. A. Reactions of Laser-ablated U atoms with (CN)2: Infrared Spectra and Electronic Structure Calculations of UNC, U(NC)2, and U(NC)4 in Solid Argon. Chem. Commun. 2015, 51, 3899-3902. (24) Chen, X.; Li, Q.; Gong, Y.; Andrews, L.; Liebov, B. K.; Fang, Z.; Dixon, D. A. Formation and Characterization of Homoleptic Thorium Isocyanide Complexes. Inorg. Chem. 2017, 56, 5060-5068. (25) Fang, Z.; Garner, E. B., III; Dixon, D. A. Gong, Y.; Andrews, L. Laser-ablated U Atom Reactions with (CN)2 to Form UNC, U(NC)2, and U(NC)4: Matrix Infrared Spectra and Quantum Chemical Calculations. J. Phys. Chem. A.2018, 122, 516528. 50 ACS Paragon Plus Environment

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(26) Andrews, L.; Cho, H. G. Matrix Preparation and Spectroscopic and Theoretical Investigations of Simple Methylidene and Methylidyne Complexes of Group 4-6 Transition Metals. Organometallics 2006, 25, 4040-4053. (27) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648-5652. (28) Energy Formula Into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785-789. (29) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007-1023. (30) Kendall, R. A.; Dunning, T. H. Jr.; Harrison, R. J., Electron Affinities of the First Row Atoms Revisited. Systematic Basis Sets and Wave Functions. J. Chem. Phys. 1992, 96, 6796-6806. (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.02, Gaussian, Inc.: Wallingford, CT, 2009. (32) Fuentealba, P.; v. Szentpaly, L.; Preuss, H.; Stoll, H. Psuedopotential Calculations for Alkaline Earth Atoms, J. Phys. B, 1985, 18, 1287-1296. (33) Glendening, E. D.; Landis, C. R.; Weinhold, F. NBO 6.0: Natural Bond Orbital Analysis Program. J. Comp. Chem. 2013, 34, 1429-1437. (34) Scott, A. P.; Radom, L. An Evaluation of Hartree-Fock, Møller-Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors. J. Phys. Chem. 1996, 100, 16502-16513. 51 ACS Paragon Plus Environment


(35) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. Quadratic Configuration Interaction - A General technique for Determining Electron Correlation Energies. J. (36) Jacox, M. E.; Thompson, W. E. Infrared Spectroscopy and Photochemistry of NCCN+ and CNCN+ Trapped in Solid Neon. J. Chem. Phys., 2007, 126, 244311. (37) Brom, J. M. Jr.; Hewett, W. D, Jr.; Weltner, W. Jr. Optical Spectra of Be Atoms and Be2 Molecules in Rare Gas Matrices, J. Chem. Phys. 1975, 62, 3122-3130. (38) Jacox, M. E. The Spectroscopy of Molecular Reaction Intermediates Trapped in the Solid Rare Gases, Chem Soc. Rev. 2002. 31, 108-115. (39) Kaupp, M.; Schleyer, P. von R. The Structural Variations of Monomeric Alkaline Earth MX2 Compounds (M = Calcium, Strontium, Barium; X = Li, BeH, BH2, CH3, NH2, OH, F). An ab initio Pseudopotential Study, J. Am. Chem. Soc. 1992, 114, 491-497 and

references therein. (40) Green, S. P.; Jones, C.; Stasch, A. Stable Magnesium (I) Compounds with Mg-Mg Bonds, Science 2007, 318, 1754-1757 and references therein.


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Figure 1. Infrared spectra for the products of laser-ablated Be atom reactions with cyanogen in excess argon condensed at 4 K. Spectra recorded after (a) Be and 0.5% (12CN)2 co-deposited for 42 min, (b) irradiation at 380-580 nm for 10 min, (c) full arc irradiation at 220-580 nm for 10 min, (d) annealing (temperature cycling) to 25 K, (e) annealing to 30 K, (f) annealing to 35 K, and (g) annealing to 40 K. Spectra recorded after (h) Be and 1 % (13CN)2 co-deposited for 42 min, (i) annealing (temperature cycling) to 25 K, (j) irradiation at 290-580 nm for 10 min, (k) full arc irradiation at 220-580 nm for 10 min, (l) annealing to 30 K, and (m) annealing to 35 K. The * denotes the CNCN photoisomer for (CN)2.



Figure 2. Infrared spectra for the products of laser-ablated Be atom reactions with cyanogen in excess argon condensed at 4 K. Spectra recorded after (a) Be and 3% (12CN)2 co-deposited for 34 min and after 220-580 nm photolysis and annealing to 30 K. (b) Be and 1% (12,13CN)2 codeposited for 60 min, (c) annealing (temperature cycling) to 25 K, (d) irradiation at 220-580 nm for 10 min, (e) annealing to 30 K, (e) annealing to 30 K, (f) annealing to 35K (g) Be and 1% (13CN)2 co-deposited for 42 min, after 220-580 nm photolysis and annealing to 30 K. The lower frequency region is plotted with a 2 x expanded absorbance scale. The sharp 1234.3 cm-1 band is due to an impurity in the reagent sample. B


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Figure 3. Infrared spectra for the products of laser-ablated Be atom reactions with cyanogen in excess neon and argon condensed at 4 K. Blue spectra using neon matrix, recorded after (a) Be and 0.7% (13CN)2 in neon co-deposited for 50 min, (b) irradiation at 220-580 nm for 10 min, (c) annealing (temperature cycling) to 8 K, and (d) annealing to 10 K. (e) Spectra recorded after Be and 1% 13CN)2 in argon co-deposited for 42 min and annealed to 25 K, (f) irradiation at 290-580 nm for 10 min, (g) irradiation at 220-580 nm for 10 min, and (h) annealing) to 30 K. (i) Be and 1.5% (C14,15N)2 co-deposited for 64 min and annealed to 25 K, which produced no reaction product, followed by (j) irradiation at 290-580 nm for 10 min, (j) after irradiation at 220-580 nm for 10 min, (k) after annealing to 30K, (l) after annealing to 35K, (m) after a second irradiation at 220-580 nm for 10 min and (n) after annealing to 40 K.



Figure 4. Infrared spectra for the products of laser-ablated Be atom reactions with cyanogen at two different concentrations in excess argon condensed at 4 K. Spectra recorded after (a) Be and 3% (12CN)2 co-deposited for 34 min, (b) annealing to 25 K, (c) full arc irradiation at 220-580 nm for 10 min, (d) annealing (temperature cycling) to 30 K, (e) annealing to 35 K, (f) annealing to 40 K. Spectra recorded after (g) Be and 0.1% (12CN)2 co-deposited for 38 min, (h) annealing to 25 K. (i) irradiation at 220-580 nm for 10 min, (l) annealing to 30 K, a second irradiation at 220580 nm for 10 min and annealing to 35 K.


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Figure 5. Infrared spectra for the products of laser-ablated Group 2 metal atom reactions with cyanogen using 3% (CN)2 in argon for Be and 2 % for the others. Spectra were recorded after annealing to 25 K, irradiation at 220-580 nm, and annealing to 30 K to reach the maximum product intensities. The Sr spectrum is from Figure 9 (e). The bands labeled d are assigned in this work to the antisymmetric C-N stretching modes of the metal di-isocyanides, and the absorptions labeled r are identified as the hexagonal rings (MCN)2. Again an * denotes the CNCN photoisomer of (CN)2.



Figure 6. Infrared spectra for the products of laser-ablated Mg atom reactions with cyanogen in excess neon and argon condensed at 4 K. Spectra recorded after (a) Blue scans for Mg and 0.7% (CN)2 in neon co-deposited for 44 min, (b) annealing (temperature cycling) to 8 K, (c) irradiation at 220-580 nm for 10 min, and (d) annealing to 10 K. (e) Spectra recorded after Mg and 2.0% (CN)2 in argon co-deposited for 42 min, annealing to 25 K, (g) irradiation at 220-580 nm for 10 min and (h) annealing) to 30 K. Spectra recorded for (i) 2% (13CN)2 in argon co-deposited for 69 min, and irradiation at 220-580 nm for 10 min, (j) annealing) to 25 K and (k) and annealing to 35 K. B


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Figure 7. Infrared spectra for the products of laser-ablated Mg atom reactions with (C14,15N )2 cyanogen at 2% in excess argon condensed at 4 K. (a) spectra recorded after deposition, (b) after annealing to 25K, (C) after irradiation at 220-580 nm for 10 min, and (d, e, f) spectra recorded after annealing to 30, 35 and 40 K.



Figure 8. Infrared spectra for laser-ablated Ca atom reaction products with (C14,15N )2 cyanogen in excess neon or argon condensed at 4 K. (a) spectra recorded after deposition with 0.5% (CN)2 in neon for 50 min, (b) spectra recorded after deposition of 2% (CN)2 in argon for 60 min, (c) spectrum after annealing to 25K, (d) spectrum after irradiation at 220-580 nm for 10 min, and (e, f, g) spectra recorded after annealing to 30, 35 and 40 K.(h) spectrum after deposition with 1% (C14,15N )2 for 64 min, (i, j, k) spectra after irradiation > 380, > 290 and > 220 nm for 10 min each, (l, m) spectra after annealing to 25 and 30 K. The label r denotes the hexagonal ring.


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Figure 9. Infrared spectra for products of laser-ablated Sr atom reaction products with (CN)2 in excess argon condensed at 4 K. (a) spectra recorded after deposition with 1% (CN)2 in argon for 56 min, (b, c, d) spectra recorded after annealing to 25, 30 and 35K, (e) spectrum after irradiation at 220-580 nm for 10 min, and (f, g) spectra recorded after annealing to 35 and 40 K. The label r denotes the hexagonal ring, and c indicates a complex of Sr(NC)2 and NCCN.




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Figure 10. Structures calculated for linear, twisted bowtie, and rhombic ring structures for Group 2 metal atom and cyanogen reaction products. The D2h structures have 3, 3, and 2 imaginary frequencies less than 62 cm-1 and are higher in energy than the hexagonal ring structures in Figure 12.

Figure 11. Color figure of the bonding molecular orbitals for CNBeNC (top four) and CNMgNC (bottom four) based on B3LYP/aug-cc-pVTZ calculations (iso value = 0.03 e/Å3).




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Figure 12. B3LYP/ aug-cc-pVTZ/SDD structures for the triplet cyclic (MNC)2 molecules. The molecular symmetries are C2h and ground electronic states are 3Bu. Natural charges are given for one MNC, which are the same for the other MNC in that hexagonal ring. NBO analysis shows that the C-N bonds are triple bonds and the metals are independent cations, meaning that the CaN and Ca-C bonds are ionic. Therefore the triplet M-(NC)2-M or (MNC)2 molecules are formed as ionic products of 2M and (CN)2 reactions. The natural charges are given for one set of atoms in each structure. Atomic spin densities are given in the SI. 65 ACS Paragon Plus Environment


TOC graphic

1.164

N

C

Ca

86.8

2

122.7 150.5 N

54

7 .53

2.3

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Ca

C


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Matrix Infrared Spectra and Electronic Structure Calculations of Linear

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