Reactions of Laser-Ablated Aluminum Atoms with Cyanogen: Matrix

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Reactions of Laser-Ablated Aluminum Atoms with Cyanogen: Matrix Infrared Spectra and Electronic Structure Calculations for Aluminum Isocyanides Al(NC) and Their Novel Dimers 1,2,3

Lester Andrews, Han-Gook Cho, and Yu Gong J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02036 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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The Journal of Physical Chemistry

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 Novel Dimers

Lester Andrews,* a Han-Gook Choa,b, and Yu Gonga,c a

Department of Chemistry, University of Virginia, P. O. Box 400319, Charlottesville, Virginia

22904-4319 United States, bDepartment of Chemistry, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon, 22012, South Korea, cDepartment of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China

in memory of George A. Olah

ABSTRACT: Laser-ablated Al atoms react with (CN)2 in excess argon during condensation at 4 K to produce AlNC, Al(NC)2 and Al(NC)3,which were computed (B3LYP) to be 27, 16 and 28 kJ/mol lower in energy, respectively, than their cyanide counterparts. Irradiation at 220-580 nm increased absorptions for the above molecules and the very stable Al(NC)4− anion. Annealing to 30, 35, and 40 K allowed for diffusion and reaction of trapped species and produced new bands for the Al(NC)1,2,3 dimers including a rhombic ring core (C)(AlN)2(C) with C’s attached to the N’s, a (NC)2Al(II)-Al(II)(NC)2 dimer with computed Al-Al length 2.557 Å, and the dibridged Al2(NC)6 molecule with a calculated D2h structure and rhombic ring core like Al2H6. In contrast the Al(NC)4− anion was destroyed on annealing presumably due to neutralization by Al+. B3LYP calculations also show that aluminum chlorides form the analogous molecules and dimers. In our search for possible new products, we calculated Al(NC)4 and found it to be a stable molecule, but it was not detected here. 1 ACS Paragon Plus Environment

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INTRODUCTION Aluminum forms both lower (AlX and AlX2) as well as higher (AlX3) halides.1,2 The monohalides are known as diatomic molecules in the gas phase3 and in solid argon.4 The simple dihalides and trihalides have been observed as matrix isolated molecules from metal atom reactions with halogen molecules.4,5 The trihalides in pure form exist as crystalline solids, and AlCl3 is used extensively as a Friedel-Crafts catalyst.1 The association energy of two AlCl3 molecules as a dibridged dimer has been computed in the 110-125 kJ/mol range using two different density functionals and the small 6-31G(d,p) basis set,6 and the Al2Cl6 molecule has been observed by matrix isolation.4 Likewise the association energy for two AlF3 molecules to form the analogous Al2F6 dimer has been computed as 210 to 290 kJ/mol,6 and the AlF1,2,3 and Al2F6 molecules have been observed in solid argon.4 The simple hydrides AlH1,2,3 have been observed in matrix isolation by several groups.7-10 The dibridged dialane molecule was finally characterized in solid hydrogen as earlier attempts with hydrogen in argon could not trap enough hydrogen to produce sufficient Al2H6 for a definitive identification from the infrared spectrum.11-14

Following the above halides it could be expected that aluminum cyanides might be observed, since polymeric [Ga(CN)3] has been prepared and characterized by X-ray powder diffraction, and the [Ga(CN)4] anion has been synthesized with the [PPh4] counterion.15,16 B3LYP calculations have shown that donor–acceptor complexes between nitriles or isonitriles and Al, Ga, or In hydrides or chlorides react to form monomeric cyanides and larger ring species.17 2 ACS Paragon Plus Environment

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However, TZ2P+f CCSD(T) quantum mechanical methods have reported that AlNC is 23 kJ/mol lower in energy than AlCN.18 Accordingly, the elemental reaction of laser-ablated Al with (CN)2 was investigated in this laboratory and found to form predominantly the simple Al(NC)1,2,3 molecules and their dimers including a symmetrical Al(II)-Al(II) core species and the dibridged Al2(III)(NC)6 molecule with a computed D2h structure similar to dialane, Al2H6.11-14 The very stable Al(NC)4− anion like AlH4− also contributes to the spectrum. There is considerable chemical interest in large bulky ligand complexes containing the Al-Al linkage, and their reactivity with small molecules.19,20 The more stable isocyanide isomer AlNC dominated the gas phase observations of millimeter-wave and microwave spectra of the reaction products from laser-ablated Al and cyanogen or trimethylsilyl cyanide.21,22 Finally, this new metal-containing molecule, AlNC, has been detected toward the circumstellar envelope of the late-type carbon star IRC + 10216, using the IRAM 30 m telescope.23

Cyanogen has proven to be a good source of CN radicals for reactions and matrix infrared spectroscopic investigations. Reactions with laser ablated U, Th, Mn, and Fe have provided the binary isocyanide molecules M(NC)1,2,3,4.24-27 Although Al is different from transition metals, its similar small halogen and hydrogen molecule chemistry prompted us to react cyanogen with aluminum atoms to seek new reaction products.1,2 Earlier experiments with the group 13 metals reacting with HCN gave infrared spectra of the major MNC product, but the higher energy minor product BCN and AlCN isomers were observed with about 1/8 of the BNC and AlNC band intensities.28,29 Interestingly the cyanides increased in intensity relative to the isocyanides using the heavier metals Ga, In and Tl.29 We report here the details of our aluminum isocyanide investigation.

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EXPERIMENTAL AND COMPUTATIONAL METHODS

Experimental. Aluminum atoms were laser-ablated from the solid and reacted with cyanogen (synthesized in this laboratory,24,26 stored in a stainless steel container, condensed at 77 K and evacuated to remove volatile impurities) in excess argon during condensation for 1 h on a 4 K CsI window cooled by a closed-cycle refrigerator (Sumitomo).5,7,12,13,29,30 Reagent gas mixtures ranged from 0.3% to 1% in argon (Spectra Gases). The Nd:YAG laser fundamental (Continuum Minilite II, 1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused using 10-20 mJ/pulse onto a rotating 1x1 cm piece of Al metal (Johnson Matthey, 99.998%). The emission plume from the laser focal point extended to the cold window and was monitored as a diagnostic for the experiment. 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 (lower limit 620 cm-1) after sample deposition. Next, samples were annealed (warmed and cooled back to 4 K) using 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 220-580 nm with the globe removed).

Computational. The initial structures and harmonic vibrational frequencies of potential 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, and Al and the Gaussian 09 package.31-35 Atomic orbital occupancies were determined using NBO6 for the natural bond orbital (NBO) population analysis at the B3LYP level.36 Such DFT calculations predict vibrational frequencies with reasonable accuracy for metal compounds.26,29,30,37 Geometries were fully relaxed during optimization, and the optimized geometries were confirmed by vibrational analysis. Selected


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calculations with the CCSD and CCSD(T) methods 38,39 were also performed with the aug-ccpVTZ basis sets to supplement the B3LYP results. The zero-point energy is included in the calculation of binding energy for a metal bearing molecule.

RESULTS AND DISCUSSION Infrared spectra. Figure 1 compares infrared spectra from two experiments using different concentrations of cyanogen in argon where the bottom set of spectra were recorded in a study using 1% and the top set employed 0.4% under the same laser-ablation conditions, and thus the aluminum concentrations were as nearly identical as possible. Figure 2 compares the same 0.4 % (CN)2 spectra with those from an experiment using 0.3% (13CN)2. A weak band at 2138.6 cm-1 is due to CO observed in these experiments along with 13CO at 2091.4 cm-1, which shows that some carbon is produced by photodissociation of cyanogen with the laser ablation plume. Strong features due to the photoisomer CNCN and its 13C counterpart (labeled with * at 2054.4 and 2017.7 cm-1) demonstrate further that some photochemistry of the reagent does occur during sample deposition. These and other product absorptions are collected and identified in Table 1. A trace of CNNC (marked **) and the diatomic radicals CN and 13CN were also observed.25,27,29,40 The strong NCCN precursor band at 2154.0 cm-1exhibits a weak blue satellite band at 2158.6 cm-1 upon deposition. This band increases on annealing along with two other absorptions at 2155.4 and 2158.5 cm-1. The N13C13CN counterparts and 12/13 frequency ratios are listed in Table 1. Notice that these ratios all agree within experimental error, which tells us that they are due to the same nearby absorbing molecule and mode. It follows that the first band to appear at 2158.6 cm-1 is probably due to the (NCCN)2 dimer and the other two satellite peaks are due to higher multimers. 5 ACS Paragon Plus Environment


Aluminum bearing product absorptions were observed at 2051.7 and 2049.2 cm-1 (labeled AlNC), at 2080.5 (labeled 3 for Al(NC)3), at 2071.5 and 2068.3 cm-1 (marked 2 for Al(NC)2) and at 2062.0 cm-1 (marked 2x2 for Al2(NC)4 with four NC groups). The former band agrees within experimental error (± 0.3 cm-1) with the major AlNC product observed at 2051.4 cm-1 from the Al reaction with HCN.29 Annealing to 25 K removed the lower band and increased the higher band for AlNC (Figure 1,b,h). Next irradiation at 220-580 nm increased both AlNC peaks, produced a new broad band at 2102.8 cm-1 (labeled 4− for the Al(NC)4−anion), the doublet at 2071.5 and 2068.3 cm-1 (marked 2), and the weak 2080.5 cm-1 absorption (labeled 3) increased substantially. Weak peaks were also produced at 2107.1, 2092.3 and 1987.1 cm-1 (marked 3x2, for six NC groups in the dimer of Al(NC)3) Figure 1(c,i). Then annealing to 30 K increased the latter three bands in concert and produced similar substructure on the higher two peaks. The pair of bands at 2065.4 and 2062.0 cm-1 (noted 2x2) and the absorption at 2057.6 cm-1 (marked c) also increased. Further annealing to 35 and 40 K (Figure 1(e,f,j,k) decreased AlNC, 3, and 4− bands and increased the 2, 2x2, and 2x3 absorptions. The major differences between the 1.0% and 0.4% sample spectra are that the smaller products labeled AlNC, CN, 2, and 2x2 are stronger with the lower concentration of precursor, and the larger products marked 3, and 3x2 are more intense with the higher concentration. The c band, which is just 3.2 cm-1 above the CNCN absorption, increased on annealing at the expense of that CNCN absorption, like the NCCN precursor satellites. The c band appeared in experiments with other laser ablated metals, and it exhibited almost the same 12/13 frequency ratio as the stronger CNCN band. Hence is related to the common CNCN product and most likely an asymmetric dimer of CNCN and the most abundant molecule present, namely NCCN.


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Figure 1. Infrared spectra of the reaction products from laser-ablated Al atoms and (CN)2 in excess argon. (a) after co-deposition with 1% (CN)2 in argon for 1 h at 4 K, (b) after annealing to 25 K, (c) after irradiation at 220-580 nm for 10 min, (d) after annealing to 30 K, (e) after annealing to 35 K, (f) after annealing to 40 K. (g) after a similar repeat co-deposition with 0.4% (CN)2 at 4 K, (h) after annealing to 25 K, (i) after 220-580 nm irradiation for 10 min, (j) after annealing to 30 K, and (k) after annealing to 35 K. Table 1. Frequencies (cm-1) Observed from the Products of Reactions of Laser-Ablated Aluminum with Cyanogen Trapped in Solid Argon at 4 K.



(12CN)2 2170.7 2143.9 2158.5,2156.8,2155.4

(13CN)2

12/13 freq ratio

2097.5 2113.8,2112.2,2110.8

2154.0, 2127.6 2108.4,2107.1 2095.1,2092.3,2090.3

2127.6, 2109.4 2068.0,2066.5 2055.1,2053.1,2051.2

2102.8 2080.5 2071.5,2068.3

2063.1 2041.4 2032.3,2028.9

1.02207 1.02115,1.02112, 1.02113 1.02114 1.01954,1.01965 1.01946,1.01909, 1.01916 1.01924 1.01935 1.01929,1.01942

2065.4, 2062.0 2057.6 2054.4 2051.7,2049.2 2044.5 1997.2 1987.1 1979.6

2026.1,2022.6 2021.2 2017.7 2011.5,2009.3 2002.1 1955.8 1933.4 1925.8

1.01941,1.01948 1.01800 1.01819 1.01999,1.01986 1.02118 1.02117 1.02777 1.02794


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assignments Al(CN)2 AlCN [(CN)2]2,n (CN)2,(12,13CN)2,(13CN)2 b3u, 3x2, Al2(NC)6, b2u, 3x2, Al2(NC)6 t2,4−, Al(NC)4− e, 3, Al(NC)3 b1,2, Al(NC) 2x2, (CN)2Al -Al(NC)2 c for common (CNCN)X *CNCN AlNC CN **CNNC b1u, 3x2, Al2(NC)6 1x2, Al2(NC)2

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Figure 2. Infrared spectra of the reaction products from laser-ablated aluminum atoms and cyanogen in excess argon. (g-k) same spectra from Figure 1for comparison here with (l) a similar codeposition with 0.3% (13CN)2 at 4 K, (m) after annealing to 25 K, (n) after irradiation at 220580 nm for 10 min, (o) after annealing to 30 K, and (p) after annealing to 35 K. _________ Assignments. AlNC. The sharp peak at 2051.7 cm-1 is just 0.3 cm-1 higher than the 2051.4 cm-1 band assigned to AlNC from the reaction of laser-ablated Al with HCN.24 Our B3LYP calculations predict this absorption at 2113.9 cm-1 with substantial intensity (438 km/mol) given in the first line in Table 2. This B3LYP computed harmonic frequency is 62 cm-1 or 3 % higher than the argon matrix absorption, which is typical of metal bearing molecules such as the 9 ACS Paragon Plus Environment


aluminum hydrides.12,13,14 The effect of anharmonicity and the argon matrix both tend to reduce the frequency observed here relative to the computed value so the B3LYP value is the better predictor. The weaker peak at 2049.2 cm-1 is probably due to a different argon matrix configuration around the guest AlNC molecule. The B3LYP calculated bond lengths for the linear molecule are shown in Figure 3 for the single aluminum products. The Al-N bonds become shorter going up the oxidation state series from AlNC to Al(NC)2 to Al(NC)3. The experimental values from rotational spectroscopy21 (1.849 and 1.171 Å) for AlNC are slightly shorter than our B3LYP values, but the previous CCSD(T) values are lower 1.861Å for the Al-N length and higher 1.187 Å for the N-C length.18 Our CCSD(T)/aug-cc-pVTZ bondlengths (Figure 5) are slightly longer than the previous values which can be attributed to the different basis sets employed. AlCN. Although AlCN was predicted earlier by CCSD(T) to be 23.0 kJ/mol higher in energy18 and here to be 27.2, 25.0 and 22.6 kJ/mol higher in energy using the B3LYP, CCSD and CCSD(T) methods, respectively, than AlNC, the AlCN absorption is computed to be 133 cm-1 higher than AlNC and about 20% as intense (Table 2). This follows the decrease in C-N bond length in the corresponding cyanides (Figure S1). The absorption assigned earlier to AlCN at 2143.7 cm-1 supports assignment of the present weak band at 2143.9 cm-1 to AlCN. This weak peak at 2143.9 cm-1 and its carbon-13 counterpart at 2097.5 cm-1 are due to the higher energy AlCN isomer: both are 0.2 cm-1 higher than stronger absorptions in the Al experiments with HCN,29 and the relative absorbances of the AlNC and AlCN bands were the same 8/1 in our experiments with both HCN and (CN)2 reactions.


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The detection of the higher energy AlCN isomer produced during sample deposition suggests that some of the primary reaction (3), Table 3, may go through insertion into the weaker N≡C—C≡N single bond rather than the stronger triple bond first forming AlCN. We presume that most of the higher energy AlCN product formed from energized Al atom reactions will be relaxed by the condensing matrix into the more stable AlNC isomer. Remember that the AlCN calculated absorption intensity is 1/5 that of AlNC so that an 8/1 relative intensity for AlNC/AlCN translates into an 8/5 population ratio for these two product molecules. Al(NC)2. Our B3LYP calculation predicts the stronger antisymmetric N-C stretching mode of Al(NC)2 to be 8.5 cm-1 higher than the -N-C mode for AlNC, and the symmetric stretching mode to be another 13cm-1 higher with only 20% of the intensity (Table 2). Guided by this information and the expectation that Al(NC)2 should increase on photolysis activating reaction (5) (Table 3) and decrease on annealing to form higher species, the pair at 2071.5, 2068.3 cm-1, which is favored at lower reagent concentration, is assigned to two matrix sites for Al(NC)2. Our B3LYP calculation predicts this mode 51 cm-1 (2.4%) higher than observed. Al(CN)2. A weak peak at 2170.7 cm-1 (not shown here) with about half the intensity of the 2143.9 cm-1 AlCN band is assigned to Al(CN)2 based on near agreement with adding the 22.6 cm-1 computed difference between frequencies for Al(CN)2 and AlCN to the 2143.9 cm-1 peak observed here for AlCN. This predicts 2166.5 cm-1 for the strongest mode of Al(CN)2 , which is in excellent agreement with the observed peak. Al(NC)3. The next strong absorption computed at higher frequency (Table 2) for singlet Al(NC)3 is at 2138.6 cm-1, just 16 cm-1 higher than that for doublet Al(NC)2, which brings us to the 2080.5 cm-1 band labeled 3 in the spectra. This stronger absorption also increases on uv



irradiation, but decreases steadily on annealing to 30, 35, 40 K and is favored by the higher concentration. The observed 12/13 isotopic frequency ratio, given in Table 1 as 1.01935, matches the ratio from computed frequencies (Table 2). This ratio observed for Al(NC)3 falls between the ratios for the two matrix site absorptions for Al(NC)2 (Table 1). Notice that the isotopic frequency ratio for isolated CN itself is slightly larger than the ratios for the N bonded N-C modes because the N vibrates more (asymmetrically) between Al and C and the carbon moves correspondingly less, hence the lower 12/13 isotopic frequency ratio for the N bonded NC group. The observation here of the major Al(NC)1,2,3 absorptions for Al reactions with (CN)2 corresponds to the previous observation of AlCl2,3 from Al reactions with Cl24,5 and Al(OH)1,2,3 absorptions from Al reactions with H2O2.41 In contrast to our Al(NC)3 isocyanide molecule and Al(NC)4− anion, the solid materials Ga(CN)3 and MGa(CN)4 are cyanides.15 Al(NC)4−. In our earlier laser ablation experiments with Al and H2, the anion AlH4− was a substantial product.12,13 The cyanide anion is a stable species in chemistry,1,2 but we do not detect it in these experiments. Our harmonic frequency prediction for CN − is 2127 cm-1 (intensity 22 km/mol). The CN radical frequency is computed higher at 2150 cm-1 (20 km/mol) and observed as a weak band at 2044.5 cm-1 in solid argon, but we observe no product absorptions in the region where CN− is expected below CN down to 2000 cm-1. Free electrons are produced in the laser ablation process along with Al+, and many of our investigations have observed anions with very high infrared intensities.30 For example, laser ablated Nb and Ta provide electrons for capture and lead to the formation of HC≡MH3− and CH3C≡MH3− in experiments with methane and ethane.42,43 Numerous transition metal carbonyl cations and anions have also been observed.44 Similar laser ablation of aluminum produced a plasma plume with optical emissions from both Al and Al+, which looks like our plume to the naked eye.30, 45 12 ACS Paragon Plus Environment

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It is quite likely that CN− is a very reactive species with Al(NC)3 present in the condensing matrix: their reaction is exothermic by about 401 kJ/mol, reaction (8), which is even more than the chloride ion affinity for AlCl3, 291 kJ/mol, reaction (22), but less than the fluoride anion affinity of AlF3, 459 kJ/mol, reaction (28) , Table 3. We calculated these Cl− and F− affinities here as complementary examples, and they nicely bracket our calculated CN−affinity for Al(NC)3. We compute that Al(NC)4− is a very stable tetrahedral species with an exceptionally strong triply degenerate antisymmetric N-C stretching mode at 2180.2 cm-1 (330 x 3 km/mol) (Table 2), which is the highest of any of the products computed here. This tetra-isocyanide anion is 16.4 kJ/m lower in energy than its cyanide counterpart. We assign the broad feature (full width at half maximum, 8 cm-1) at 2102.8 cm-1 to the Al(NC)4− anion. It is barely detected upon sample deposition, destroyed on subsequent annealing to 25 K, but increased markedly on irradiation at 220-580 nm, and decreased stepwise on annealing to 30, 35 and 40 K. (Figure 1). The 12/13C isotopic frequency ratio, 1.01924, is toward the lower end of the range of values observed here. The harmonic ratio from the computed frequencies is slightly higher, at 1.02002, as expected. The disappearance of Al(NC)4− on annealing probably arises from neutralization by Al+, which must be in the matrix from laser ablation with free electrons in the ablation process.30,45 Photolysis markedly increases the absorption for Al(NC)3 and slightly increases the CN radical absorption. This surely stirs up electrons that are trapped in the solid argon matrix, which sustains reaction (8). Al(NC)4. After forming Al(NC)3, the pair of electrons needed for coordination of another NC would have to be provided by further polarization of the -N≡C groups. In our search for possible new product molecules, we calculated Al(NC)4 and found it to be a stable molecule of T2d symmetry with very intense absorptions in the congested N-C stretching region (B2, 2124.5 cm-1, 13 ACS Paragon Plus Environment


1222 km/mol; E, 2120.4 cm-1, 1340 x 2 km/mol, Table 2) although it was not detected here. Addition of another NC group to Al(NC)3 is a 133 kJ/mol exothermic process, reaction (20), Table 3. The bond lengths computed for Al(NC)4 and its anion are virtually the same: for Al(NC)4 the Al-N = 1.842 and N-C = 1.163 Å and for the anion the Al-N = 1.846 and N-C = 1.168 Å and all six of the angles are 109.5°. Furthermore, all of the Al-N and N-C bond lengths are the same within the radical, but there is a difference in the angles: four are 108.9° and two are 110.6°. The radical is 2B2 symmetry in the D2h point group. Our NBO6 calculation36 shows that adding one electron to Al(NC)4 reduces the natural positive charge on C from 0.426 to 0.207 and increases the negative charge on N from 0.896 to 0.925 while the charge on Al from 1.883 to 1.871 is essentially unchanged (Supporting Information). The N’s are sp hybridized and the smaller negative charge on N in the radical is due to the transfer of negative charge to the bond with Al. Calculations to describe other examples of polarized substituent stabilized bonds with Al have employed the Cl and F ligands. B3LYP calculations found almost the same nearly tetrahedral structure for AlCl4 as the tetrahedral structure for AlCl4− (Supporting Information). The bond lengths are 2.156 and 2.177 Å for AlCl4 and for AlCl4−, and the Mulliken charges are the same for the four chlorines in each species (SI). The NC affinity for Al(NC)3 is 133 kJ/mol, which is larger than both Cl for AlCl3, 60.3 kJ/mol, and F for AlF3, 90.5 kJ/mol. This difference is probably due to the greater ability of the diatomic NC molecule to transfer charge to the Al centers than the halogen atoms. Again Al(NC)4 is a conjugated system and N is an effective electron donor, more effective than Cl and F. The AlH4− anion is stable and tetrahedral, but similar

calculations for AlH4 converged to the weak H2--AlH2 complex as hydrogen is not capable of any stabilization through substituent polarization.13 14 ACS Paragon Plus Environment

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Al2(NC)6. Sample annealing produces two similar broad absorptions at 2107.1 and 2092.3 cm-1 with superimposed splittings (of about 2 cm-1) that are listed in Table 1. A lower frequency band at 1987.1 cm-1, which matches the former bands on annealing, is due to the same product. Remembering that dialane increased on annealing in hydrogen matrices containing AlH3,12,13 we calculated the dibridged D2h structure (Figure 4) formed upon the exothermic association reaction (11, Table 3) of two Al(NC)3 molecules. This dimer has two very strong b2u and b3u antisymmetric Al-N-C stretching modes (Table 2) computed as 2147.9 and 2135.1 cm-1 and a strong b1u mode computed lower at 2011.8 cm-1 for the bridging N-C group. These B3LYP calculated harmonic frequencies are 42.3, 50.7, and 24.7 cm-1 (or 2.0, 2.4 and 1.2% ) higher than the observed anharmonic values, which is excellent agreement. The pure density functional gave the same pattern of frequencies at lower wavenumbers. (These frequency calculations and the modes labeled 3x2 for 3 pairs of NC groups, are described in detail in Supporting Information.) The strongest of these is barely detected on sample deposition, and it increases slightly on irradiation with the full arc. Final annealing to 30, 35 and 40 K increases these bands substantially in concert following the behavior of dialane.12,13 Our B3LYP calculations find the association energy of 2 Al(NC)3 molecules to be 89.2 kJ/mol, which is lower than computed for 2 AlH3 to form dialane (about 150 kJ/mol).11-13 It is interesting to note that the association energy for 2 Al(CN)3 is only 20 kJ/mol. We tried to calculate Al2(NC)6 by bonding between the Al atoms of two planar Al(NC)3 molecules, but this always led to dissociation. However; the side on approach converged to the dibridged structure with an (AlN)2 core rhombus as given in Figure 4. _______________



Table 2. Calculated (B3LYP/aug-cc-pVTZ) Higher Harmonic Frequencies (cm-1) and Intensities (km/mol) for Products of Al and (CN)2 Reactionsa

AlNC: 2113.9 (438), 534.9 (188), 98.3 (3), 98.3 (3) AlN13C: 2072.0 (438), 529.2 (183), 97.2 (2), 97.1 (2) AlCN: 2247.3 (84), 449.3 (169), 130.5 (7), 130.5 (7). Al13CN: 2198.0 (74), 445.5 (167), 126.8 (7), 126.8(7) Al(NC)2: 2135.8 (113), 2122.4 (665), 681.1 (204), 578.8 (78), 273.4 (13), 163 (0), 146.0 (0), 135.6 (1) Al(N13C)2: 2094.7 (112), 2081.2 (660), 674.9 (198), 572.1 (78), 271.5 (12), 161.6 (0), 144.3 (0), 133.9 (1) Al(NC)3: 2158.8 (0), 2138.6 (643), 2138.4 (642), 751.3 (250), 751.2 (250), 504.5 (0), 298.8 (64), 272.7 (17), 272.7 (17), 162.8 (0) Al(N13C)3: 2118.3 (0), 2098.0 (638), 2097.7 (637), 744.4 (245), 744.3 (245), 494.0 (0), 298.6 (64), 271.7 (17), 271.7 (17), 160.9 (0) Al(NC)4−: 2195.4 (0), 2180.2 (330), 2180.2 (330), 2180.2 (330), 622.1 (255), 622.1 (255), 622.1 (155), 450.2 (0), 308.3 (11), 308.3 (11), 308.3 (11) Al(N13C)4−: 2153.1 (0), 2137.5 (331), 2137.5 (331), 2137.5 (331), 616.9 (252), 616.9 (252), 616.9 (252), 442.0 (0), 306.7 (11), 306.7 (11), 306.7 (11)

Al2-(NC)2: 2036.6 (0), 2022.4 (439), 332.0 (0), 324.0 (149), 321.9 (333), 258.6 (0) . Al2-(N13C)2: 1995.0 (0), 1980.1 (435), 327.2 (0), 321.6 (332), 320.9 (146), 257.4 (0). (CN)2Al-Al(NC)2[D2d]: 2147.9 (0), 2135.1 (515), 2128.7 (661), 2128.7 (661), 699.4 (172), 699.4 (172), 659.9 (0) (13CN)2Al-Al(N13C)2 [D2d]: 2107.4 (0), 2093.4 (535), 2088.3 (1295), 2086.1 (0), 698.9 (334), 692.9 (0), 654.5 Al2(NC)6: 2159.1 (0), 2149.4 (454), 2143.9 (1201.5), 2141.7 (0), 2034.4 (0), 2011.8 (699), 757.2 (431) Al2(N13C)6: 2118.5 (0), 2108.6 (449), 2103.1 (1191), 2101.0 (0), 1994.3 (0), 1970.8 (687), 750.7 (422)


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Al(NC)4: 2209.3 (0), 2124.5 (1222), 2120.4 (1340), 2120.4 (1340), 567.2 (68), 558.7 (157), 558.7 (157), 452.00 (0), 295.6 (0), 269.0 (27), 269.0 (27), 256.7 (0), 245.4 (0), 180.9 (0), 180.9 (0), 179.4 (0), 90.0 (0). Al(N13C)4: 2166.6 (0), 2081.3 (1188), 2077.2 (1305), 2077.2 (1305), 562.3 (66), 553.8 (154), 553.7 (154), 442.8 (0), 294.4 (0), 267.7 (27), 267.7 (27), 255.8 (0), 244.7 (0), 178.8 (0), 177.3 (0), 87.3 (0), 87.2 (0.)

a

Complete lists given in SI.

Table 3. Reactions Considered and Anticipated between Laser Ablated Al Atoms and Electrons with Cyanogen: Energies Calculated with B3LYP/ aug-cc-pVTZ

(1) NCCN → NC + CN

+588.4 kJ/mol

(2) NCCN + e− → CN + CN− +198.0 kJ/mol (3) Al* + NCCN → AlNC + CN -87.6 kJ/mol (4) Al + CN → AlNC

-500.9 kJ/mol

(5) Al + NCCN → Al(NC)2

-218.5 kJ/mol

(6) Al + 2 NCCN → Al(NC)3 + CN -122.2 kJ/mol (7) AlNC + NCCN → Al(NC)3

-209.7 kJ/mol

(8) Al(NC)3 + CN− → Al(NC)4− (9) 2 AlNC → (C)(AlN)2(C)

-400.8 kJ/mol

-26.3 kJ/mol

(10) 2 (2A1) Al(NC)2 → (NC)2Al-Al(NC)2 (11) 2 Al(NC)3 → Al2(NC)6

-89.2 kJ/mol

(12) 2 Al(CN)3 → Al2(CN)6

-20.1 kJ/mol

-254.5 kJ/mol

(13) 2 Al + 2 (NCCN) → (D2d) (CN)2Al-Al(NC)2

-691.6 kJ/mol

(14) 2 Al + 3 (NCCN) → (D2h) (CN)2Al-(NC)2-Al(NC)2 (15) Al2(NC)4 + NCCN → Al2(NC)6

-921.9 kJ/mol

-230.3 kJ/mol



(16) Al + 2 NCCN → Al(NC)4

-255.1 kJ/mol

(17) Al2(CN)4 + NCCN → Al2(CN)6

-147.7 kJ/mol

(18) Al2(NC)4 + NCCN → Al2(NC)6

-230.3 kJ/mol

(19) Al + 2 NCCN → Al(NC)4 (20) Al(NC)3 + CN → Al(NC)4

-255.1 kJ/mol -133 kJ/mol

(21) Al(NC)4− + Al+ → (CN)2Al-Al(NC)2 -358.8 kJ/mol (22) AlCl3 + Cl− → AlCl4− -291.1 kJ/mol (23) AlCl3 + Cl → AlCl4 -60.3 kJ/mol (24) 2 AlCl3 → Al2Cl6

-89.2 kJ/mol

(25) AlCl2 + AlCl2 → Cl2Al-AlCl2 (26) AlH2 + AlH2 → H2Al-AlH2 (27) AlCl + AlCl → (AlCl)2

-240.7 kJ/mol -232.5 kJ/mol

-23.6 kJ/mol

(28) AlF3 + F− → AlF4− - 458.9 kJ/mol (29) AlF3 + F → AlF4

- 90.5 kJ/mol

(30) Al + Al → Al-Al

-128.4 kJ/mol


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Figure 3. Structures of the primary single aluminum AlNC, Al(NC)2 , Al(NC)3 and Al(NC)4− reaction products calculated at the B3LYP/ aug-cc-pVTZ level of theory. Bond lengths are in angstroms and angles in degrees. The symmetries of the molecular structures and ground electronic states are also shown. (The CCSD(T) method gave 1.187 and 1.784 Å bond lengths for Al(NC)3.)

.



Figure 4. Structures of the symmetric dimers of AlNC, Al(NC)2 and Al(NC)3 calculated at the B3LYP/ aug-cc-pVTZ level of theory. Bond lengths are in angstroms and angles in degrees. The symmetries of the molecular structures and ground electronic states are also shown.


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Al2(NC)4: The stability the above Al2(NC)6 derivative of the novel Al2H6 molecule, and the known stability for the Al2H4 molecule12-14 suggested an attempt to compute Al2(NC)4. Although the side by side approach of two Al(NC)2 molecules was not successful, two Al(NC)2 molecules in the Al to Al arrangement for input to a B3LYP calculation converged to the Al—Al bonded structures (Figure 4) with the D2d form being 2.2 kJ/mol lower energy than the planar D2h dimer and all real frequencies (Table S1). The strong computed N-C stretching frequencies for Al2(NC)4 at 2135.1, 2129.3 cm-1 are close to the strong 2135.8, 2122.4 cm-1 N-C bands calculated for the Al(NC)2 monomer. The pair of bands observed at 2065.4, 2062.0 cm-1 increases on annealing at the expense of the Al(NC)2 bands at 2071.5, 2068.3 cm-1 (Figure 1d,e,f). Following this evidence the lower pair of bands which increases on annealing is assigned to the most stable dimer of Al(NC)2 namely (CN)2Al—Al(NC)2.

Al2(NC)2: The cyclic structure calculated for the AlNC dimer around the rhombic (AlN)2 core is illustrated in Figure 4, and its frequencies are given in Table 2. Notice that the calculated antisymmetric N-C stretching mode is about 92 cm-1 lower than the N-C stretching mode computed for AlNC, and the only other absorption band in this region is a sharp feature at 1979.6 cm-1 which shifts to 1925.7 cm-1 on 13C substitution with a 12/13 isotopic frequency ratio of 1.02794. This is slightly higher than the ratio for AlNC itself, 1.0199. The N atoms in the rhombic structure are damped through bonding to two Al’s thus the C’s are forced to move more in this vibration. Again following aluminum hydrides, the two lowest energy Al2H2 structures are planar dibridged (D2h) and monobridged,47 and our computed structure for the AlNC dimer also contains a dibridged rhombus. The polar AlCl molecule forms a simple rhombic dimer.



The B3LYP energy of association for two Al(I)NC molecules, reaction (9), is 26.3 kJ/mol exothermic, which is much less than 89.2 kJ/mol value, reaction (11), for two Al(III)(NC)3 molecules discussed above. In addition the (AlN)2 rhombic core for the Al(I)NC dimer has longer bonds by 0.157 Å than the core rhombus for the Al(III)(NC)3 dimer (Figures 3,4). Given the natural charges for the AlNC molecule Al[+0.86]N[-1.15]C[+0.29] (SI), AlNC behaves like a diatomic molecule with a large dipole moment (2.73 D) and forms a rhombic (AlN)2 core with C’s attached to the N’s. Such is the fate of two AlCl molecules that form the (AlCl)2 rhombus dimer.

Aluminum Chloride Molecules. In order to understand better the aluminum isocyanide products observed here calculations on the analogous aluminum chloride molecules have been performed at the B3LYP/aug-cc-pVTZ level of theory. Structures of these molecules are shown in Figure S2. The AlCl molecule, known in the gas phase and in solid argon,3,4 forms a rhombic dimer in an association reaction (27), which is exothermic by 23.6 kJ/mol: calculations do not find a ClAl-AlCl structure. However, aluminum dichloride forms the single bonded dimer with a 2.553 Å Al-Al bond length for Cl2Al-AlCl2, which is exothermic by 241 kJ/mol reaction (25). This D2d molecule with a strong Al-Al bond is more stable by 232.9 kJ/mol than the dimer with a rhombic ring core, which has three imaginary frequencies. It is surprising that the association dimerization of planar AlCl3 is much less, namely 89 kJ/mol, until one realizes that there is little change in the AlCl2 structure on dimerization but a considerable change in the structure of AlCl3 to form the dibridged D2h dimer. Our B3LYP calculations for Cl2Al-AlCl2 find all real frequencies (SI) in contrast to the more primitive calculations performed earlier.4


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We also computed the chloride anion and chlorine atom affinities for AlCl3, and found them to be 291.1 and 60.3 kJ/mol, respectively, reactions (22) and (23), Table 3.

Aluminum Fluoride Molecules. After computing the above latter chlorine products we realized the importance of computing their AlF4− and AlF4 analogs. At the B3LYP/ aug-cc-pVTZ level of theory the fluoride anion and fluorine atom affinities of AlF3 are 459.9 and 90.5 kJ/mol, respectively, reactions (28) and (29), Table 3. Although the bond lengths and angles are all the same for the anion, the radical has different bond lengths and angles. These structures are shown in Supporting Information.

Bonding: There are several interesting bonding situations to consider here. First examine the bonding molecular orbitals for AlNC shown in Figure 5. There is a single σ bond for the Al-N linkage and a single σ plus πx and πy bonds for a full triple N≡C bond. Next for (C)(AlN)2(C) Figures 3 and 4 illustrate that forming this dimer increases the Al-N bond length in the rhombic ring by 0.261 Å or 14% and the terminal NC bond by 0.008 Å or 4.5%. The molecular orbitals for AlNC combine to form the dimer MO’s in Figure 6.

The structure Figures 3 and 4 show that the N-C bond is not changed, but the Al-N bond is decreased by 0.010 Å when two Al(NC)2 radicals form their most stable dimer, the singlet Al-Al bonded structure. (Initial calculations found a higher energy ring structure (SI) with five imaginary frequencies.) This reaction is 254 kJ/mol exothermic, which is about double the Al-Al bond strength in the metal dimer (measure of the bond strength. Figure 7 illustrates the bonding MO’s for the singlet Al-Al bonded structure with a strong σ Al-Al bond. Al provides sp2 hybridized orbitals for the Al-Al (36.5% s and 63.4% p) and Al-N (31.8% s and 68.0% p) bonds according to the NBO calculations. The s and p characters come mostly from Al 3s and 3p 23 ACS Paragon Plus Environment


atomic orbitals. The Al-N bonds look somewhat like the Al-Al sigma bond, increasing the electron density between the two Al atoms. The NC bonds of (CN)2Al-Al(NC)2 are true triple bonds, a sigma bond (sp hybridizations of N and C) and two pi bonds. The Al-N bonds are largely polar in both AlNC and Al(NC)2 and their dimers. Of more importance the MO’s for Al(NC)2 compare favorably with those for the Al(NC)2 subunit in the (CN)2Al-Al(NC)2 molecule (Figure7).

As we go down in Figure 3 the oxidation state of Al increases from I to III, and the B3LYP Al-N distance decreases by 0.063, 0.031, and 0.008 Å, respectively. However the Al-N distance increases by 0.068 Å in the Al(NC)4− anion relative to Al(NC)3. The Al-N bonds in Al(NC)4− are made of orbitals from sp3 hybridization of Al and sp hybridization of N. NBO results show that Al contributes 25% s and 75% p, and N contributes 50% s and 49% p, perfect examples of sp3 and sp hybridization. The Al-N molecular orbitals are shown in Figure 8 where the sp3 hybridization is clearly observed. The symmetric interactions of the bonding electron pairs lead to the tetrahedral structure of Al(NC)4−. The p-orbitals of N and C yield the π(N-C) orbitals, and provide a good example of a tetrahedral chemical species with π bonds.

The last but not least dimer is the dibridged dialane derivative Al2(NC)6 formed through the exothermic (89 kJ/mol, reaction 11) dimerization of the Al(III)(NC)3 monomer. The Al-N distance in the monomer is increased by 0.198 Å (11%) to form the rhombus core while the N-C distance with N in the ring is increased by 0.013 Å and the N-C on the Al is decreased by 0.001 Å. The MO’s for the (AlN)2 core ring are illustrated in Figure 9. Dimerization of AlH3 itself is


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almost 5/3 times as exothermic as is the tri-isocyanide, but dialane (6) is difficult to investigate because in the pure form it spontaneously reverts to solid alane, (AlH3)n.12,13

Reaction 10, Table 3, is exothermic by 254.5 kJ/mol to give the lower energy D2d dimer. This is almost double the energy of the Al-Al bond that is formed in the gaseous Al2 dimer.46 Although there is very little change in the 2A1 (C2v) Al(NC)2 structure on dimerization. The structures of this monomer and dimer are given in Figures 3 and 4.

There is considerable interest in Al-Al bonded complexes, but these require large bulky ligands which also allow asymmetric compounds to be synthesized.19 We reported earlier the simplest such compound, the D2d dimer of AlH2, namely H2Al-AlH2, which has a computed 2.592 Å Al-Al bond length.13 Figure 4 presents a 2.557 Å (CN)2Al-Al(NC)2 single bond for the diisocyanide dimer, and Figure S3 in Supporting Information illustrates the structure and 2.553 Å Al-Al bond length for Cl2Al-AlCl2. These computed Al-Al single bond lengths are in the range of values reported for a number of large ligand stabilized R2Al-AlR2 complexes.19



Figure 5: The CCSD(T)/ aug-cc-pVTZ computed structure of AlNC and highest occupied Hartree-Fock MO’s of AlNC plotted with an isodensity of 0.04e/Å3. The orbital energies in atomic units are shown below the MO’s. The non-bonding electron pairs are located on Al and C and the four bonding orbitals [πx(N-C), πy(N-C), σ(Al-N), and σ(N-C)] are also shown.


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Figure 6: The π(N-C), σ(Al-N) and σ(N-C) bonding Hartree-Fock MO’s for (C)(AlN)2(C) plotted with an isodensity of 0.04e/Å3 with the CCSD(T) structure at the top. The orbital energies in atomic units are shown below the MO’s. The CCSD(T) structure is given at the top.



Figure 7: The B3LYP/aug-cc-pVTZ calculated σ(Al-Al) and σ(Al-N) bonding MO’s of (CN)2AlAl(NC)2 compared with the n(Al) and σ(Al-N) MO’s for Al(NC)2 plotted with an isodensity of 0.04e/Å3. The orbital energies in atomic units are shown below the MO’s. The σ(Al-Al) bond of (CN)2Al-Al(NC)2 evidently results from pairing of the non-bonding electron on the Al atoms of two Al(NC)2 radicals. 28 ACS Paragon Plus Environment

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Figure 8.The B3LYP/aug-cc-pVTZ calculated π(N-C), σ(Al-N) and σ(N-C) bonding MO’s of Al(NC)4− plotted with an isodensity of 0.04 e/Å3. The orbital energies in atomic units are shown below the MO’s. The corresponding NBO calculations show that these σ(Al-N) bonds are produced from linear combinations of four sp3 hybridized (25% s and 75% p each) Al atomic 29 ACS Paragon Plus Environment


orbitals and sp hydridized (50% s and 49% p) orbitals of the four N atoms. The symmetric interactions of the bonding electron pairs lead to the tetrahedral structure of Al(NC)4−. The porbitals of N and C yield the π(N-C) orbitals and provide a good example of a tetrahedral chemical species with π bonds.


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Figure 9. The B3LYP/aug-cc-pVTZ calculated bonding MO’s for Al2(NC)6 plotted with an isodensity of 0.04 e/Å3. The orbital energies in atomic units are shown below the MO’s.

CONCLUSIONS In the reaction of laser-ablated Al with (CN)2 we have observed the straightforward single aluminum products Al(NC)1,2,3 and the Al(NC)4− anion following the analogous aluminum and hydrogen reactions to form aluminum hydrides.12,13 These isocyanides are more stable by 27, 16, 28 and 16 kJ/mol lower in energy, respectively, than their cyanide isomers at the B3LYP/ augcc-pVTZ level of theory. Gas phase rotational spectroscopy of laser ablated Al and cyanide bearing reaction products characterized AlNC, but not AlCN.21, 22 We observed and computed important dimers for Al(NC)1,2,3 that follow their hydrogen analogs. The first is the dimer of AlNC which gives a (C)(AlN)2(C) rhombic ring structure. Next Al(NC)2 dimer forms an Al-Al single bonded structure by association which is exothermic by 254 kJ/mol for the D2d isomer. The dimer of Al(NC)3 has the D2h structure for Al2(NC)6 analogous to AlH3 association to provide the D2h structure for Al2H6 . Calculations for aluminum chlorides characterize the analogous products including AlCl2, AlCl3, Cl2Al-AlCl2, Al2Cl6 and Al(Cl) 4−. Calculations to describe other examples of a polarized substituent stabilized 31 ACS Paragon Plus Environment


bonds with Al employed the Cl and F ligands. B3LYP calculations found almost the same nearly tetrahedral structure for AlCl4 as the tetrahedral structure for AlCl4− (Supporting Information). The bond lengths are 2.156 and 2.177 Å for AlCl4 and for AlCl4−, and the Mulliken charges are the same for the four chlorines in each species (SI). The bond lengths in AlF4, 1.779 Ằ, are slightly longer than in AlF4−, 1.762 Ằ. The NC affinity for Al(NC)3 is 133 kJ/mol, which is larger than both Cl for AlCl3, 60.3 kJ/mol, and F for AlF3, 90.5 kJ/mol. This difference is probably due to the greater ability of the diatomic NC molecule to transfer charge to the Al centers than the halogen atoms. Again Al(NC)4 is a conjugated system and N is an effective electron donor, more effective than Cl and F atoms.

Finally, a comparison with the Al2 diatomic molecule is of interest. Gas phase high resolution spectroscopy of jet-cooled Al2 diatomic molecules revealed a 2.701 ± 0.002 Å bond length and a 129 ± 6 kJ/mol bond dissociation energy.46 Our B3LYP calculation of 2.758 Å and 128.4 kJ/mol for the 3∏u ground state gas phase Al2 standard are in satisfactory agreement with the gas phase standard. If we compare the 2.5 to 2.6 Å range of our computed and the large ligand stabilized single bonded R2Al—AlR2 complex19 single bond lengths to the 2.70 Å value for Al2 in the gas phase, we can conclude that there is some ligand stabilization for the Al—Al single bond in ligated complexes.

■ ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Complete citation for ref 35. Tables of calculated 32 ACS Paragon Plus Environment

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isotopic vibrational frequencies for Al isocyanides and cyanides with the B3LYP and BPW91functionals; NBO population analysis and NPA charges for Al isocyanides. Calculated structures for aluminum chlorides and fluorides. Cartesian coordinates for aluminum isocyanide products. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (L.A.) ORCID Han-Gook Cho: 0000-0003-0579-376X Yu Gong: 0000-0002-8847-1047 Lester Andrews: 0000-0001-6306-0340 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS We gratefully acknowledge support from KISTI supercomputing center (KSC-2017-C10016).and the “Strategic Priority Research Program” and “Frontier Science Key Program” (Grant Nos. XDA02030000 and QYZDY-SSW-JSC016) of the Chinese Academy of Sciences.

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(5) Olah, G. A.; Farooq, O.; Farnia, S. M.F.; Bruce, M. R.; Clouet, F. L.; Morton, P. R.; Prakash, G. K. S.; Stevens, R. C.; et al.; Suzer, S.; Andrews, L. Aluminum Dichloride and Dibromide. J. Am. Chem. Soc. 1988, 110, 3231-3238. (6) Willis, B. G.; Jensen, K. F. An Evaluation of Density Functional Theory and ab Initio Predictions for Bridge-Bonded Aluminum Compounds. J. Phys. Chem.1998, 102, 26132623. (7) Chertihin, G. V.; Andrews, L. Reactions of Pulsed-Laser Ablated Al Atoms with H2. Infrared Spectra of AlH, AlH2 AlH3 and Al2H2 Species. J. Phys. Chem. 1993, 97, 10295-10300. (8) Kurth, F. A.; Eberlein, R. A.;Schnockel, H.; Downs, A. J.Pulham, C. R. Molecular Aluminium Trihydride, AlH3: Generation in a Solid Noble Gas Matrix and Characterization by its Infrared Spectrum and Ab Initio Calculations. Chem. Commun. 1993, 1302-1304. (9) Pullumbi, P.; Mijoule, C.; Manceron, L.; Bouteiller, Y. Aluminium, Gallium and Indium Dihydrides. An IR Matrix Isolation and ab Initio Study. Chem Phys. 1994,185, 13-24. (10) Pullumbi, P.; Mijoule, C.; Manceron, L.; Bouteiller, Y. Aluminium, Gallium and Indium Trihydrides. An IR Matrix Isolation and ab Initio Study. Chem. Phys. 1994,185, 2537. (11) Shen, M.; Schaefer, III, H. F. The Known and Unknown Group 13 Hydride Molecules M2H6: Diborane (6), Dialane (6) and Digallane (6). J. Chem. Phys. 1992, 96, 2868-2876 and references therein. (12) Andrews, L; Wang, X.-F. The Infrared Spectrum of Al2H6 in Solid Hydrogen. Science 2003, 299, 2049-2052.


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(22) Walker, K. A.; Gerry, C. L. Nuclear Hyperfine Interactions in the Microwave Spectrum of Aluminium Isocyanide. Chem. Phys. Letts. 1997, 278, 9-15. (23) Zuirys, L. M.; Savage, C.; Highberger, J. L.; Apponi, A. J. More Metal Cyanide Species: Detection of AlNC (X 1∑+) Toward IRC +10216. Astrophys. J. 2002, 564, L45L48. (24) 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. (25) 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. (26) 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, 516-528. (27) Chen, X.; Li, Q.; Andrews, L.; Gong, Y. Matrix Infrared Spectra of Manganese and Iron Isocyanide Complexes. J. Phys. Chem. A, 2017, 121, 8835-8842. (28) Lanzisera, D. V.; Andrews, L.; Taylor, P. R. Reactions of Laser-Ablated Boron Atoms with HCN during Condensation in Argon. A Comparison of Matrix Infrared and DFT, CCSD(T), and CASSCF Frequencies of BNC, BCN, HBNC, and HBCN. J. Phys. Chem. A, 1977, 101, 7134-7140. (29) Lanzisera, D. V.; Andrews, L. Reactions of Laser-Ablated Al, Ga, In, and Tl Atoms with Hydrogen Cyanide in Excess Argon. Matrix Infrared Spectra and Density


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Reactions of Laser-Ablated Aluminum Atoms with Cyanogen: Matrix

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