Vibrational Spectroscopy of Cr+(NH3)n (n=1-6) Reveals Coordination

May 22, 2019 - Vibrational spectra are measured for Cr+(NH3)1-6 in the N-H stretching region (2950-3600 cm−1) using photofragment spectroscopy and a...
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Vibrational Spectroscopy of Cr+(NH3)n (n = 1−6) Reveals Coordination and Hydrogen-Bonding Motifs Justine Kozubal, Tristan R. Heck, and Ricardo B. Metz* Department of Chemistry, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States

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ABSTRACT: Vibrational spectra are obtained for Cr+(NH3)1−6 in the N−H stretching region (2950−3600 cm−1) using photofragment spectroscopy and complemented by calculations at the M11L/6-311++G(3df,3pd) level of theory. Because of the high bond dissociation energies of Cr+(NH3) and Cr+(NH3)2, their spectra are obtained via N2 tagging; the spectrum of Cr+(NH3) is also obtained by vibrationally mediated photodissociation. The spectra all show intense peaks near 3380 cm−1 due to the antisymmetric N−H stretch. Peaks due to the symmetric N−H stretch (∼3300 cm−1) are intense for n = 1−2, weak for n = 3, and not observed for n > 3. The spectrum of Cr+(NH3) and those of Cr+(NH3)(N2)2 and Cr+(NH3)2(N2) show two peaks near 3200 and 3225 cm−1 due to bend overtones. The spectra indicate that the coordination number of Cr+(NH3)n is 4. In the spectra of Cr+(NH3)5−6 intense, broad peaks appear in the 3080−3280 cm−1 region. Peaks at 3080−3180 cm−1 are due to one first-shell NH3 donating to a second-shell NH3; peaks at 3180−3280 cm−1 are produced by two first-shell NH3 donating to a second-shell NH3. The calculations indicate that the double-donor complexes are energetically favored, while single-donor complexes are entropically favored. or three ligands for Cu; four or five for Mg; and six for Na. Zero electron kinetic energy spectroscopy of Ag(NH3)32,33 and Cu(NH3)1−233,34 has determined the ionization energies of the complexes and the metal cation-ligand stretching frequencies. In this study, we report the vibrational spectra of Cr+(NH3)n (n = 1−6) obtained via photofragment spectroscopy. In addition to studying how Cr+ affects the N−H bonds in NH3, the experimental results are compared with simulations of spectra of possible isomers to determine the geometries and coordination of these molecules.

1. INTRODUCTION The interaction of transition-metal ions with ammonia is important in catalysis, materials synthesis, and solvation. From a coordination chemistry viewpoint, ammonia is a strong-field, σ-donating ligand.1 Studies of metal ion interactions with ammonia thus complement those on weaker-field, π-donating ligands such as water. The desire to understand M+−NH3 interactions has spurred numerous computational studies2−9 and measurements of M+−NH3 bond strengths via collisioninduced dissociation (CID)10−13 and photodissociation.14 These studies find that the bond dissociation energies (BDEs) of first-row transition-metal cations with NH3 are around 200 kJ/mol,10,11 about twice as large as those with H2O. The interactions of metal ions with NH3 are so strong that, for larger clusters of some metals, the electronic structure of M+(NH3)n corresponds to a metal dication core solvated by ammonia, with a diffuse, Rydberg-like orbital at the surface of the cluster. These solvated electron precursors have been predicted for several metals15,16 and observed in sizedependent electronic spectroscopy of Mg+(NH3)n17−19 for n ≥ 4 and Sr+(NH3)n20−23 for n ≥ 6. The coordination number of M+(NH3)n reflects the competition between M+−NH3 dative bonding and NH3− NH3 hydrogen bonding. Vibrational spectroscopy is sensitive both to the presence of hydrogen bonding and to the extent to which the interaction with the metal weakens the N−H bonds in the first-shell ligands. The vibrational spectroscopy of Na + (NH 3 ) 6 − 1 2 , 2 4 Mg + (NH 3 ) 3 − 6 , 2 5 Al + (NH 3 ) 1 − 5 , 2 6 V+(NH3)4−8,27 Co+(NH3)1−8,28 Ni+(NH3)3−8,28 Cu+(NH3)3−9,29,30 and Ag+(NH3)3−830,31 reveals that the first shell contains four ligands for M = Al, V, Co, Ni, and Ag; two © XXXX American Chemical Society

2. EXPERIMENTAL AND COMPUTATIONAL METHODS The molecules are produced in a laser ablation source on a home-built dual time-of-flight reflectron mass spectrometer, which is described in detail elsewhere.35 The Cr+(NH3)1−6 complexes are produced by ablating a chromium rod in a gas mixture containing NH3 and He. The ablation pulse is the second harmonic (532 nm) of a Nd/YAG laser operating at 4−6 mJ/pulse with a repetition rate of 20 Hz. The gas is introduced through a pulsed valve and consists of 0.02−0.5% NH3 in pure He and, for tagged complexes, 15−25% N2 with backing pressures ranging from 80 to 120 psi. The sequential Cr+−NH3 bond strengths are high, so when the clusters are formed, they have a large amount of internal vibrational energy. Cooling the ions requires numerous collisions with the buffer gas. This is accomplished by using an extended postReceived: April 5, 2019 Revised: May 17, 2019 Published: May 22, 2019 A

DOI: 10.1021/acs.jpca.9b03196 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A ablation mixing region consisting of a 0.1″ ID × 1.47″ long tube. The last 0.26″ of the tube flares out at a 10° angle to increase the ion signal. The spectra were taken multiple times to optimize conditions for formation of vibrationally cold ions, which leads to sharper vibrational spectra than in most previous studies of M+(NH3)n. Ions expand into vacuum and further cool, creating a molecular beam with a rotational temperature of about 15 K.36 The ion beam is skimmed, extracted into the time-of-flight mass spectrometer, accelerated, and re-referenced to ground potential. The ions are mass selected and dissociated at the turning point of the reflectron by a Nd/YAG-pumped OPO/ OPA IR laser system, which produces about 6−8 mJ/pulse near 3400 cm−1 and has a line width of 1.8 cm−1. The laser is calibrated using the absorption spectrum of ammonia.37 A multipass Perry cell38 is used to enhance the dissociation yield; two >98% T concave mirrors allow the IR beam to make 21 passes through the ion cloud. The parent and photofragment ions are re-accelerated in the second time-of-flight stage and hit a 40 mm dual microchannel plate detector. The ion masses are determined from their characteristic flight times. The ion signal is amplified and collected on a gated integrator and a LabVIEW-based program is used to record the data. The photodissociation spectrum is obtained by normalizing the fragment to parent ion signal ratio to laser power as a function of wavelength. The observed fragments correspond to loss of one or more intact NH3 from Cr+(NH3)n (n = 3−6) and N2 from Cr+(NH3)n(N2)3−n (n = 1−2). Calculations are carried out with the Gaussian09 program package.39 Optimized geometries of the ions are computed using the M11L40 density functional and the 6-311+ +G(3df,3pd) basis set, using the superfine integration grid. All reported energies include zero point energies. Vibrational frequencies are scaled by 0.958, based on the ratio of the experimental and computed symmetric and antisymmetric stretching frequencies of NH3.

Figure 1. Vibrational spectra of Cr+(NH3)(N2)2, Cr+(NH3)2(N2), and Cr+(NH3)3−6 in the N−H stretching region (2950−3600 cm−1). The y-axis shows the normalized photofragment yield.

bonds in the ligand, which is the first step in catalytic activation. No photodissociation is observed for Cr+(NH3) and Cr+(NH3)2, which is not surprising because the calculated BDEs of the first and second ammonia are greater than 14 000 cm −1 (171 kJ/mol) and 13 700 cm−1 (164 kJ/mol), respectively (Table 1). Sequential M+(NH3)n−1−NH3 BDEs have been measured using CID by Marinelli and Squires10 (MS) for n = 1,2 and by Walter and Armentrout11 (WA) for n = 1−4. Our group recently measured the BDE of Cr+(NH3) to be 177.4 ± 1.2 kJ/mol based on its photodissociation onset.14 This value refines the WA measurement of 183 ± 10 kJ/mol11 and is just outside the error bars of the MS value, 157 ± 19 kJ/ mol.10 Placing tags on molecules is a form of spectator spectroscopy developed by Lee and co-workers.42 The tags are weakly interacting ligands, such as N2 or Ar, which generally only have a small effect on the vibrational frequencies. The calculated frequencies of the tagged and untagged molecules predict the shift induced by tagging, which can then be used to estimate the vibrational frequencies of the untagged molecule. We have used Ar tagging to measure vibrations of M+(CH4)1−2 (M = Co, Ni, Cu).43,44 The spectrum of Cr+(NH3) was first taken with an Ar tag; however, the Cr+(NH3)(Ar) yield is quite low. The nitrogen-tagged ions are much easier to make. The Cr+(NH3)(N2) binding energy is calculated to be 6212 cm−1 and no dissociation of Cr+(NH3)(N2) is observed. A second N2 is predicted to be bound by only 1929 cm−1 and Cr+(NH3)(N2)2 dissociates readily. The Cr+(NH3)2 is tagged with a single N2 as its binding energy is calculated to be 870 cm−1. The vibrational spectrum of Cr+(NH3)(N2)2 is shown in Figure 2, along with its calculated geometry and simulated vibrational spectrum. The spectrum shows the classic signature of ammonia bound to a metal, with the N−H symmetric stretch observed at 3303 cm−1 and the antisymmetric stretch at 3382 cm−1. These are red-shifted by 34 and 62 cm−1,

3. RESULTS AND DISCUSSION Photofragment spectroscopy is used to obtain the vibrational spectra of Cr+(NH3)(N2)2, Cr+(NH3)2(N2), and Cr+(NH3)3−6 in the N−H stretching region (2950−3600 cm−1). N2 tagging is used for the smaller clusters because five photons in the N− H stretching region would be required to dissociate Cr+(NH3)1,2. The spectra (Figure 1) show peaks slightly to the red of the symmetric (v1 = 3337 cm−1) and antisymmetric (v3 = 3444 cm−1) N−H stretching frequencies in bare NH3. The intensity of the symmetric stretch relative to the antisymmetric stretch decreases as more NH3 is added to the metal. The spectra of Cr+(NH3)(N2)2 and Cr+(NH3)2(N2) also have two smaller peaks near 3200 cm−1. Intense, broad peaks below 3300 cm−1 are observed for Cr+(NH3)5,6. The spectra, structures, and corresponding simulations of each cluster will be discussed in the following sections. 3.1. N2-Tagged Cr+(NH3)n (n = 1−2). Although groundstate Cr+ only forms an adduct with NH3, translationally and electronically excited ions react, forming CrNH2+ and CrNH+.41 Calculations predict that insertion into the N−H bond has a large barrier and these reactions are endothermic for ground-state Cr+ (6S, 3d5) but exothermic and barrierless for the 4D (3d44s) excited state.6 Vibrational spectroscopy can reveal how interaction with the metal weakens the covalent B

DOI: 10.1021/acs.jpca.9b03196 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 1. Experimental and Calculated BDEs of Cr+(NH3)(N2)2, Cr+(NH3)2(N2), and Cr+(NH3)3−6a calculated species

kJ/mol

cm−1

experimental kJ/mol

Cr -NH3 Cr+(NH3)−N2 Cr+(NH3)(N2)−N2 Cr+(NH3)−NH3 Cr+(NH3)2−N2 Cr+(NH3)2−NH3

171.0 74.3 23.1 164.2 10.4 62.1 45.1 54.7 40.9 39.2 36.5 35.9 35.3 33.6

14 292 6212 1929 13 723 870 5193 3769 4574 3416 3274 3052 2997 2947 2810

177.4 ± 1.2b, 183 ± 10c, 157 ± 19d

+

Cr+(NH3)3−NH3

Cr+(NH3)4−NH3 Cr+(NH3)5−NH3

geometry

179 ± 9c, 171 ± 19d 54 ± 6c 30 ± 9c

[3 [2 [4 [3 [3 [4 [4 [4 [4

+ + + + + + + + +

0] 1] 0] 1] 1] 1] 1] 2] 2]

double single double single double mix

a

Calculations are at 0 K and use the M11L functional with the 6-311++G(3df,3pd) basis set. bResult from electronic spectroscopy of Cr+(NH3) at 0 K.14 cCID results from Walter and Armentrout at 298 K.11,13 dCID results from Marinelli and Squires at 298 K.10

3298 cm−1 and the antisymmetric stretch at 3370 cm−1. These are red-shifted by 39 and 74 cm−1, respectively, from the values in bare NH3. The calculations predict that the tag induces a slight bend as the tagged H3N−Cr−NH3 bond angle is 177° and the untagged complex is linear. The complexes tagged with N2 have very similar geometries to the untagged ones, with the Cr−NH3 bond length increasing by 0.078 and 0.012 Å upon tagging for n = 1 and 2, respectively. The simulated spectra of the tagged and untagged molecules for n = 1 and 2 have predicted shifts of less than 10 cm−1 for both the symmetric and antisymmetric N−H stretches. The Cr+(NH3)(N2)2 and Cr+(NH3)2(N2) spectra also show two peaks around 3200 cm−1, which do not correspond to any fundamental vibrational frequencies. They are at 3203 and 3227 cm−1 in Cr+(NH3)(N2)2 and at 3199 and 3222 cm−1 in Cr+(NH3)2(N2). These peaks are due to the 2v4 degenerate bending overtone of the NH3. For bare NH3, v4 = 1627 cm−1 and 2v4 = 3217 cm−1 (l = 0) and 3241 cm−1 (l = 2).45 Spectroscopy of NH3 clusters in helium droplets show bend overtones in the same region of the spectrum;46,47 bend overtones are also observed in the spectra of MCH2+48,49 and M+(CH4)n.43,44,50 The red shifts of the 2v4 (l = 0, 2) bend overtones in Cr+(NH3)(N2)2 and Cr+(NH3)2(N2) are smaller (14−19 cm−1) than those in v1 (34 and 39 cm−1) and v3 (62 and 74 cm−1). The calculations at this level of theory underestimate the red shift of the antisymmetric N−H stretch. The red shifts of metals previously studied vary but are very similar to the red shifts we observe in Cr+(NH3)n. Co+(NH3)1−2 has red shifts of 35 cm−1 for the symmetric stretch and 70 and 64 cm−1 for the antisymmetric stretch. The red shifts for Ni+(NH3)1−2 are 42 cm−1 for the symmetric stretch and 63 cm−1 for the antisymmetric stretch.28 3.1.1. VMP of Cr+(NH3). Vibrationally mediated photodissociation (VMP) can, in favorable cases, be used to obtain the vibrational spectrum of strongly bound ions without tagging. This is accomplished by combining IR excitation of a vibration with selective photodissociation in the ultraviolet or visible region of the excited molecule.51−54 VMP can also be used to differentiate vibrational spectra of structural isomers.55 The electronic photodissociation spectra for the vibrationally excited and unexcited molecules need to be different for VMP

Figure 2. Vibrational spectra of Cr+(NH3) obtained via vibrationally mediated photodissociation (VMP) (green) and N2 loss from Cr+(NH3)(N2)2 (blue). The optimized geometry of Cr+(NH3)(N2)2 at the M11L/6-311++G(3df,3pd) level and its simulated spectrum (red) are also shown.

respectively, from the values in bare NH3, as the interaction of NH3 with the metal ion reduces the electron density of nitrogen and slightly weakens the N−H bonds. The Cr+(NH3)(N2)2 ion adopts a trigonal planar geometry with the N2 binding end-on to the metal. The Cr+(NH3)2(N2) spectra and optimized geometry are shown in Figure 3. The symmetric N−H stretch is observed at

Figure 3. Experimental photodissociation spectrum (blue), simulated spectrum (red), and optimized geometry of Cr+(NH3)2(N2) at the M11L/6-311++G(3df,3pd) level. C

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The Journal of Physical Chemistry A to work. Typically, this occurs because the electronic spectrum has well-resolved peaks as in our VMP studies of V+(OCO)56,57 and Co+(H2O).36 In the case of Cr+(NH3), we also take advantage of the fact that the photodissociation onset is thermodynamic, rather than spectroscopic, and set the visible laser to 14 670 cm−1, which is about 200 cm−1 below the one-photon dissociation onset.14 Figure 2 compares the vibrational spectra of Cr+(NH3) obtained using N2 tagging and VMP. The N2 tag induces only a small ( 1. This is now due to hindered rotation of the NH3 group. This has been observed and modeled in detail in the spectrum of Cu+(H2O)Ar2.58 3.2. Cr+(NH3)3 and Cr+(NH3)4. Our calculations predict that the binding energies of Cr+(NH3)3−4 are significantly smaller than those of the n = 1 and 2 complexes, with values of 5193 cm−1 and 4574 cm−1, respectively. This decrease is also observed in M+(NH3)n complexes of M = V,27 Co,28 Ni,28 Cu,29 and Ag31 and in numerous other studies.10,11,13 Our calculated n = 3 binding energy is close to the value of WA11 (4500 ± 500 cm−1), but the calculated n = 4 value is significantly higher than their measured value of 2500 ± 700 cm−1 (Table 1). Although the NH3 binding energy exceeds the photon energy, Cr+(NH3)3 photodissociates readily, losing NH3. The spectrum of Cr+(NH3)3, shown in Figure 4, is dominated by the N−H antisymmetric stretch at 3392 cm−1; the much weaker symmetric stretch is observed at 3300 cm−1. The symmetric and antisymmetric stretches are red shifted by 37 and 52 cm−1, respectively, from the values in bare NH3. There is a very small, broad peak near 3200 cm−1 which is likely due to bend overtones. The simulated spectrum of the [3 + 0] isomer, in which all three NH3 are bound to the metal, provides a good match to the experimental spectrum, although the red shift in the antisymmetric N−H stretch is slightly underestimated. The simulated spectrum correctly predicts the relative peak intensities of the symmetric and antisymmetric stretches. As with the smaller clusters, the added breadth and additional small peaks in the antisymmetric stretch band are likely due to transitions to hindered rotor states. A [2 + 1] isomer, in which one NH3 is in the second solvent shell, is a local minimum, which is calculated to lie 1424 cm−1 above the [3 + 0] isomer and to have a dissociation energy of only 3769

Figure 4. Experimental photodissociation spectrum (blue), simulated spectrum (red), and optimized geometry of Cr+(NH3)3 at the M11L/ 6-311++G(3df,3pd) level.

cm−1. Its simulated spectrum has an intense peak around 3100 cm−1 as well as the higher wavenumber peaks. No dissociation is observed below ∼3150 cm−1, indicating that only the [3 + 0] isomer is observed. The calculations predict that Cr+(NH3)3 adopts a T-shape geometry with two Cr−N bonds at 2.111 Å, only very slightly longer than in Cr+(NH3)2, and one at 2.391 Å. The two shorter bonds are bent, with a N−Cr−N angle of 166°. All of the non-hydrogen atoms are coplanar. Geometry optimizations with initial structures with equal Cr−N bond lengths and with one short and two long Cr−N bonds all converge to the structure shown in Figure 4. One would expect a trigonal planar geometry because of a half-filled 3d5 shell, which would lead to a spherically symmetric distribution of electron density; however, there is some mixing with the 4s orbital and the Tshape geometry is adopted to minimize the metal−ligand repulsions. The singly occupied molecular orbitals of Cr+(NH3)3 are shown in Figure S1. The spectrum of Cr+(NH3)4 is shown in Figure 5. The antisymmetric stretch peak is nearly unchanged at 3394 cm−1. However, there are no clear peaks due to the symmetric stretch or bend overtones, and there is now a broad peak near 3100 cm−1. The calculations predict that the lowest energy structure has all four ligands bound to the metal in a [4 + 0] configuration. The vibrational spectrum of the [4 + 0] isomer nicely captures the major antisymmetric stretch peak. However, it does not predict any absorptions near 3100 cm−1. Bands in this wavenumber region have been observed for M+(NH3)n for sufficiently large n for M = Al,26 V,27 Co,28 Ni,28 Cu,29,30 and Ag.30,31 They have been assigned to structures in which one first-shell NH3 is a hydrogen bond donor to a second-shell NH3 (single donor) or two first-shell NH3 donate to a single second-shell NH3 (double donor). For Cr+(NH3)4, the double-donor and single-donor [3 + 1] structures are calculated to lie 1158 and 1300 cm−1 above the [4 + 0] isomer, respectively. The spectrum of the double-donor structure is dominated by a peak at 3257 cm−1; there is no corresponding peak in the measured spectrum. However, the single donor is expected to have a very intense peak at 3142 cm−1, which matches the observed broad peak around 3100 cm−1. Simulations with 90% [4 + 0] and 10% [3 + 1] single-donor match the experiment. Although the [3 + 1] single-donor structure is at fairly high energy, it is readily produced by addition of NH3 to Cr+(NH3)3. The [4 + 0] structure has a sawhorse geometry with two longer Cr−N bonds at 2.388 Å and two shorter Cr−N bonds at 2.119 Å. The two shorter bonds are slightly bent, with a N− D

DOI: 10.1021/acs.jpca.9b03196 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Figure 5. Experimental photodissociation spectrum (blue), simulated spectra (red), and optimized geometries of Cr+(NH3)4 at the M11L/6-311+ +G(3df,3pd) level. The single-donor isomer spectrum has been divided by a factor of 3.

Figure 6. Experimental photodissociation spectrum (blue), simulated spectra (red), and optimized geometries of Cr+(NH3)5 at the M11L/6-311+ +G(3df,3pd) level. The single-donor isomer spectrum has been divided by a factor of 3.

Figure 7. Experimental photodissociation spectrum (blue), simulated spectra (red), and optimized geometries of Cr+(NH3)6 at the M11L/6-311+ +G(3df,3pd) level. The mixed-donor isomer spectrum has been divided by a factor of 2.

cm−1 from bare NH3. There is a broad feature near 3250 cm−1 and a very intense, broad peak near 3100 cm−1. At the M11L/ 6-311++G(3df,3pd) level of theory, a [5 + 0] structure is not stable. The two Cr+(NH3)5 structures have a four-coordinate sawhorse core with the fifth NH3 hydrogen bonded to either one or two first-shell NH3. On the basis of the simulations, the intense, broad peak around 3150 cm−1 corresponds to the N− H stretch from the single-donor geometry and the weaker, broad peak around 3250 cm−1 corresponds to the N−H stretch from the double-donor geometry. In this case, it is clear that there are double-donor complexes present and that the population of the single-donor complex has increased from those present for n = 4. As expected, the N−H stretches of the hydrogen bond donors are calculated to be very intense. Moreover, the intensity of this stretch for the single donor (near 3150 cm−1) is about a factor of 4 larger than that for the double donor (3220 cm−1). Experimentally, the single-donor peak is more intense than the double-donor peak, implying that similar amounts of the two isomers are present; the relative energy of the single-donor isomer is calculated to be only 55 cm−1 higher than the double-donor isomer. Simulations with 61% single donor and 39% double donor

Cr−N angle of 161°. This is very similar to the structure of the Cr+(NH3)3, with the fourth ligand at long rCr−N. Cr+ is a d5 metal and one would expect the geometry to be tetrahedral; however, population analysis shows that there is some 3d−4s hybridization in the singly occupied molecular orbitals of Cr+(NH3)4, which leads to a sawhorse geometry rather than a tetrahedral geometry. The singly occupied molecular orbitals of Cr+(NH3)4 are shown in Figure S2. The ligands have 96° and 98° N−Cr−N bond angles rather than the 109.5° angles of a tetrahedral geometry to minimize repulsive overlap with the singly occupied metal molecular orbitals. Apparently, the sawhorse geometry is preferred over distorted square planar as it involves less steric overlap between the NH3 ligands. There is no evidence for a Rydberg-like delocalized orbital, as is predicted for Be+(NH3)416 and V+(NH3)615 presumably because of the high second ionization energy of Cr. 3.3. Cr+(NH3)5 and Cr+(NH3)6. The spectra of Cr+(NH3)n (n = 5−6) have intense, broad peaks in the lower wavenumber region, which indicate the onset of the second solvent shell. Figure 6 shows the experimental spectra and simulations for Cr+(NH3)5. The antisymmetric stretch is now a doublet, with peaks at 3358 and 3390 cm−1, which are red-shifted 86 and 54 E

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Figure 8. Experimental photodissociation spectrum (blue), simulated spectra (red), and optimized geometries of Cr+(NH3)6 at the M11L/6-311+ +G(d,p) level. The single- and mixed-donor spectra have been divided by a factor of 4. The double- and mixed-donor geometries are very similar to those shown in Figure 7.

Our studies show that Cr+(NH3)n complexes have a maximum coordination number of four. The onset of the second shell starts with Cr+(NH3)5 and both the n = 5 and 6 complexes have a four-coordinate core with a sawhorse geometry. Vibrational spectroscopy has revealed that V+(NH3)n and Ni+(NH3)n are also four-coordinate complexes, but with a square planar geometry,27,28 while Co+(NH3)n and Ag+(NH3)n are four-coordinate with a tetrahedral core.28,31 Although Koga et al. calculate the six-coordinate complex to be more stable than the four-coordinate geometry for V+(NH3)6, the six-coordinate complex is not observed under their experimental conditions. 27 The first solvent shell of Na+(NH3)n and Cu+(NH3)n contains six and two NH3, respectively, before hydrogen bonding occurs.24,29 It is not clear from the spectra whether ammonia complexes of Mg+ adopt a four- or five-coordinate geometry.25 The situation is slightly more complex for Al+, which forms solvated structures with the first three NH 3 ; N−H insertion complexes predominate starting with the fourth.26

match the experiment. The calculations suggest that the peak at 3390 cm−1 is due to the antisymmetric N−H stretch of NH3 bound to Cr+ and not involved in hydrogen bonding, while the 3358 cm−1 peak is the corresponding vibration in the two inner shell NH3 bound to second-shell NH3 in the double-donor complexes. Figure 7 shows the experimental spectrum and simulations for Cr+(NH3)6. The antisymmetric stretch is again a doublet at 3355 and 3388 cm−1. As with the n = 5 complexes, the intense, broad peak at 3130 cm−1 in the n = 6 spectrum is due to the N−H stretch of a single donor and the 3230 cm−1 peak is due to the N−H stretch of a double donor. There are now two NH3 in the second solvent shell. The simulations predict two stable geometries, one where both second-shell NH3 are hydrogen bound in a double-donor geometry and the other where one NH3 is a single donor and the other is a double donor. The double-donor geometry is the most stable isomer, with the mixed donor at a relative energy of 137 cm−1. Simulations at the M11L/6-311++G(3df,3pd) level of theory do not find a stable geometry with two single donors; however, simulations done with a smaller basis set do. Figure 8 compares the spectra of all three isomers at the M11L/6-311++G(d,p) level of theory. The spectra of the double-donor and mixed-donor structures are very similar to those calculated with the larger basis set. The single-donor structure is predicted to produce a very intense peak near 3150 cm−1. The spectrum of the mixed complex is essentially the average of those of the single-donor and double-donor complexes. This is not surprising, as the second-shell NH3 are well separated and do not interact with each other. Simulations with 69% single donor and 31% double donor overall match the experiment. Assuming a binomial distribution, this works out to 47% single, 43% mixed, and 10% double donor contribution to the spectrum. At the M11L/6-311++G(d,p) level, the double-donor structure is predicted to be most stable, 69 cm−1 below the mixed-donor structure and 181 cm−1 below the single-donor structure. An additional calculation, using the complete basis set CBS-QB3 method,59 gives a nearly identical result, with relative energies of 0, 80, and 163 cm−1 for the three isomers. However, the single-donor motif is less rigid than the double donor motif, resulting in a higher vibrational entropy. As a result, even at 298 K, the single-donor structure is predicted to have the lowest free energy, 460 cm−1 below the mixed-donor structure and 600 cm−1 below the double-donor structure.

4. CONCLUSIONS Photofragment spectroscopy is used to obtain the vibrational spectra of Cr+(NH3)(N2)2, Cr+(NH3)2(N2), and Cr+(NH3)3−6 in the N−H stretching region (2950−3600 cm−1). The spectra are obtained by monitoring loss of N2 for the tagged complexes (n = 1−2) and NH3 for the untagged complexes (n = 3−6). Nitrogen tagging is used for the smaller clusters because five photons in the N−H stretching region are required to dissociate Cr + (NH 3 ) 1,2 . The vibrational spectrum of Cr+(NH3) is also obtained using VMP. Our calculations at the M11L/6-311++(3df,3pd) level of theory generally agree with BDEs measured by Marinelli and Squires10 and Walter and Armentrout.11 The spectra show peaks due to the symmetric and antisymmetric N−H stretching frequencies. These are redshifted from the values in bare NH3 by 34−37 cm−1 for the symmetric stretch and by 50−87 cm−1 for the antisymmetric stretch. The relative intensity of the symmetric stretch decreases as more NH3 binds to the metal. Bend overtones are observed near 3200 and 3230 cm−1 in the spectra of Cr+(NH3), Cr+(NH3)(N2)2, and Cr+(NH3)2(N2). Cr+(NH3)4 is a four-coordinate complex, with a predicted sawhorse geometry. The appearance of intense peaks below 3300 cm−1 for Cr+(NH3)5 indicates the onset of the second solvent shell. These peaks are due to hydrogen-bonded N−H stretches from one and two inner shell NH3 donating to second-shell NH3 and F

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The Journal of Physical Chemistry A are observed around 3100 and 3250 cm−1, respectively. The predicted core geometry of Cr+(NH3)n (n = 5−6) is of a fourcoordinate sawhorse geometry. Although the double-donor isomer is calculated to be more energetically stable than the single-donor isomer, the single donor is entropically favored. Similar amounts of the two isomers are observed in the experiment.

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Interacting with Water, Ammonia, and Methane. J. Phys. Chem. A 2004, 108, 1069−1081. (10) Marinelli, P. J.; Squires, R. R. Sequential Solvation of Atomic Transition-Metal Ions. The Second Solvent Molecule Can Bind More Strongly Than the First. J. Am. Chem. Soc. 1989, 111, 4101−4103. (11) Walter, D.; Armentrout, P. B. Sequential Bond Dissociation Energies of M+(NH3)x (x=1-4) for M=Ti-Cu. J. Am. Chem. Soc. 1998, 120, 3176−3187. (12) Liyanage, R.; Styles, M. L.; O’Hair, R. A. J.; Armentrout, P. B. Sequential Bond Energies of Pt+(NH3)x (x=1-4) Determined by Collision-Induced Dissociation and Theory. Int. J. Mass Spectrom. 2003, 227, 47−62. (13) Rodgers, M. T.; Armentrout, P. B. Cationic Noncovalent Interactions: Energetics and Periodic Trends. Chem. Rev. 2016, 116, 5642−5687. (14) Ashraf, M. A.; Kozubal, J.; Metz, R. B. Bond Dissociation Energy and Electronic Spectroscopy of Cr + (NH 3 ) and its Isotopomers. J. Chem. Phys. 2018, 149, 174301. (15) Almeida, N. M. S.; Pawłowski, F.; Ortiz, J. V.; Miliordos, E. Transition-Metal Solvated-Electron Precursors: Diffuse and 3d Electrons in V(NH3)60,±. Phys. Chem. Chem. Phys. 2019, 21, 7090− 7097. (16) Ariyarathna, I. R.; Khan, S. N.; Pawłowski, F.; Ortiz, J. V.; Miliordos, E. Aufbau Rules for Solvated Electron Precursors: Be(NH3)40,± Complexes and Beyond. J. Phys. Chem. Lett. 2018, 9, 84−88. (17) Yoshida, S.; Daigoku, K.; Okai, N.; Takahata, A.; Sabu, A.; Hashimoto, K.; Fuke, K. Photodissociation and ab initio Studies of Mg+(NH3)n, n=1-4: Electronic Structure and Photoinduced Reaction. J. Chem. Phys. 2002, 117, 8657−8669. (18) Yoshida, S.; Okai, N.; Fuke, K. Photodissociation of Mg+(NH3) Ion. Chem. Phys. Lett. 2001, 347, 93−100. (19) Lee, J. I.; Sperry, D. C.; Farrar, J. M. Spectroscopy and Reactivity of Size-Selected Mg+-Ammonia Clusters. J. Chem. Phys. 2004, 121, 8375−8384. (20) Shen, M. H.; Farrar, J. M. Absorption Spectra of Size-Selected Solvated Metal Cations: Electronic States, Symmetries, and Orbitals in Sr+(NH3)1,2 and Sr+(H2O)1,2. J. Chem. Phys. 1991, 94, 3322−3331. (21) Donnelly, S. G.; Schmuttenmaer, C. A.; Qian, J.; Farrar, J. M. Frequency- and Time-Resolved Cluster Photodissociation Dynamics in Sr+(H2O)n, Sr+(NH3)n and Sr+(CH3OH)n. J. Chem. Soc., Faraday Trans. 1993, 89, 1457−1465. (22) Shen, M. H.; Farrar, J. M. Size-Dependent Effects in the Photodissociation Spectra of Strontium Ammine Complexes (Sr+(NH3)n, n=1-4). J. Phys. Chem. 1989, 93, 4386−4389. (23) Farrar, J. M. Size-Dependent Reactivity in Open Shell MetalIon Polar Solvent Clusters: Spectroscopic Probes of ElectronicVibration Coupling, Oxidation and Ionization. Int. Rev. Phys. Chem. 2003, 22, 593−640. (24) Selegue, T. J.; Lisy, J. M. Vibrational Spectroscopy of Ammoniated Sodium Ions: Na+(NH3)M, M = 6-12. J. Phys. Chem. 1992, 96, 4143−4145. (25) Ohashi, K.; Terabaru, K.; Inokuchi, Y.; Mune, Y.; Machinaga, H.; Nishi, N.; Sekiya, H. Infrared Photodissociation Spectroscopy of Mg+(NH3)n (n=3-6): Direct Coordination or Solvation through Hydrogen Bonding. Chem. Phys. Lett. 2004, 393, 264−270. (26) Mune, Y.; Ohashi, K.; Iino, T.; Inokuchi, Y.; Judai, K.; Nishi, N.; Sekiya, H. Infrared Photodissociation Spectroscopy of [Al(NH3)n]+ (n=1-5): Solvation Structures and Insertion Reactions of Al+ into NH3. Chem. Phys. Lett. 2006, 419, 201−206. (27) Koga, N.; Ohashi, K.; Furukawa, K.; Imamura, T.; Judai, K.; Nishi, N.; Sekiya, H. Coordination and Solvation of V+ with Ammonia Molecules: Infrared Photodissociation Spectroscopy of V+(NH3)n (n=4-8). Chem. Phys. Lett. 2012, 539−540, 1−6. (28) Imamura, T.; Ohashi, K.; Sasaki, J.; Inoue, K.; Furukawa, K.; Judai, K.; Nishi, N.; Sekiya, H. Infrared Photodissociation Spectroscopy of Co+(NH3)n and Ni+(NH3)n: Preference for Tetrahedral or Square-Planar Coordination. Phys. Chem. Chem. Phys. 2010, 12, 11647−11656.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b03196. Full refs 37 and 39; singly occupied molecular orbitals for Cr+(NH3)3−4; and energies, geometries, vibrational frequencies, and intensities of each Cr+(NH3)n complex at the M11L/6-311++G(3df,3pd) level of theory and Cr+(NH3)6 complexes at the M11L/6-311++G(d,p) level of theory (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-413-545-6089. Fax: +1-413-545-4490. ORCID

Ricardo B. Metz: 0000-0003-1933-058X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation under award no. CHE-1566407 is gratefully acknowledged. The authors are grateful for computational resources provided by the Massachusetts Green High-Performance Computing Center (MGHPCC).

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REFERENCES

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