How the nature and charge of metal cations affect vibrations in acetone solvent molecules.

Vibrational spectra of a series of gas-phase metal 1+ and 2+ ions solvated by acetone molecules are collected to investigate how the metal charge, number of solvent molecules and nature of the metal affect the acetone. The spectra of Cu+(Ace)(N2)2, Cu+(Ace)4, and M2+(Ace)4, where M = Co, Ni, Cu, and Zn are measured via photodissociation by monitoring fragment ion signal as a function of IR wavenumber. The spectra show a red shift of the CO stretch and a blue shift of the C-C antisymmetric stretch. DFT calculations are carried out to provide the simulated spectra of possible isomers to be compared with the observed vibrational spectra, and specific structures are proposed. The red shift of the CO stretch increases as the number of acetone molecules decreases. Higher charge on the metal leads to a larger red shift in the CO stretch. Although all of the M2+ complexes have very similar red shifts, they are predicted to have different geometries due to their different electron configurations. Unexpectedly, we find that the calculated red shift in the CO stretch in M+/2+(Ace) is highly linearly correlated with the ionization energy of the metal for a wide range of metal cations and dications.

Vibrational Spectroscopy and Structural Analysis of V+(C2H6)n Clusters (n = 1-4).

The vibrational structure and binding motifs of vanadium cation-ethane clusters, V+(C2H6)n, for n = 1-4 are probed using infrared photodissociation spectroscopy in the C-H stretching region (2550-3100 cm-1). Comparison of spectra to scaled harmonic frequency spectra obtained using density functional theory suggests that ethane exhibits two primary binding motifs when interacting with the vanadium cation: an end-on η2 configuration and a side-on configuration. Determining the denticity of the side on isomer is complicated by the rotational motion of ethane, implying that structural analysis based solely on Born-Oppenheimer potential energy surface minimizations is insufficient and that a more sophisticated vibrationally adiabatic approach is necessary to interpret spectra. The lower-energy side-on configuration predominates in smaller clusters, but the end-on configuration becomes important for larger clusters as it helps to maintain a roughly square-planar geometry about the central vanadium. Proximate C-H bonds exhibit elongation and large red-shifts when compared to bare ethane, particularly in the case of the side-on isomer, demonstrating initial effects of C-H bond activation, which are underestimated by scaled harmonic frequency calculations. Tagging several of the clusters with argon and nitrogen results in nontrivial effects. The high binding energy of N2 can lead to the displacement of ethane from a side-on configuration into an end-on configuration. The presence of either one or two Ar or N2 can impact the overall symmetry of the cluster, which can alter the potential energy surface for ethane rotation in the side-on isomer and may affect the accessibility of low-lying electronic excited states of V+.

Photofragment imaging differentiates between one- and two-photon dissociation pathways in MgI.

The bond strength and photodissociation dynamics of MgI+ are determined by a combination of theory, photodissociation spectroscopy, and photofragment velocity map imaging. From 17 000 to 21 500 cm-1, the photodissociation spectrum of MgI+ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, which is consistent with absorption to the repulsive wall of the (1) Ω = 1 or (2) Ω = 1 states followed by direct dissociation to ground state products Mg+ (2S) + I (2P3/2). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D0(Mg+-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02 eV; 17 000 ± 150 cm-1). At photon energies of 33 000-41 000 cm-1, exclusively I+ fragments are formed. Over most of this region, the formation of I+ is not energetically allowed via one-photon absorption from the ground state of MgI+. Images show the observed product is due to resonance enhanced two-photon dissociation. The photodissociation spectrum from 33 000 to 38 500 cm-1 shows vibrational structure, giving an average excited state vibrational spacing of 227 cm-1. This is consistent with absorption to the (3) Ω = 0+ state from ν = 0, 1 of the (1) Ω = 0+ ground state; from the (3) Ω = 0+ state, absorption of a second photon results in dissociation to Mg* (3P° J) + I+ (3PJ). From 38 500 to 41 000 cm-1, the spectrum is broad and unstructured. We attribute this region of the spectrum to one-photon dissociation of vibrationally hot MgI+ at low energy and ground state MgI+ at higher energy to form Mg (1S) + I+ (3PJ) products.

Probing adsorption of methane onto vanadium cluster cations via vibrational spectroscopy.

Photofragment spectroscopy is used to measure the vibrational spectra of V2+(CH4)n (n = 1-4), V3+(CH4)n (n = 1-3), and Vx+(CH4) (x = 4-8) in the C-H stretching region (2550-3100 cm-1). Spectra are measured by monitoring loss of CH4. The experimental spectra are compared to simulations at the B3LYP+D3/6-311++G(3df,3pd) level of theory to identify the geometry of the ions. Multi-reference configuration interaction with Davidson correction (MRCI+Q) calculations are also carried out on V2+ and V3+. The methane binding orientation in V2+(CH4)n (n = 1-4) evolves from η3 to η2 as more methane molecules are added. The IR spectra of metal-methane clusters can give information on the structure of metal clusters that may otherwise be hard to obtain from isolated clusters. For example, the V3+(CH4)n (n = 1-3) experimental spectra show an additional peak as the second and third methane molecules are added to V3+, which indicates that the metal atoms are not equivalent. The Vx+(CH4) show a larger red shift in the symmetric C-H stretch for larger clusters with x = 5-8 than for the small clusters with x = 2, 3, indicating increased covalency in the interaction of larger vanadium clusters with methane.

Bonding, Thermodynamics, and Dissociation Dynamics of NiO+ and NiS+ Determined by Photofragment Imaging and Theory.

We use photofragment ion imaging and ab initio calculations to determine the bond strength and photodissociation dynamics of the nickel oxide (NiO+) and nickel sulfide (NiS+) cations. NiO+ photodissociates broadly from 20350 to 32000 cm-1, forming ground state products Ni+(2D) + O(3P) below ∼29000 cm-1. Above this energy, Ni+(4F) + O(3P) products become accessible and dominate over the ground state channel. In certain images, product spin-orbit levels are resolved, and spin-orbit propensities are determined. Image anisotropy and the results of MRCI calculations suggest NiO+ photodissociates via a 3 4Σ- ← X 4Σ- transition above the Ni+(4F) threshold and via 3 4Σ-, 2 4Σ-, and/or 2 4Π and 3 4Π excited states below the 4F threshold. The photodissociation spectrum of NiS+ from 19900 to 23200 cm-1 is highly structured, with ∼12 distinct vibronic peaks, each containing underlying substructure. Above 21600 cm-1, the Ni+(2D5/2) + S(3P) and Ni+(2D3/2) + S(3P) product spin-orbit channels compete, with a branching ratio of ∼2:1. At lower energy, Ni+(2D5/2) is formed exclusively, and S(3P2) and S(3P1) spin-orbit channels are resolved. MRCI calculations predict the ground state of NiS+ to be one of two nearly degenerate states, the 1 4Σ- and 1 4Δ. Based on images and spectra, the ground state of NiS+ is assigned as 4Δ7/2, with the 1 4Σ3/2- and 1 4Σ1/2- states 81 ± 30 and 166 ± 50 cm-1 higher in energy, respectively. The majority of the photodissociation spectrum is assigned to transitions from the 1 4Δ state to two overlapping, predissociative excited 4Δ states. Our D0 measurements for NiO+ (D0 = 244.6 ± 2.4 kJ/mol) and NiS+ (D0 = 240.3 ± 1.4 kJ/mol) are more precise and closer to each other than previously reported values. Finally, using a recent measurement of D0(NiS), we derive a more precise value for IE (NiS): 8.80 ± 0.02 eV (849 ± 1.7 kJ/mol).

Structures of M+(CH4)n (M = Ti, V) Based on Vibrational Spectroscopy and Density Functional Theory.

Photofragment spectroscopy is used to measure the vibrational spectra of M+(CH4)(Ar) and M+(CH4)n (M = Ti, V; n = 1-4) in the C-H stretching region (2550-3100 cm-1). Spectra were measured by monitoring the loss of Ar from M+(CH4)(Ar) and loss of CH4 from the larger clusters. The experimental spectra are then compared to simulations done at the B3LYP/6-311++G(3df,3pd) level of theory to identify the structures of the ions. The spectra all have a peak near 2800 cm-1 due to the symmetric C-H stretch of the hydrogens adjacent to the metal. Some complexes also have a smaller peak due to the corresponding antisymmetric stretch. Most complexes also have a peak near 3000 cm-1 due to the C-H stretch of hydrogens pointing away from the metal. The symmetric proximate C-H stretches of M+(CH4)(Ar) to M+(CH4)4 are red-shifted from the symmetric stretch in bare CH4 by 149, 152, 128, and 107 cm-1 for the titanium complexes and 164, 175, 158, and 146 cm-1, respectively, for the vanadium complexes. In M+(CH4)(Ar) (M = Ti, V), the heavy atoms are collinear. Ti+(CH4)(Ar) has η3 methane hydrogen coordination (∠M-C-H = 180°), while V+(CH4)(Ar) has η2 (∠M-C-H = 124°). The n = 2 complexes have C-M-C linear. Ti+(CH4)2 has C2h symmetry with η3 CH4 while V+(CH4)2 has methane coordination intermediate between η2 and η3 (∠M-C-H = 156°). Both the M+(CH4)3 (M = Ti, V) complexes have C2v symmetry with one methane farther away from the metal in an η2 binding orientation and two methanes close to the metal with a nearly η2 methane for vanadium and coordination between η2 and η3 CH4 for titanium (∠M-C-H = 150°). In Ti+(CH4)4 and V+(CH4)4 all of the methanes have η2 coordination. The titanium complex has a distorted square planar geometry with two different Ti-C bond lengths and the vanadium complex is square planar.

Exciton energy transfer reveals spectral signatures of excited states in clusters.

Electronic excitation and concomitant energy transfer leading to Penning ionization in argon-acetylene clusters generated in a supersonic expansion are investigated with synchrotron-based photoionization mass spectrometry and electronic structure calculations. Spectral features in the photoionization efficiency of the mixed argon-acetylene clusters reveal a blue shift from the 2P1/2 and 2P3/2 excited states of atomic argon. Analysis of this feature suggests that excited states of argon clusters transfer energy to acetylene, resulting in its ionization and successive evaporation of argon. Theoretically calculated Arn (n = 2-6) cluster spectra are in excellent agreement with experimental observations, and provide insight into the structure and ionization dynamics of the clusters. A comparison between argon-acetylene and argon-water clusters reveals that argon solvates water better, allowing for higher-order excitons and Rydberg states to be populated. These results are explained by theoretical calculations of respective binding energies and structures.

Vibrational Spectroscopy of Intermediates and C-H Activation Products of Sequential Zr+ Reactions with CH4.

Vibrational spectra of the entrance channel complexes Zr+(CH4)n(Ar) (n = 1-2) and Zr+(CH4)n (n = 3-4) and the reaction products ZrC3H10+, ZrC4H13+, ZrC4H14+, and ZrC4H15+ in the C-H stretching region (2550-3100 cm-1) are obtained using photofragment spectroscopy. The experimental spectra and simulations based on calculations at the B3LYP/aug-cc-pVTZ level of theory work together to identify the structures of the ions. The n = 1-3 entrance channel complexes show peaks around 2800 and 3000 cm-1 which indicates methane η3 hydrogen coordination, while the n = 4 complex has two peaks around 2800 cm-1 indicative of methane η2 hydrogen coordination. Observation of the low-frequency C-H stretch of an agostic carbene group, as well as the high-frequency H-H stretch, also confirms production of (H2)ZrCH2+(CH4)n-1 (n = 1-2) exit channel complexes. The observed C-H activation products formally correspond to loss of H2 from Zr+(CH4)3 and loss of H, H2, and H2 + H from Zr+(CH4)4. Comparison of experiment and simulations indicates that the activation products are Zr(CH3)2+(CH4), Zr(CH3)3+(CH4), Zr(CH3)2+(CH4)2, and HZrCH2+(CH4)3 and/or ZrCH3+(CH4)3.

Vibrational Spectroscopy of Cr+(NH3) n ( n = 1-6) Reveals Coordination and Hydrogen-Bonding Motifs.

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.

Probing Reactivity of Gold Atoms with Acetylene and Ethylene with VUV Photoionization Mass Spectrometry and Ab Initio Studies.

Reaction of gold atoms with acetylene and ethylene in a laser ablation source produces a number of gold-containing species. Their photoionization efficiency (PIE) curves are measured using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. Their structures are assigned by comparing the experimental ionization energies and PIE curves to those of potential isomers calculated at the CAM-B3LYP/aug-cc-pVTZ level. For smaller molecules, the contribution of ionization to excited electronic states of the cation is also included using photoionization cross sections calculated using ePolyScat. Reaction with acetylene produces adducts Au(C2H2) and Au(C2H2)2, as well as HAu(C4H2). Reaction with ethylene leads to adducts Au(C2H4), Au(C2H4)2, an adduct with a gold dimer, Au2(C2H4), as well as the gold hydrides AuH, HAu(C2H4), and HAu(C4H4). [Au,C4,H7] is also observed, and it likely corresponds to a gold alkyl, H2C═C(H)-Au(C2H4). Reactions leading to production of odd-hydrogen species are endothermic and are likely due to translationally or electronically excited gold atoms. These measurements provide the first directly measured ionization energy for gold hydride, IE(AuH) = 10.25 ± 0.05 eV. Combining this value with the dissociation energy of AuH+ gives a dissociation energy D0(AuH) = 3.15 ± 0.12 eV. Several other ionization energies are measured: IE(Au2(C2H4)) = 8.42 ± 0.05 eV, IE(HAu(C2H4)) = 9.35 ± 0.05 eV, IE(HAu(C4H2)) = 8.8 ± 0.1 eV, and IE(HAu(C4H4)) = 8.8 ± 0.1 eV.

Photofragment Imaging, Spectroscopy, and Theory of MnO.

Density functional and ab initio calculations, along with photodissociation spectroscopy and ion imaging of MnO+ from 21,300 to 33,900 cm-1, are used to probe the photodissociation dynamics and bond strength of the manganese oxide cation (MnO+). These studies confirm the theoretical ground state (5Π) and determine the spin-orbit constant ( A' = 14 cm-1) of the dominant optically accessible excited state (5Π) in the region. Photodissociation via this excited 5Π state results in ground state Mn+ (7S) + O (3P) products. At energies above 30,000 cm-1, the Mn+ (5S) + O (3P) channel is energetically accessible and becomes the preferred dissociation pathway. The bond dissociation energy ( D0 = 242 ± 5 kJ/mol) of MnO+ is measured from several images of each photofragmentation channel and compared to theory, resolving a disagreement in previous measurements. MRCI+Q calculations are much more successful in predicting the observed spectrum than TD-DFT or EOM-CCSD calculations.

Photofragment imaging and electronic spectroscopy of Al2.

A combination of photodissociation spectroscopy, ion imaging, and high-level theory is employed to refine the bond strength of the aluminum dimer cation (Al2+) and elucidate the electronic structure and photodissociation dynamics between 38 500 and 42 000 cm-1. Above 40 400 cm-1, structured photodissociation is observed from an extremely anharmonic excited state, which calculations identify as the double minimum G 2Σ+u state. The photodissociation spectrum of the G 2Σ+u ← X 2Σ+g transition in Al2+ gives an average vibrational spacing of 170 cm-1 for the G 2Σ+u state and ν0 = 172 cm-1 for the ground state. Photofragment images of G 2Σ+u ← X 2Σ+g transitions indicate that once the Al (4P) + Al+ (1S) product channel is energetically accessible, it dominates the lower energy, spin-allowed pathways despite being spin-forbidden. This is explained by a proposed competition between radiative and non-radiative decay pathways from the G 2Σ+u state. The photofragment images also yield D0 (Al+-Al) = 136.6 ± 1.8 kJ/mol, the most precise measurement to date, highlighting the improved resolution achieved from imaging at near-threshold energies. Additionally, combining D0 (Al+-Al) with IE (Al) and IE (Al2) gives an improved neutral D0 (Al-Al) = 136.9 ± 1.8 kJ/mol.

Bond dissociation energy and electronic spectroscopy of Cr+(NH3) and its isotopomers.

The electronic spectra of Cr+(NH3), Cr+(ND3), and Cr+(15NH3) have been measured from 14 200 to 17 400 cm-1 using photodissociation spectroscopy. Transitions are predominantly observed from the 6A1 ground state, in which the Cr+ has a 3d 5 electronic configuration, to the B ̃ 6E (Π) state (3d 44s). There is extensive vibronic structure in the spectrum due to a long progression in the Cr-N stretch and transitions to all six spin-orbit levels in the upper state. The spin-orbit splitting in the excited state is observed to be Aso' = 39 cm-1. For the lowest spin-orbit level, the Cr-N stretching frequency in the excited state is 343 cm-1, with an anharmonicity of 4.2 cm-1. The 6E (Π) origin is predicted to lie at T0 = 14 697 cm-1. The first peak observed is due to v' = 1, so the observed photodissociation onset is thermodynamic rather than spectroscopic, giving D0(Cr+-NH3) = 14 830 ± 100 cm-1 (177.4 ± 1.2 kJ/mol) and D0(Cr+-ND3) = 15 040 ± 30 cm-1 (179.9 ± 0.4 kJ/mol). The 6E (Π) state of Cr+(NH3) is ∼2740 cm-1 less strongly bound than the ground state, and the Cr-N bond length increases by 0.23 ± 0.03 Å upon electronic excitation. Calculations at the time-dependent density functional theory (M06) and equations of motion coupled cluster, with single and double excitations (EOM-CCSD) level fairly accurately predict the energy and vibrational frequency of the excited state. Multi-reference configuration interaction calculations show how the spin-orbit states of Cr+(NH3) evolve into those of Cr+ + NH3.

Vibrational Spectroscopy of Fe3+(CH4)n (n = 1-3) and Fe4+(CH4)4.

Vibrational spectra are measured for Fe3+(CH4)n (n = 1-3) and Fe4+(CH4)4 in the C-H stretching region (2650-3100 cm-1) using photofragment spectroscopy, monitoring loss of CH4. All of the spectra are dominated by an intense peak at around 2800 cm-1 that is red-shifted by ∼120 cm-1 from free methane. This peak is due to the symmetric C-H stretch of the η3 hydrogen-coordinated methane ligands. For clusters with three iron atoms, the peak becomes less red-shifted as the number of methane ligands increases. For clusters with one methane ligand per iron atom, the red shift increases in going from Fe2+(CH4)2 (88 cm-1) to Fe3+(CH4)3 (108 cm-1) to Fe4+(CH4)4 (122 cm-1). This indicates increased covalency in the binding of methane to the larger iron clusters and parallels their increased reactivity. Density functional theory calculations, B3LYP, BPW91, and M11L, are used to identify possible structures and geometries and to predict the spectra. Results show that all three functionals tend to overestimate the methane binding energies. The M11L calculations provide the best match to the experimental spectra.

Vibrational spectroscopy and theory of Fe2(+)(CH4)(n) (n = 1-3).

Vibrational spectra are measured for Fe2(+)(CH4)n (n = 1-3) in the C-H stretching region (2650-3100 cm(-1)) using photofragment spectroscopy, by monitoring the loss of CH4. All of the spectra exhibit an intense peak corresponding to the symmetric C-H stretch around 2800 cm(-1). The presence of a single peak suggests a nearly equivalent interaction between the iron dimer and the methane ligands. The peak becomes slightly blue shifted as the number of methane ligands increases. Density functional theory calculations, B3LYP and BPW91, are used to identify possible structures and predict the spectra. Results suggest that the methane(s) bind in a terminal configuration and the complexes are in the octet spin state.

Vibrational Spectroscopy Reveals Varying Structural Motifs in Cu(+)(CH4)(n) and Ag(+)(CH4)(n) (n = 1-6).

Vibrational spectra are measured for Cu(+)(CH4)(Ar)2, Cu(+)(CH4)2(Ar), Cu(+)(CH4)n (n = 3-6), and Ag(+)(CH4)n (n = 1-6) in the C-H stretching region (2500-3100 cm(-1)) using photofragment spectroscopy. Spectra are obtained by monitoring loss of Ar or CH4. Interaction with the metal ion produces substantial red shifts in the C-H stretches of proximate hydrogens. The magnitude of the shift reflects the metal-methane distance and the coordination to the metal ion of the methane hydrogens (η(2) or η(3)). The structures of the complexes are determined by comparing the measured spectra with spectra calculated for candidate geometries using the B3LYP and CAM-B3LYP density functionals with 6-311++G(3df,3pd) and aug-cc-pVTZ-PP basis sets. Because of the d(10) electronic configuration of the metal ions, the complexes are expected to adopt symmetric structures, which is confirmed by the experiments. All of the complexes have η(2) hydrogen coordination in the first shell, in accord with theoretical predictions; second-shell ligands sometimes show η(3) hydrogen coordination. The vibrational spectrum of Cu(+)(CH4)(Ar)2 shows extensive structure due to Fermi resonance between the lowest-frequency C-H stretch and overtones of the H-C-H bends. The Cu(+)(CH4) cluster has a smaller red shift in the lowest-frequency C-H stretch than M(+)(CH4), M(+) = Co(+) (d(8)) and Ni(+) (d(9)). Although all three ions have similar binding energies, the metal-ligand electrostatic interaction is largest for Cu(+), while the contribution from covalent interactions is largest for Co(+). The larger ionic radius of Ag(+) leads to a larger metal-ligand distance and weaker interaction, resulting in substantially smaller red shifts than in the Cu(+) complexes. The Cu(+)(CH4)2 and Ag(+)(CH4)2 clusters have symmetrical structures, with the methanes on opposite sides of the metal, while Cu(+)(CH4)3 and Ag(+)(CH4)3 adopt symmetrical, trigonal planar structures with all M-C distances equal. For Cu(+)(CH4)4, the tetrahedral structure dominates the observed spectrum, although a trigonal pyramidal structure may contribute; however, only the tetrahedral structure is observed for Ag(+)(CH4)4. The structures of Cu(+)(CH4)n and Ag(+)(CH4)n differ for clusters with n > 4. For copper complexes, these are primarily formed by adding outer-shell methane ligand(s) to the tetrahedral n = 4 core. The observed spectra of the larger Ag(+) clusters are dominated by symmetrical structures in which all of the Ag-C distances are similar: Ag(+)(CH4)5 has a trigonal bipyramidal geometry and Ag(+)(CH4)6 is octahedral.

Vibrational spectroscopy of Co⁺(CH4)n and Ni⁺(CH4)n (n = 1-4).

Vibrational spectra of M(+)(CH4)m(Ar)(3-m) and M(+)(CH4)n (M = Co, Ni; m = 1, 2; n = 3, 4) in the C-H stretching region (2500-3100 cm(-1)) are measured using photofragment spectroscopy, monitoring the loss of argon or methane. Interaction with the metal leads to large red shifts in the C-H stretches for proximate hydrogens. The extent of this shift is sensitive to the coordination (η(2) vs η(3)) and to the metal-methane distance. The structures of the complexes are determined by comparing measured spectra with those calculated for candidate structures at the B3LYP/6-311++G(3df,3pd) level. Binding energies are also computed using the CAM-B3LYP functional. In all cases, CH4 shows η(2) coordination to the metal. The m = 1 complexes show very large red shifts of 370 cm(-1) (for M = Co) and 320 cm(-1) (for M = Ni) in the lowest C-H stretch, relative to the symmetric stretch of free CH4. They adopt a C2v structure with the heavy atoms and proximate hydrogen atoms coplanar. The m = 2 complexes have slightly reduced red shifts, and Tee-shaped structures. Both Tee-shaped and equilateral (or quasi-equilateral) structures are observed for the n = 3 complexes. The measured photodissociation onset and significantly reduced intensity for low-frequency C-H stretches imply a value of 2650 ± 50 cm(-1) for the binding energy of Ni(+)(CH4)2-CH4. The Co(+)(CH4)4 complexes have two low-lying structures, quasi-tetrahedral and distorted square-planar, which contribute to the rich spectrum. In contrast, the symmetrical, square-planar Ni(+)(CH4)4 complex is characterized by a very simple vibrational spectrum.

Near ultraviolet photodissociation spectroscopy of Mn(+)(H2O) and Mn(+)(D2O).

The electronic spectra of Mn(+)(H2O) and Mn(+)(D2O) have been measured from 30,000 to 35,000 cm(-1) using photodissociation spectroscopy. Transitions are observed from the (7)A1 ground state in which the Mn(+) is in a 3d(5)4s(1) electronic configuration, to the (7)B2 (3d(5)4py) and (7)B1 (3d(5)4px) excited states with T0 = 30,210 and 32,274 cm(-1), respectively. Each electronic transition has partially resolved rotational and extensive vibrational structure with an extended progression in the metal-ligand stretch at a frequency of ∼450 cm(-1). There are also progressions in the in-plane bend in the (7)B2 state, due to vibronic coupling, and the out-of-plane bend in the (7)B1 state, where the calculation illustrates that this state is slightly non-planar. Electronic structure computations at the CCSD(T)/aug-cc-pVTZ and TD-DFT B3LYP/6-311++G(3df,3pd) level are also used to characterize the ground and excited states, respectively. These calculations predict a ground state Mn-O bond length of 2.18 Å. Analysis of the experimentally observed vibrational intensities reveals that this bond length decreases by 0.15 ± 0.015 Å and 0.14 ± 0.01 Å in the excited states. The behavior is accounted for by the less repulsive px and py orbitals causing the Mn(+) to interact more strongly with water in the excited states than the ground state. The result is a decrease in the Mn-O bond length, along with an increase in the H-O-H angle. The spectra have well resolved K rotational structure. Fitting this structure gives spin-rotation constants ɛaa″ = -3 ± 1 cm(-1) for the ground state and ɛaa' = 0.5 ± 0.5 cm(-1) and εaa' = -4.2 ± 0.7 cm(-1) for the first and second excited states, respectively, and A' = 12.8 ± 0.7 cm(-1) for the first excited state. Vibrationally mediated photodissociation studies determine the O-H antisymmetric stretching frequency in the ground electronic state to be 3658 cm(-1).

Dissociation energy and electronic and vibrational spectroscopy of Co(+)(H2O) and its isotopomers.

The electronic spectra of Co(+)(H(2)O), Co(+)(HOD), and Co(+)(D(2)O) have been measured from 13,500 to 18,400 cm(-1) using photodissociation spectroscopy. Transitions to four excited electronic states with vibrational and partially resolved rotational structure are observed. Each electronic transition has an extended progression in the metal-ligand stretch, v(3), and the absolute vibrational quantum numbering is assigned by comparing isotopic shifts between Co(+)(H(2)(16)O) and Co(+)(H(2)(18)O). For the low-lying excited electronic states, the first observed transition is to v(3)' = 1. This allows the Co(+)-(H(2)O) binding energy to be determined as D(0)(0 K)(Co(+)-H(2)O) = 13730 ± 90 cm(-1) (164.2 ± 1.1 kJ/mol). The photodissociation spectrum shows a well-resolved K(a) band structure due to rotation about the Co-O axis. This permits determination of the spin rotation constants ε(aa)" = -6 cm(-1) and ε(aa)' = 4 cm(-1). However, the K(a) rotational structure depends on v(3)'. These perturbations in the spectrum make the rotational constants unreliable. From the nuclear spin statistics of the rotational structure, the ground state is assigned as (3)B(1). The electronic transitions observed are from the Co(+)(H(2)O) ground state, which correlates to the cobalt ion's (3)F, 3d(8) ground state, to excited states which correlate to the (3)F, 3d(7)4s and (3)P, 3d(8) excited states of Co(+). These excited states of Co(+) interact less strongly with water than the ground state. As a result, the excited states are less tightly bound and have longer metal-ligand bonds. Calculations at the CCSD(T)/aug-cc-pVTZ level also predict that binding to Co(+) increases the H-O-H angle in water from 104.1° to 106.8°, as the metal removes electron density from the oxygen lone pairs. The O-H stretching frequencies of the ground electronic state of Co(+)(H(2)O) and Co(+)(HOD) have been measured by combining IR excitation with visible photodissociation in a double resonance experiment. In Co(+)(H(2)O) the O-H symmetric stretch is ν(1)" = 3609.7 ± 1 cm(-1). The antisymmetric stretch is ν(5)" = 3679.5 ± 2 cm(-1). These values are 47 and 76 cm(-1), respectively, lower than those in bare H(2)O. In Co(+)(HOD) the O-H stretch is observed at 3650 cm(-1), a red shift of 57 cm(-1) relative to bare HOD.

Al+ Activates Acetone to Form Pinacolate.

The interaction between aluminum cations and acetone is studied in the gas phase via photodissociation vibrational spectroscopy from 1100 to 2000 cm-1. Spectra of Al+(acetone)(N2) and ions with the stoichiometry of Al+(acetone)n (n = 2-5) were measured. The experimental results are compared to DFT calculated vibrational spectra to determine the structures of the complexes. The spectra show a red shift of the C=O stretch and a blue shift of the CCC stretch, which decrease as the size of the clusters increases. The calculations predict that the most stable isomer for n ≥ 3 is a pinacolate, in which oxidation of the Al+ enables reductive C-C coupling between two acetone ligands. Experimentally, pinacolate formation is observed for n = 5, as evidenced by a new peak observed at 1185 cm-1 characteristic of the pinacolate C-O stretch.