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 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.

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 3d5 electronic configuration, to the B̃ 6E (Π) state (3d44s). 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 spectra are measured for Fe(CH) (n = 1-3) and Fe(CH) in the C-H stretching region (2650-3100 cm) using photofragment spectroscopy, monitoring loss of CH. All of the spectra are dominated by an intense peak at around 2800 cm that is red-shifted by ∼120 cm from free methane. This peak is due to the symmetric C-H stretch of the η 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 Fe(CH) (88 cm) to Fe(CH) (108 cm) to Fe(CH) (122 cm). 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.

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