Probing adsorption of methane onto vanadium cluster cations via vibrational spectroscopy

Photofragment spectroscopy is used to measure the vibrational spectra of V-2(+)(CH4)(n) (n = 1-4), V-3(+)(CH4)(n) (n = 1-3), and V-x(+)(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 V-2(+) and V-3. The methane binding orientation in V-2(+)(CH4)(n) (n = 1-4) evolves from eta 3 to eta 2 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 V-3(CH4)n (n = 1-3) experimental spectra show an additional peak as the second and third methane molecules are added to V-3, which indicates that the metal atoms are not equivalent. The V-x(+)(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.