Structures of M⁺(CH₄) (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) (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 C₂ 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 C₂ 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.