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

J Phys Chem A. 2021 May 20;125(19):4143-4151. doi: 10.1021/acs.jpca.1c02217. Epub 2021 May 7.

Abstract

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.