Infrared spectrum and structure of Me₂TiCl₂ and quantum mechanical calculations of geometries and force fields for MeTiCl₃ and Me₂TiCl₂

Infrared spectra have been studied over the range 4000-200 cm^-1 for (CH₃)₂TiCl₂, (CD₃)₂TiCl₂, (CH₂D)₂-TiCl₂, and (CHD₂)₂TiCl₂ in the gas and matrix phases. Some new spectral observations are also reported for CH₃TiCl₃ and CD₃TiCl₃. Equilibrium geometries and force fields are calculated for both Me₂TiCl₂ and MeTiCl₃ using both ab initio (MP2) and DFT approaches. Scale factors for the force fields were first determined in MeTiCl₃ and then transferred to Me₂TiCl₂ so as to provide accurate estimates and facilitate the assignment of the spectra of Me₂TiCl₂. Quantum mechanical (QM) calculations of infrared intensity proved to be vital in this process. A number of Fermi resonances involving skeletal bending modes below 200 cm^-1 are postulated. Combination and overtone evidence for these modes suggests that they occur close to their predicted positions in all cases except one. The infrared evidence from the C-H and C-D stretching regions indicates that the C-H bonds in each methyl group in Me₂TiCl₂ are equivalent and slightly weaker than those in MeTiCl₃. The H-C-H angle in Me₂TiCl₂ is found to be 109 ± 1°, about 1° less than in MeTiCl₃. These results are largely reproduced by the DFT calculations, whereas the ab initio values indicate little difference between the two compounds. The skeletal interbond angles in Me₂TiCl₂ are particularly sensitive to the type of QM calculation, but all calculations agree on a reduced C-Ti-C and an enlarged Cl-Ti-Cl angle, compared with the tetrahedral value. Problems arising in customary scaling procedures are addressed. The mode of thermal decomposition of the molecule is discussed.