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 (CH 3 ) 2 TiCl 2, (CD 3 ) 2 TiCl 2, (CH 2 D) 2 TiCl 2, and (CHD 2 ) 2 TiCl 2 in the gas and matrix phases. Some new spectral observations are also reported for CH 3 TiCl 3 and CD 3 TiCl 3 . Equilibrium geometries and force fields are calculated for both Me 2 TiCl 2 and MeTiCl 3 using both ab initio (MP2) and DFT approaches. Scale factors for the force fields were first determined in MeTiCl 3 and then transferred to Me 2 TiCl 2 so as to provide accurate estimates and facilitate the assignment of the spectra of Me 2 TiCl 2 . 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 Me 2 TiCl 2 are equivalent and slightly weaker than those in MeTiCl 3 . The H−C−H angle in Me 2 TiCl 2 is found to be 109 ± 1°, about 1° less than in MeTiCl 3 . 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 Me 2 TiCl 2 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.