INFRARED-SPECTRUM AND STRUCTURE OF ME(2)TICL(2) AND QUANTUM-MECHANICAL CALCULATIONS OF GEOMETRIES AND FORCE-FIELDS FOR METICL(3) AND ME(2)TICL(2)

Infrared spectra have been studied over the range 4000-200 cm(-1) for (CH3)(2)TiCl2, (CD3)(2)TiCl2, (CH2D)(2)TiCl2, and (CHD2)(2)TiCl2 in th e gas and matrix phases. Some new spectral observations are also repor ted for CH3TiCl3 and CD3TiCl3. Equilibrium geometries and force fields are calculated for both Me(2)TiCl(2) and MeTiCl(3) using both ab init io (MP2) and DFT approaches. Scale factors for the force fields were f irst determined in MeTiCl(3) and then transferred to Me(2)TiCl(2) so a s to provide accurate estimates and facilitate the assignment of the s pectra of Me(2)TiCl(2). Quantum mechanical (QM) calculations of infrar ed intensity proved to be vital in this process. A number of Fermi res onances involving skeletal bending modes below 200 cm(-1) are postulat ed. Combination and overtone evidence for these modes suggests that th ey occur close to their predicted positions in all cases except one. T he 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 equivalen t and slightly weaker than those in MeTiCl(3). The H-C-H angle in Me(2 )TiCl(2) is found to be 109 +/- 1 degrees, about, 1 degrees less than in MeTiCl(3). These results are largely reproduced by the DFT calculat ions, whereas the ab initio values indicate little difference between the two compounds. The skeletal interbond angles in Me(2)TiCl(2) are p articularly sensitive to the type of QM calculation, but all calculati ons agree on a reduced C-Ti-C and an enlarged Cl-Ti-Cl angle, compared with the tetrahedral value. Problems arising in customary scaling pro cedures are addressed. The mode of thermal decomposition of the molecu le is discussed.