DEVIATIONS FROM IDEALIZED GEOMETRY - AN AB-INITIO INVESTIGATION OF (CH3)(2)AX(2) MOLECULES [A=SI, GE, X=F, CL]

Four related compounds (CH3)(2)SiF2 (1), (CH3)(2)SiCl2 (2), (CH3)(2)Ge F2 (3), and (CH3)(2)GeCl2 (4) were studied by ab initio quantum mechan ical techniques to examine the deviation from the ''ideal'' tetrahedra l geometry. Basis sets of double-zeta (DZ) quality were used, as well as DZ with the addition of a set of d-type functions on all heavy atom s (DZd). These bases were used in conjunction with the Hartree-Fock (H F) self-consistent field method and the single-and-double-excitation c onfiguration interaction (CISD) method. Theoretical predictions corrob orate electron diffraction results in that all four molecules have C-A -C bond angles significantly larger than 109 degrees 28' but smaller t han concluded from some analyses of electron diffraction data. Specifi cally, at the DZd/CISD level of theory, C-A-C bond angles of 115.9, 11 4.3, 120.9, and 117.6 degrees were found for molecules 1-4, respective ly. An analysis also suggests that the 124 degrees experimental C-Ge-C bond angle for (CH3)(2)GeBr2 may be unrealistically large. Additional ly, harmonic vibrational analyses at the HF level of theory have been completed to aid future studies of these four molecules.