AB-INITIO AND DENSITY-FUNCTIONAL THEORETICAL-STUDIES OF STRUCTURES, VIBRATIONAL-SPECTRA, AND DIMERIZATION DYNAMICS OF SIMPLE TRANSIENT GERMENES

Structures and harmonic vibrational frequencies for several simple tra nsient germenes, H2Ge=CH2 (1), MeHGe=CH2 (2), Me2Ge=CH (3), FHGe=CH2 ( 4), and H2Ge=CHF (5), and their dimerization and transition state ener gies for the head-to-tail and head-to-head self-coupling have been cal culated by nb initio HF3-21G and 6-311G(d,p) and density functional th eory B3LYP/6-311G(d,p) methods. The effect of substituents on the Ge=C bond lengths and bond orders, as well as frequencies and force consta nts of the Ge=C stretch, is predicted to be relatively small in germen es 1-3 and be substantially stronger in the F-substituted molecules 4 and 5. Within the limits of the HF/3-21C method, the head-to-tail cycl odimerizations of all studied germenes 1-5 were found to be more exoth ermic than the head-to-head processes and to proceed with very low or zero barriers. This conclusion is confirmed for 1-4 by the higher leve l calculations with the 6-311G(d,p) basis set at HF and B3LYP levels, which in the case of 5 lends to the opposite result such as a substant ial barrier for the head-to-tail dimerization and a lesser exothermici ty of this process relative to tile head-to-head one. The comparison o f the HF/3-21G data available for the whale series of similarly substi tuted silenes with those calculated for 1-5 at the same level of theor y indicate somewhat easier dimerization of germenes with respect to th eir double-bonded silicon analogues.