Anharmonic vibrational analyses for the 1-silacyclopropenylidene molecule and its three isomers

Abstract

The global minimum among possible structures of SiC2H2 has been experimentally and theoretically determined to be 1-silacyclopropenylidene (1S). In 1994 Maier and Reisenauer reported the generation of 1-silacyclopropenylidene and its three isomers (2S–4S) by pulsed-flash pyrolysis followed by matrix-spectroscopic identification. Reliable quartic force fields for 1-silacyclopropenylidene and its three isomers are determined employing ab initio coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] and the correlation-consistent core-valence quadruple zeta (cc-pCVQZ) basis set. Second-order vibrational perturbation theory (VPT2) has been utilized to determine equilibrium and zero-point vibration corrected rotational constants, centrifugal distortion constants, and harmonic and anharmonic vibrational frequencies. The distances between the average nuclear positions (r α ) are also determined. The predicted rotational constants, centrifugal distortion constants, and anharmonic frequencies for the four lowest-lying isomers (1S-4S) of SiC2H2, as well as their 13C and deuterated isotopologues, agree well with available experiments. Excluding the CH and CD stretching modes, the mean absolute deviation between theoretical anharmonic and experimental fundamental frequencies for isomer 1-silacyclopropenylidene (1S) is 4.1 cm−1 (5 isotopologues, 25 modes). The corresponding deviation for ethynylsilanediyl (2 S) is 4.9 cm−1 (7 isotopologues, 38 modes) without the SiH and SiD stretching modes, while it is 8.6 cm−1 (5 isotopologues, 22 modes) for silacyclopropyne (4S) without the SiC s-stretching, SiH2 a-stretching and SiD2 wagging modes. By comparing the theoretical harmonic and anharmonic with the experimental fundamental vibrational frequencies for the four isomers (1S-4S), it is demonstrated that the anharmonic effects greatly improve the harmonic results. This theoretically derived spectroscopic data should aid in the experimental detection of the transitions that have yet to be observed, particularly for the vinylidensilanediyl isomer. †Present address: Department of Chemistry, Tsinghua University, Beijing, P. R. China 100084