AB-INITIO MO STUDIES OF NEUTRAL AND ANIONIC SICN CLUSTERS (N=2-5)

The geometries and energies of SiCn and SiCn- (n=2-5) were investigate d with ab initio calculations including electron correlation effects w ith the MP2/6-31G method, followed by MP4 and CCSD(T) single-point ca lculations to determine the most stable isomers. The adiabatic electro n affinities (AEA) were evaluated with the Delta MP4 method. Because o f the complexity of the electronic structure of SiC2, its AEA was calc ulated at the CCSD(T)/aug cc-pVTZ//CCSD(T)/aug cc-pVDZ level of approx imation. For the neutral SiCn clusters, the isomer having a triplet gr ound state has large EA, whereas the isomer having a singlet ground st ate has small EA. This is attributed to the bonding character between Si and C atoms in the orbital occupied by the extra electron. The calc ulated EA was used to assign the photoelectron spectra of SiCn- report ed previously. Furthermore, the effects of Renner-Teller splitting for the linear isomers of anions are discussed. (C) 1997 American Institu te of Physics. [S0021-9606(97)02247-2].