Silacyclobutadiene: singlet and triplet geometries, vibrational frequencies and electronic structures

Abstract

The geometries of the lowest singlet and triplet states of silacyclobutadiene have been optimized at the double-zeta (DZ) and double-zeta (DZ +d) self-consistent-field (SCF) level of theory. Silacyclobutadiene has a planar asymmetric closed-shell ground state. The ground-state C—C and C—Si bond lengths are in good agreement with standard values, indicating that this state is closely analogous to the ground state of cyclobutadiene. The calculated DZ +d SCF harmonic vibrational frequencies support this analogy. The lowest triplet state of silacyclobutadiene is found by configuration-interaction calculations to lie only ca. 5 kcal mol–1 above the ground state, much lower than the established value of 23 kcal mol–1 for the singlet–triplet separation in cyclobutadiene. The overall stability and singlet–triplet separation of silacyclobutadiene are discussed in terms of bond energies and π-system aromaticity.