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.