Ab initio study of singlet-triplet energy separations in C2HXSi silylenes (X=H, F, Cl and Br)

Singlet-triplet energy separations in silylenes: C2HSiX (where X=H, F, Cl and Br), are compared and contrasted, at six levels of theory: HF/6-31G*, HF/6-311++G**, B3LYP/6-311++G**, MP2/6-311++G**, MP4 (SDTQ)/6-311++G** and QCISD (T)/6-311++G**. Four possible structures are considered for each singlet (s) and triplet (t) states of C2HSiX species: silacyclopropenylidene, 1; vinylidenesilylene, 2; propargylsilylene, 3; and ethynylsilylene, 4. The order of singlet-triplet energy separations for cyclic species, 1t-X-1s-X (ΔEs-t,X), as a function of X is: H>Br>F>Cl. In contrast, the order of singlet-triplet energy gaps, ΔEs-t,X, for their acyclic analogues (2t-X-2s-X, 3t-X-3s-X and 4t-X-4s-X), as a function of X, follows electro-negativity: F>Cl>Br>H. The order of stability for six isomers of C2H2Si is: 1s-H>2s-H>3s-H>2t-H>3t-H>1t-H; with the singlet cyclic 1s-H appearing as the global minimum. These are compared to 24 divalent species of C2HSiX, with the stability order of: 3s-X>1s-X>2s-X>3t-X>4s-X>2t-X>4t-X>1t-X; which have the singlet acyclic 3s-X as their global minima. This demonstrates stabilization through resonance and/or induction, by electronegative α-halogens, being more pronounced for singlet silylenes than their reported singlet carbene analogues. Comparisons between relative stabilities, multiplicities, geometrical parameters and harmonic vibrational frequencies of 1-4 are discussed.