Ab initio study of singlet–triplet energy separations in C 2HXSi silylenes (X=H, F, Cl and Br)

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Singlet–triplet energy separations in silylenes: C 2HSiX (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 C 2HSiX species: silacyclopropenylidene, 1; vinylidenesilylene, 2; propargylsilylene, 3; and ethynylsilylene, 4. The order of singlet–triplet energy separations for cyclic species, 1 t-X – 1 s-X (Δ E s–t, X ), as a function of X is: H>Br>F>Cl. In contrast, the order of singlet–triplet energy gaps, Δ E s–t, X , for their acyclic analogues ( 2 t-X – 2 s-X , 3 t-X – 3 s-X and 4 t-X – 4 s-X ), as a function of X, follows electro-negativity: F>Cl>Br>H. The order of stability for six isomers of C 2H 2Si is: 1 s-H > 2 s-H > 3 s-H > 2 t-H > 3 t-H > 1 t-H ; with the singlet cyclic 1 s-H appearing as the global minimum. These are compared to 24 divalent species of C 2HSiX, with the stability order of: 3 s-X > 1 s-X > 2 s-X > 3 t-X > 4 s-X > 2 t-X > 4 t-X > 1 t-X ; which have the singlet acyclic 3 s-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.