Is SH4, the simplest 10-S-4 sulfurane, observable?

Abstract

The kinetic stability of SH₄ was investigated theoretically with the coupled cluster ansatz. The two possible modes of decomposition into SH₂ and H₂ through either a C_2v or a C₁ transition structure (TS) were investigated using intrinsic reaction coordinate (IRC) computations; orbital interactions along the reaction paths were analyzed. The two dissociation modes are due to differences in the electron delocalization in the TSs. While the C_2v TS is bonded rather covalently by a three center–four electron (3c–4e) interaction which is lost in a strictly synchronous way (two electrons occupy the same orbital at a time along the reaction coordinate), the bonding orbital in the C₁ TS is merely occupied by a single electron. Surprisingly, this highly polarized TS has a lower barrier. Computations at the CCSD(T)/cc-pVQZ level of theory show that the zero-point corrected enthalpy (ΔH₀^‡) of the C₁ TS is 16 kcal mol⁻¹ above the C_4v symmetric ground state; the barrier along the C_2v path is 40 kcal mol⁻¹. The overall exothermicity for the dissociation into SH₂ and H₂ was estimated to be ΔH₀=−76 kcal mol⁻¹. The fundamental IR absorptions of SH₄ (obtained by scaling the computed harmonic vibrational frequencies taken from the CCSD(T)/cc-pVQZ level of theory) are 1432 and 2037 cm⁻¹.