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

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 C2v 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 C2v 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 C4v symmetric ground state; the barrier along the C2v 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.