Theoretical study of the ring opening of phosphirane and silirane: contrasting mechanisms of hydrogen migration

Abstract

Ab initio quantum chemical calculations including HF, MP2, CCSD(T), CASSCF(12,12), CASPT2 and B3LYP methods with the basis sets ranging from 6-31G(d,p) to 6-311++G(3df,2p) were used to establish the contrasting mechanism of the ring–chain rearrangement of both three-membered phosphirane and silirane rings. It is confirmed that the phosphirane ring opening induced by C–P bond cleavage is accompanied by a hydrogen migration from C to P yielding vinylphosphine (H₂CCHPH₂); both motions occur concertedly in a single step with an energy barrier of about 200 ± 15 kJ mol⁻¹. In contrast, the preferred ring opening of silirane by C–Si bond cleavage involves a downgrade hydrogen migration from Si to C giving rise to ethylsilylene (H₃C–CH₂–SiH) and is associated with a smaller energy barrier of 110 ± 15 kJ mol⁻¹ (experimental: about 100 kJ mol⁻¹ for substituted siliranes). There are no significant variations in transition structures geometries obtained either from single determinantal HF-based or multi-configurational CASSCF methods concerning the advance of H-transfer. The solvent effect is also probed using a polarizable continuum model (PCM). Full geometry optimizations within the continuum show that solvation enthalpies are rather small and do not modify the relative ordering of the energy barriers. The contrasting behaviour can be understood by the fact that ethylsilylene is a stable singlet isomer whereas singlet ethylphosphinidene has a high-energy content and does not exist as an equilibrium structure. Evolution of the Boys localized orbitals suggests that the H-atom migrates as a hydride from C to P and C to Si and as a proton from Si to C. Profiles of static polarizabilities and hardnesses along the IRC pathways are also constructed. In one case, the hardness profile does not follow the “principle of maximum hardness”.