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

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 t he ring-chain rearrangement of both three-membered phosphirane and silirane rings. It is confirmed that the phosphirane ring opening induced by C-P bo nd cleavage is accompanied by a hydrogen migration from C to P yielding vin ylphosphine (H2C=CHPH2); both motions occur concertedly in a single step wi th an energy barrier of about 200 +/- 15 kJ mol(-1). In contrast, the prefe rred ring opening of silirane by C-Si bond cleavage involves a downgrade hy drogen migration from Si to C giving rise to ethylsilylene (H3C-CH2-SiH) an d is associated with a smaller energy barrier of 110 +/- 15 kJ mol(-1) (exp erimental: about 100 kJ mol(-1) for substituted siliranes). There are no si gnificant variations in transition structures geometries obtained either fr om single determinantal HF-based or multi-configurational CASSCF methods co ncerning the advance of H-transfer. The solvent effect is also probed using a polarizable continuum model (PCM). Full geometry optimizations within th e continuum show that solvation enthalpies are rather small and do not modi fy 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 dagger has a high-energy content and do es not exist as an equilibrium structure. Evolution of the Boys localized o rbitals 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 h ardnesses along the IRC pathways are also constructed. In one case, the har dness profile does not follow the "principle of maximum hardness".