Theoretical study of the ring opening of phosphirane and silirane: contrasting mechanisms of hydrogen migration
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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 CHPH2); 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
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