Nn. Pham-tran et al., Theoretical study of the ring opening of phosphirane and silirane: contrasting mechanisms of hydrogen migration, J CHEM S P2, (5), 2001, pp. 766-773
Citations number
39
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
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".