A theoretical study of the CSH₄ and CPH₅ hypersurfaces. Geometries, tautomerization, and dissociation of sulfonium and phosphonium ylides

Abstract

. Can. J. Chem. 59,3280 (1981). Calculations have been performed at four basis set levels (STO-3G, STO-3G*, 4-31Ci, 4-31G*) on the model ylides rnethylenesulfurane (CH,SH,) and methylenephosphorane (CH,PH,), their stable tautomers (CH,SH and CH,PH,), their dissociation products (SH,, PH, and CH,), and the protonated species CH,SH,+ and CH,PH,+. At each basis set level all geometries have been optimized fully, using the FORCE method. The conformational behaviour of the ylides as a function of C-X bond-stretching, C-X torsion. and CH, (or SH,) bending has been examined in some detail. The experimental properties (i.e., geometries, relative stabilities, proton affinities, rotation-inversion behaviour) of sulfonium and phosphonium ylides are reproduced well by the model calculations with the 4-31G* basis set, which contains d-type functions on both carbon and sulfur (or phosphorus). All other basis sets are deficient in different ways and for different reasons, whichare discussed in detail. The principal result of this work is the conclusion that d-type functions are essential for a proper description of the hypervalent species CH,SH, and CH,PH,, but not for the normal-valent species SH,, PH,, CH,SH, and CH,PH,. The conclusion concerning hypervalent species reverses our earlier views. The role of the d-type functions is to concentrate charge into the C-X region of the ylides, and thus to stabilize the system. Evidence for this effect has been obtained from quantitative perturbational molecular orbital (PMO) analyses of interactions associated with the carbon lone pair. as well as comparisons of electron density plots with and without the d AO's. A second conclusion is that the imposition of various geometrical constraints such as assumed C-X, C-H, or X-H bond lengths, and HCH or XH, bond angles, as was necessary for computational reasons in all previous work on such systems, has major and previously unrecognized consequences. For example. the assumption that the CH, moiety is planar in CH,SH, leads to very similar geometries with and without d AO's, although only in the latter case does such a geometry at carbon correspond to the true energy minimum: in the absence of d AO's the C-S bond length is maintained by a symmetry-enforced barrier to dissociation. These and other consequences of geometrical constraints at carbon, sulfur, or phosphorus are analyzed in detail.DAVID JOHN MITCHELL, SAUL WOLFE et H . BERNHARD SCHLEGEL. Can. J. Chem. 59, 3280 (1981). Utilisant quatre ensembles de base (STO-3G, STO-3G*, 4-31G, 4-31G*), on a effectue des calculs sur des modeles d'ylures de methylenesulfurane (CH,SH,) et de methylenephophorane (CH,PH,), sur leurs tautomeres stables (CH,SH et CH,PH,), sur leurs produits de dissociation (SH,, PH, et CH,) et sur leurs especes protonees CH,SH,, CH3PH3+. Faisant appel a la methode de FORCE, on a optimise pleinement toutes les geometries pour tous les ensembles de base. On a examine soigneusement le comportement conformationnel des ylures en fonction de l'elongation de la liaison C-X, de l...

A theoretical survey of singly bonded silicon compounds. Comparison of the structures and bond energies of silyl and methyl derivatives

“…The staggered form 6 of methyl mercaptan has a C-S bond distance8 of 1.822 A, in good agreement with the experimental values of 1.819144 and 1.818 A145 and the HF/4-31G* separation of 1.815 A. 125 The C-S-H bond angle is small, 97.6°; experiments give 96.50144 and 110.3°. 145 The C-S distance, larger than the C-O bond length, compensates for the smaller angle at sulfur and yields a rotational barrier of 1.1 kcal/mol in H3CSH, approximately the same as that in methanol.…”

Stretching the P–C Bond. Variations on Carbenes and Phosphanes

“…On the contrary, the bonding between a carbene R 2 C: and a phosphane :PR 3 is traditionally described as an ylidic double bond (λ 5 -PC bond). − While the charge separation (P + –C – character) can clearly be seen in the photoelectron spectrum, the bond distance is matching that of the λ 3 -PC bond. − …”

Are RR′C–PR′′(BH3)2‘electron poor’ phosphorus ylides?— an ab initio–NMR study

“…On the one hand, the d-AO at phosphorus, which is symmetry appropriate to mix with the e symmetric σ* orbitals of the PH 3 fragment (p x (P) combined antibonding with the set of s(H ϩy ), s(H Ϫy ), s(H x ) AOs) is normally neglected. 21, 24 Secondly, the phosphorus 3p AO is often 24 simplified to a 2p AO (fewer nodal planes) due to the lack of appropriate visualising tools. The MO figures in this study (generated with the MOLDEN 41 program) use correct AOs.…”