Calculations have been performed at four basis set levels (STO-3G, STO-3G*, 4-31G, 4-31G*) on the model ylides methylenesulfurane (CH2SH2) and methylenephosphorane (CH2PH3), their stable tautomers (CH3SH and CH3PH2), their dissociation products (SH2, PH3 and CH2), and the protonated species CH3SH2+ and CH3PH3+. 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 CH2 (or SH2) 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, which are 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 CH2SH2 and CH2PH3, but not for the normal-valent species SH2, PH3, CH3SH, and CH3PH2. 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 XHn 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 CH2 moiety is planar in CH2SH2 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.
A G2(MP2) theoretical study of substituent effects on H3BNHnCl3−n (n= 3-0) donor-acceptor complexes