On the π-Donating Abilities of Sulfur and Oxygen. A Comparative Quantum Chemical Investigation of the Static and Dynamic Properties and Gas Phase Acidities of and

Abstract

Nonempirical SCF-MO computations have been performed on CH2O, CH2S, and their protonated derivatives [Formula: see text] and [Formula: see text]. The methylene group of CH2O is strongly positive and that of CH2S is slightly negative, because oxygen in CH2O behaves towards carbon as a π-donor and σ-acceptor, but sulfur in CH2S behaves as both a π- and σ-donor. The total π-overlap in CH2O is greater than that in CH2S.The stable conformations of [Formula: see text] and [Formula: see text] correspond in each case to a structure in which all atoms lie in the same plane. In this conformation, both the C—O and C—S bond lengths are substantially shorter than those of methanol and methanethiol, indicative of conjugative interaction between the cationic center and the adjacent heteroatom. From the results of a Mulliken-type population analysis it is found that, relative to hydrogen, OH and SH behave towards an adjacent [Formula: see text] group as electron-withdrawing and electron-releasing ligands, respectively. When [Formula: see text] is considered to form from [Formula: see text] and OH fragments, π-donation from O to C amounts to 0.38 electron, but σ-donation in the opposite direction amounts to 0.35 electron; the π-overlap population is 0.1496. A similar analysis of [Formula: see text] reveals π- and σ-donation from S to C of 0.53 and 0.15 electron, respectively, and a π-overlap population of 0.1734, so that sulfurformsastronger π-bondtotheadjacentcationiccenter.Geometrical isomerization of [Formula: see text] proceeds by linear inversion at oxygen, with a barrier of 13.98 kcal/mol; but that of [Formula: see text] proceeds by rotation about the C—S bond, with a barrier of 36.47 kcal/mol. This latter value is substantially higher than that (23 kcal/mol ) computed for [Formula: see text] with a rigid rotor model, and reflects the greater strength of the π-bond between sulfur and the adjacent cationic center.