Effect of Substituents on the Stability of Sulfur-Centered Radicals.

Abstract

High-level ab initio calculations have been used to calculate the standard and inherent radical stabilities (RSEs) of a test set of 41 sulfur-centered radicals, chosen for their relevance in fields as diverse as combustion, atmospheric chemistry, polymer chemistry, and biochemistry. Radical stability was shown to be profoundly affected by substituents, varying over a 30 kcal mol(-1) range for the test set studied. Like carbon-centered radicals, substituent effects on sulfur-centered radical stabilities result from the competition between the stabilizing effect of electron delocalization by lone pair donation and π-acceptance, and the destabilizing effect of σ withdrawal. However, in contrast to carbon-centered radicals, the heavier thiyl radicals are better able to undergo resonance and lone-pair donor interactions with heavier substituents. In particular, sulfur-containing lone pair donor and π-acceptor substituents have the greatest stabilizing effect, whereas σ-withdrawing substituents such as carbonyls and pyridines are the least stabilizing. The stabilities predicted using the standard definition and Zavitsas's inherent RSEZ scheme are shown to be in surprisingly good agreement with one another for most species tested. The RSEZ values have also been shown to be capable of making chemically accurate estimates of bond energies by comparing our calculated values with 34 currently available experimental ones.