Nature and origin of γ-gauche effect in sulfoxides: A density functional theory and information-theoretic approach study

Abstract

Bulky vicinal groups around a single bond usually prefer to be in the opposite (anti) conformation but when extra interactions come into play, more stable could be the gauche conformer, where the neighboring groups embrace a dihedral angle around 60°. This is called the gauche effect. The γ-gauche effect historically features the upfield shift of 13C NMR spectra when a carbon atom and its nonprotonic γ-substituent are in a gauche arrangement. In this work, using sulfoxides as illustrative examples, we show that the nature of the γ-gauche effect is the same as that of the gauche effect, i.e., the gauche conformation is more stable than the anti conformation. To unveil the origin of this effect, we employ two total energy partition schemes in density functional theory and four quantities from the information-theoretic approach. Our results unambiguously show that the γ-gauche effect is of the stereoelectronic origin with a multifaceted nature. Its origin can be understood from a few different perspectives including energetic contributions, dipole moment, charge distribution, hyperconjugation, and information theory. Substantially different from other stereoelectronic effects such as anomeric and generalized anomeric effects, this effect has its unique features as demonstrated from our results in the present work. Having successfully established the link of the γ-gauche with other gauche effects and unveiled its multi-faceted origin, this study should shed new light towards the comprehensive understanding about this important effect of conformational stability in sulfoxides and other systems alike.