Quantum effects on conformational equilibria

Abstract

Some relatively preliminary investigations are made into the nature of quantum effects on conformational equilibria with the aid of simple gas phase models and a semiclassical method. The case of ethane is first used to test the semiclassical method and to define the concepts of an effective rotational barrier and an effective zero point energy. Internal rotation in n‐butane and some alkyl radicals is then examined. At liquid nitrogen temperatures, butane is predicted to have its trans/gauche equilibrium constant shifted relative to its classical value by an amount roughly equal to (and in the opposite direction from) the shift induced by a hypothetical hard sphere solvent. In the case of certain alkyl radicals, it is pointed out how the occurrence of quantum effects may actually help resolve the current disagreement between solution ESR experiments and theoretical predictions.