Thermodynamic functions of conformational changes I. A comparative first principles study of 1,2-disubstituted ethanes

Abstract

The conformational changes of ethane and disubstituted ethanes, X–CH2–CH2–X, where X were hydrogen isotopes of higher masses, −F atoms and −CH3 groups, were investigated by CBS-4M and MP2/6-311G(d,p) computational levels. Thermodynamic functions such as ΔH, ΔG and S were computed along the approximate reaction coordinate of conformational changes. The lowest frequency associated with the rotation about the central C–C bond was omitted along the whole reaction path. The thermodynamic functions were continuous in the case when only 3N–7 internal degrees of freedom were taken into consideration. Identical results were obtained using projected frequency calculations in the surroundings of rotational transition states. The effect of the mass and electronic difference on the entropy of the two key compounds (X = F and X = CH3) were separated and assessed. The periodicity of reaction coordinates determined the periodicity of the thermodynamic functions. The second derivatives of the potential energy functions had a shape similar to the entropy functions. The enthalpy was plotted as a function of entropy and rotational reaction coordinate. The concept of ‘entropy funnel’ used by protein chemists was recognized even in the cases of the simple compounds investigated. We concluded that the bottom of the entropy funnel could correspond to the gauche conformers, in accordance with the concept of protein folding.