Thermodynamic functions of conformational changes, Part IV: Functional analysis of conformational entropy of substituted ethane and methanol

摘要

The thermodynamic functions Delta H and Delta S, were computed along the dihedral angle (x) of conformational change for a few selected single rotors: YCH2-CH2Y and YCH2-OH. The computations were carried out at the B3LYP/6-31G(d,p) level of theory, with 5 degrees increments in x, under very fight optimization condition. The entropy function, S(x), was generated for the full range (0 degrees to 360 degrees) of internal rotation and was subjected to functional analysis. We found the general rule that S(x) had critical points at or near the critical points of Delta H(x); therefore, (dS/dx) vanished for all critical points (minima and transition states) of Delta H(x). However, the entropy derivative also vanished at certain characteristic conformations that were not manifested on the potential energy or enthalpy function. These geometries were classified as '' latent minima '' or "latent critical points.'' The conformational entropy change was related. to the relative information (I/I-180) of the same state, where the reference state was chosen to be the anti conformation (x = 180 degrees). According to the relationship ln(I/I-180) = -Delta S/R, information accumulation was observed in the case of 1,2-disubtituted ethane and information depletion occurred in the case of fluoromethanol and ethyl alcohol. (c) 2007 Wiley Periodicals, Inc.