Infrared Studies on Rotational Isomerism. VI. Normal-Coordinate Analysis of 2-Chloro-, 2-Bromo-, and 2-Cyanoethanol

Abstract

A normal-coordinate analysis of three substituted ethanol molecules X—CH2—CH2—OH (where X = Cl, Br, or CN) was carried out using the simple valence force field model, and a computer program of Schachtschneider translated into Fortran language by Brooks. The effect of internal hydrogen bonding was studied by adding one extra symmetry coordinate, H … X (two, H … C and H … N, in the case of 2-cyanoethanol). The corresponding force constants show a good correlation with the hydrogen bond strengths. Most of the other force constants maintain the same value in the gauche and the trans rotamers. In general the calculated fundamental frequencies agree closely with the observed ones. The potential energy distribution confirms the extensive coupling between certain vibrations; particularly the skeleton stretching and the CH2 twisting and rocking. Only the C—X stretching and the skeleton bending vibrations are strongly affected by rotational isomerism.