Molecular mechanics and molecular shape. Part 1. van der Waals descriptors of simple molecules

Abstract

A computer program was written to calculate and display graphically the van der Waals volume (V_w) and surface area (S_w) of molecules. Using van der Waals radii of C, H, Cl, and Br, and geometries optimised by the force-field method, V_w and S_w were obtained for about a hundred simple molecules: alkanes, cycloalkanes, and chlorinated and brominated derivatives thereof. S_w is more sensitive than V_w to stereochemical and positional isomerism. It was found that V_w correlates with the molar refraction and with the constant b in the equation of state, and can be used to estimate the molar volume. For very similar species, S_w correlates with the free volume that encapsulates the molecule in the bulk, so that a combined usage of V_w and S_w leads to the difference in molar volume between conformers. Numbers obtained by the procedure bear upon the ‘conformational rule’ of organic stereochemistry, and suggest a novel interpretation of the torsional term in the molecular-mechanical force field.