Molecular mechanics (MM4) calculations on conjugated hydrocarbons

Abstract

The MM4 force field has been extended to conjugated hydrocarbon systems. It retains most of the formalism and computational schemes that were present in MM3. Several cross-terms have been added in MM4 that were not present in MM3, mainly to improve vibrational frequencies, but also to improve structures and energies. Additional bond order dependence equations have been included. About 80 molecules have been examined, some in multiple conformations, and others with more complete experimental profiles. Conformational energy differences/barriers are generally fit to within 0.5 kcal/mol unless they are very large. The MM4 method for calculating heats of formation has been modified to include vibrational as well as steric energy contributions. The heats of formation for 35 alkenes and 57 conjugated hydrocarbons were determined to have an overall root mean square (rms) deviation of 0.68 kcal/mol from experiment (0.47 kcal/mol weighted rms). The vibrational frequency rms error for eight conjugated hydrocarbons is 31 cm1. Geometries are fit for the most part to within the following ranges: 0.004 Å for bond lengths, 1° for bond angles, 4° for torsion angles, and 0.5% for moments of inertia (r2). © 1996 by John Wiley & Sons, Inc.