MOLECULAR MECHANICS (MM4) CALCULATIONS ON CONJUGATED HYDROCARBONS

The MM4 force field has been extended to conjugated hydrocarbon system s. It retains most of the formalism and computational schemes that wer e present in MM3. Several cross-terms have been added in MM4 that were not present in MM3, mainly to improve vibrational frequencies, but al so to improve structures and energies. Additional bond order dependenc e equations have been included. About 80 molecules have been examined, some in multiple conformations, and others with more complete experim ental profiles. Conformational energy differences/barriers are general ly fit to within 0.5 kcal/mol unless they are very large. The MM4 meth od for calculating heats of formation has been modified to include vib rational as well as steric energy contributions. The heats of formatio n for 35 alkenes and 57 conjugated hydrocarbons were determined to hav e an overall root mean square (rms) deviation of 0.68 kcal/mol from ex periment (0.47 kcal/mol weighted rms). The vibrational frequency rms e rror for eight conjugated hydrocarbons is 31 cm(-1). Geometries are fi t for the most part to within the following ranges: 0.004 Angstrom for bond lengths, 1 degrees for bond angles, 4 degrees for torsion angles , and 0.5% for moments of inertia (r(z)). (C) 1996 by John Wiley & Son s, Inc.