AN AB INITIO STUDY OF INTERNAL ROTATION

Ethane, propane, chloroethane, n-butane, 1-chloropropane, 1,2-dichloroethane and the meso and dl isomers of 2,3-dichlorobutane (DCBs) were used as model compounds in an ab initio study of single internal rotations around the central carbon-carbon bonds. Hartree-Fock calculations using the 6-31G* basis set were performed for rotation angles at 30-degrees intervals (phi(i)) with relaxation of all other molecular coordinates. The stationary points on the potential energy profiles were determined separately with full geometry optimization. The resulting potential energies V(phi(i)) and derivatives V'(phi(i)) were then fitted to Fourier-series expansions to different levels of accuracy. potential energy functions interpolated from only the energies were found to be reasonably accurate representations; however, adding derivatives to the input is recommended for enhancing the accuracy of the fitted functions. Numerous theoretical techniques for assessing the accuracy of a potential function are discussed and demonstrated. The ab initio energy data were subsequently employed to test the vicinal-pair-energy method, a simple approach for simulating torsional energies of large molecules from those of smaller ones. The approach is found to be moderately successful in reproducing the HF/6-31G* optimized-rotor potential curves for the DCBs.