Intermolecular Potential Variation due to Intramolecular Rotation in Molecules

Abstract

The changes in charge and polarization densities of ethane, methanol and formamide upon internal rotation is studied using SCF-HF calculations. For formamide the charge density changes with dihedral angle are seen in the rapidly varying molecular dipole moment, whereas these changes are only moderate in the methanol molecule. In ethane the quadrupole moment variation is about 15%. The barriers to rotation are found to be 3.47 kcal/mole for ethane, 0.84 kcal/mole for methanol and 14.94 kcal/mol for formamide. The total molecular polarizability, which is estimated using perturbation theory, is varying slowly with rotation angle, but the individual atomic components change rapidly for all three molecules investigated. The calculations indicate that solvent effects upon intramolecular configuration are reflected in variations of intermolecular interactions. These implications are discussed in connection with modelling internally flexible molecules for use in molecular dynamics and Monte Carlo simulations. Similarly, water-water interactions are affected by small vibrations which alter the equilibrium charge density and polarizability of the individual molecule.

Key Words:

• Internal rotation
• multipole moment variations
• intermolecular interactions