Polar bonds and polarizability in conformational energy calculations: Application of a polarization model to alkyl halides

Abstract

Conformational energy models traditionally attempt to simulate molecular properties by means of a total molecular energy assembled from empirically determined transferable energy functions that include bond stretching, bending, and torsion as well as nonbonded interactions. In extending the model to polar molecules through the addition of bond-centered dipoles or atom-centered charges, questions arise concerning inductive effects on assumed bond moments and the effect of intervening atoms on the electrostatic interaction between polar groups. We investigate here the use of a polarization–mutual induction model in representing these effects. Each bond is assumed to carry an intrinsic dipole moment and a polarization center. The induced moment at each polarization center is computed from the electric field arising from all sources (the other permanent and induced moments in the molecule and an external field). The model is parameterized for calculations on haloalkanes by determining intrinsic C–X bond monents, C–X, C–H, and C–C bond polarizabilities as well as stretching and bending constants involving halo atoms from experimental data. Other C, H parameters are taken from a previous hydrocarbon force field. It is found that molecular polarizabilities and dipole moments are reproduced very well including moments in molecules where inductive effects render a fixed bond moment scheme inaccurate. Conformational energies are generally satisfactorily accounted for. Electrostatic contributions to the latter are found to be important in determining conformer stabilities.