Conformations of 2,4-dichloropentane and 2,4,6-trichloroheptane and a force field for poly(vinyl chloride) based upon ab initio electronic structure calculations

Abstract

The conformational properties of 2,4-dichloropentane (DCP) and 2,4,6-trichloroheptane (TCH) have been investigated in detail through ab initio electronic structure calculations as a critical step in the development of an accurate classical force field for poly(vinyl chloride). The conformational energies for DCP, obtained with a 6-31G*(*) basis set with electron correlation effects considered at the MP2 level, yield a value of Eη = -0.9 kcal/mol, denoting the energy of the racemic tt conformer relative to the meso tg conformer. This value is in excellent agreement with ca. -0.85 kcal/mol deduced previously from stereochemical epimerization experiments. The conformational energies calculated for TCH with a 6-31G*basis set at the MP2 level also yield stereochemical equilibrium results in good agreement with experiment. The analysis of TCH conformational energies in terms of a rotational isomeric state (RIS) model shows that a second-order RIS representation is not adequate due to significant Coulombic interactions involving chlorine atoms that depend on the conformations of four intervening skeletal bonds. The intermolecular interactions for model complexes of 2-chloropropane/2-chloropropane and 1,3-dichloropropane/2-chloropropane, determined at the MP2 level using a 6-31G*(*) basis set, indicate that the strength of the chlorine-methylene hydrogen interaction is comparable to that of the chlorine-methine hydrogen interaction. Moreover, the ab initio calculated energies, geometries, and dipole moments of DCP, together with the complex energies, have been employed to derive a conformational energy force field. The force field was found to reproduce well the conformational energies of TCH as determined from ab initio calculations and has been applied in simulations of poly(vinyl chloride) melt chains. © 1995 American Chemical Society.