Interatomic potential functions for hydrocarbons from crystal data: transferability of the empirical parameters

Abstract

The transferability of (6-exp) interatomic n0n-bonded potential functions for hydrocarbons previously derived from low-temperature structures extrapolated to 0 K, sublimation heats and elasticity data of model molecular crystals, was tested by calculation of properties of other crystals not involved in the earlier optimization process. Equilibrium structures and sublimation heats were calculated, in agreement with experiment, for methane CH4, adamantane (CH2)6(CH)4, 2,2-paracyclophane (CH2)4(C6H4)2 and 3,3-paracyclophane (CH2)6(C6H4)2 crystals. The solid-solid phase-transition characteristics of adamantane were described consistently with the same potential force field. For the cubic phase of adamantane the orientationally disordered molecular arrangement, suggested by X-ray experiment, was predicted theoretically. The calculation of the structures and heats of sublimation of the same crystals were repeated with a set of potential functions derived recently [Williams, Acta Cryst. (1974). A30, 71-77] on the basis of experimental data not extrapolated to 0 K. The difference between the two approaches is discussed and illustrated by comparing the calculated results.

Crystal structures of polynuclear aromatic hydrocarbons. Classification, rationalization and prediction from molecular structure

“…Our line of thought is in agreement with recent views (Pertsin & Kitaigorodskii, 1987) on nonbonded potentials in general; the use of optimized potentials which do not include explicit Coulomb terms does not mean that electrostatic interactions are altogether neglected. The potentials we use here have been shown to fit the static properties of aromatic crystals reasonably well and their adequacy is established from previous work (Mirsky, 1976;Gavezzotti, 1982;Gavezzotti & Desiraju, 1988).…”

“…While he was able to reproduce the observed orientation of molecules in hydrocarbon crystals through the device of pairwise atom potentials and generate new packings, given the unit-cell dimensions (Kitaigorodskii & Mirskaya, 1972), he did not really address the question of crystal structure prediction from molecular structure. The atom-potential method of Kitaigorodskii was further formalized (Williams, 1967(Williams, , 1974Williams & Starr, 1977;Mirsky, 1976). Using energetic as well as geometrical criteria and incorporating the observations of Kitaigorodskii and Williams, we now modify Robertson's classification to define four basic structural types for aromatic hydrocarbons.…”

Crystal packing of hydrocarbons. Effects of molecular size, shape and stoichiometry

“…The packing potential energy (PPE) is here calculated as: Mirsky (1976) for numerical values, also reported and discussed in Gavezzotti & Simonetta (1982) and in Gavezzotti (1989)]. Although PE can be related to the crystal sublimation energy (Pertsin & Kitaigorodski, 1987;Williams & Starr, 1977;Gavezzotti, 1989) we regard it in this context as no more than a weighted sum of intermolecular separations in the crystal, or an overall index of the packing propensity for a given molecule -in other words, the one readily available link between molecular and crystal structures.…”

Structure of and energy calculations for 1,2,9,10,17,18-hexadehydro[2.2.2]paracyclophane

“…The same non-bonded potential parameters for C and H (Mirsky, 1976) were used in four separate calculations. Parameters were those of the low-temperature molecule, and the low-temperature space group, in the roomtemperature unit cell (corresponding to the minimum-energy position in Fig.…”

“…For the packing-energy (lattice-energy) calculations, the atom-atom potential method was used (Kitaigorodsky, 1973), with the same C and H non-bonded potential parameters as for [2.2]paracyclophane, [3.3]-paracyclophane and other hydrocarbon crystals (Mirsky, 1976). The C-atom coordinates in Table 2(a) (L 1) and (b) (Rm) were used; the C-H bond lengths were adjusted to 1.08A.…”