Nonbonded Potential Parameters Derived from Crystalline Hydrocarbons

Abstract

The previously described least-squares derivation of nonbonded potential parameters from crystalline aromatic hydrocarbons was extended to include nonaromatic hydrocarbons. Further evidence was obtained that no specially large energy effects are present in the aromatic crystal structures with their π-electron systems. A better separation of the nonbonded energy into C···C, C···H, and H···H components was obtained when the observational equations for the aromatic and nonaromatic structures were combined. The potentials obtained from the combined observational equations gave better fits to the nonaromatics than to the aromatics. Evidence is presented favoring an H–C–H angle of less than 106° in crystalline n-pentane and n-hexane. The parallel packing of molecular chains in the n-hexane crystal and the nonparallel packing in the n-pentane crystal were reproduced by a steepest descent minimization of the lattice energy using the observed lattice constants.

Temperature dependence of thermal motion in crystalline naphthalene

“…The differences among the three sets of calculated results are partly due to differences in the potential functions; Pawley used Kitaigorodskii potentials while the other two groups used the somewhat softer Williams potentials [details of FGS's calculations are the same as given in Filippini, Gramaccioli, Simonetta & Suffritti (1973) except that the summation limit was increased from 5.5 to 15 A, and the potentials were Williams's (1967) set IV]. Unfortunately, VCD do not make it clear which of the two sets of Williams IV potentials (Williams, 1966(Williams, , 1967 Williams's (1967) set IV potentials but reducing the summation limit from 15 to 5.5 A give results essentially identical to those of VCD. It is interesting that the agreement with experiment worsens as the cut-off limit is increased, presumably because in this parametrization the effects of the long-range, electrostatic interactions are not included explicitly but are absorbed into the other parameters.…”

High-pressure phases of cyclohexane-d 12

“…7. The minimum potential energy structure for phase IV, obtained using the molecular geometry of Trew et al (1990) combined with the potential parameters of Williams (1967). calculation. Those peaks not observed at 250 K (and therefore absent from Table 1) which are apparent in the 175 K data, are the (131) and (131) In a manner similar to that described for phase III, we have performed energy-minimization calculations for the phase IV structure in an attempt to locate a molecular orientation sufficiently close to the real configuration to permit Rietveld refinement of our powder data.…”

Lattice-dynamical evaluation of temperature factors in non-rigid molecular crystals: a first application to aromatic hydrocarbons

“…For van der Waals interactions we used the Williams IVa functions (Williams, 1967), with a maximum a u packing distance of 5.5 A and a C-H distance b, standardized to 1.09 A, just as in GFS: the use of a, b~ semiempirical potentials of this kind has proved to be b, particularly valuable for reproducing the experimental a, au dispersion curves for naphthalene Pawley et al, 1980), and of anthracene-d10 'q (Dorner et al, 1982).…”