On simple, accurate calculations of atomic charges, bond properties and molecular energies.: V.: Conformational features of alkylcyclohexanes and related polycyclic hydrocarbons

Atomic charges of selected alkylcyclohexanesand related polycyclic molecules are calculated with the help of a simple approximation due to Del Re. The carbon charges duplicate those inferred from C-13 nuclear magnetic resonance shifts within 0.30 X 10(-3) electron (root-mean-square deviation). The bond-energy formulas which depend explicitly on the charges of the bond-forming atoms, and proper consideration of butane-gauche interactions enable the calculation of the energy of atomization, DeltaE(a)(*) of molecules at their potential minimum. A convenient approximation for zero-point plus heat-content energies, supported by extended basis set Hartree-Fock and density functional calculations, simplifies the evaluation of standard enthalpies of formation, well within the uncertainties of the experimental data used for comparison. (C) 2004 Elsevier B.V. All rights reserved.