XVIII. On the Relation Between the Spatial Electron Density and the Conformational Properties of Molecular Systems

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Abstract
The increasing power of computer simulation experiments for molecular systems is discussed. These include such apparently disparate calculations as Monte Carlo simulation of solvent structure in biological systems, conformational analysis and folding of oligopeptide chains, and lattice‐energy calculations and the effect of lattice forces on molecular geometry. All such calculations have in common the requirement of an analytic expression of the energy of the system as a function of the inter‐ and intramolecular coordinates of interest.The nature and derivation of these force fields and their relation to the spatial electron densities in relevant molecular systems is reviewed. The method of the derivation of intermolecular force fields from crystal data of amides is outlined. It is shown that information which may be obtained from ab‐initio molecular orbital or X‐ray diffraction studies, population analysis, and total and difference electron density maps, may be used in conjunction with the crystal data both to verify (or test) assumptions made in the derivation and to help in the formulation of the models on which the analytical expressions for the energy are based. Finally, the relation between intramolecular forces and the electron distribution as represented in difference electron density maps is discussed. Several examples involving carbonyl compounds are presented, in which changes in the molecular geometry, intramolecular forces and vibrational frequencies on protonation and substitution are discussed both in terms of “valence or Urey—Bradley” type interactions and the changes which occur in the difference electron density maps.