New Empirical Correlations between Molecular Structure and Energies in Hydrocarbons and Their Stereochemical Interpretations

Abstract

Several new empirical correlations between molecular structure and energies in hydrocarbons are established based on the fundamental principles of structural chemistry. Thus, on the basis of the postulate of the constancy of C-C bonds (or covalent binding energies in general) alone, a simple but highly improved bond energy system can be constructed empirically, using, for example, four parameters and few structural corrections for saturated hydrocarbons (Method 1).On further investigation of the structural relations of molecular energy, the principle of the constancy of isomeric variations in long-chain paraffins is discovered, and the concept of the unstabilization energies is set up, which denotes all the intramolecular interactions between bonds or non-bonded atoms in empirical terms such that the bond-to-bond unstabilization parameters Rn's are expressed by Rn=20.48/4nKcal/mole where (n-1) is the number of intervening C-C bonds, and which enables us to calculate most satisfactorily (mean and maximum errors ±0.2∼0.3 and ±0.5∼0.6Kcal/mole, respectively) the isomerization energies of complex paraffin and olefin hydrocarbons with four or six structural parameters (for paraffins) and two additional parameters (for olefins) (Method 2).A consideration of the contribution of kinetic energy terms makes it possible to analyse the unstabilization factors into more detailed structural terms and to estimate the structural contributions of zero-point energy with excellent accuracy, using three parameters (for paraffins). It is now possible to evaluate the covalent binding energies of C-H and C-C bonds on the basis of the heat of atomization of methane and the methylene increment in long-chain paraffins as standards and to construct a general function of exact additivity for the molecular energies of all paraffin hydrocarbons at any temperature, containing all the structural contributions in terms of 9 parameters including three temperature-dependent ones (Method 3). At this stage a correlation between the molecular energy and the dissociation energy can be established by using the structural parameters of Method 3 with a single plausible assumption which is derived from any one of observed dissociation data, in order to estimate the extent of the contributions of the unstabilization energies and zero-point energies to the dissociation process. Thus, taking the C-H dissociation in methane as the standard, the C-H dissociation energies of various paraffin molecules are calculated in sufficient agreement with the observed data. The general applicability of the present method to molecular problems is also indicated.