On quasi-tranferable molecular fragments. Part III: Parameters for use in pluri-fragment models

Abstract

A simple formula for the standard enthalpy of perfect-gas molecules, $\Delta H_f^{\circ }={\sum }_{K}F\left(K\right)+\mathrm{ZPE}+H_T-H_0-{\sum }_{k<l}{\epsilon }_{\mathrm{kl}}-\left(\mathrm{CNE}-E_{\mathrm{nb}}^{\mathrm{KL}}\right)$, is illustrated by novel applications and over 100 examples and comparisons with experimental results. The propounded model views molecules as constructs of chemical groups, K, L, … , etc. (such as CH₃, COOH, for example) characterized once and for all, independently of their belonging to one or another host, by fixed numbers $F\left(K\right)\text{,}F\left(L\right)\text{,}\dots$, etc. ZPE + H_T − H₀ is the familiar sum of zero-point + heat-content energy, ε_kl is the intrinsic energy (at 0 K) of the bond connecting K and L, CNE (for Charge Neutralization Energy) takes care of the fact that K, L, … , are usually not electroneutral in the host molecule, and $E_{\mathrm{nb}}^{\mathrm{KL}}$ measures nonbonded interactions summed over all pairs of groups K and L. New parameters, $F\left[\mathrm{CH}\right(X\left)\right]$ with X = CH₃, NH₂, OH, CHO, COOH, Cl, Br and SH, are described and an amazingly simple formula for carbon–hydrogen bonds, giving ${\epsilon }_{\mathrm{CH}}^{\ast }={\epsilon }_{\mathrm{CH}}+\mathrm{CNE}$, turns out to be most useful any time a fragment CH(X) is bonded to one hydrogen atom at least.