On quasi-tranferable molecular fragments. Part IV. Bond energies and bond dissociation: Novel approaches and comparisons with classical results

Abstract

Results given by the new formula for the standard perfect-gas enthalpy of formation, $\Delta H_{\text{f}}^{\circ }={\sum }_{\text{K}}F\left(\text{K}\right)+\text{ZPE}+H_T-H_0-{\sum }_{k<l}{\epsilon }_{\mathrm{kl}}-(\text{CNE}-E_{\text{nb}})$, are compared with experiment. $F\left(\text{K}\right)\text{,}F\left(\text{L}\right)\text{,}\dots$ are fixed parameters of chemical groups K, L, etc. and ${\epsilon }_{\mathrm{kl}}$ is the intrinsic energy of the link between K and L. $Z=\text{ZPE}+H_T-H_0$ is the familiar sum of zero-point + heat-content energies and CNE accounts for the fact that the fragments K, L, etc. are not individually electroneutral in their host molecule. $E_{\text{nb}}$ stands for nonbonded interactions between the fragments. The reduction of the 3-fragment formula applicable to molecules written K-[CH(X)]-L to get its equivalent for the 2-fragment form [CHK(X)]-L reveals a most useful relationship between the functions $F$[CH(X)] and $F$[${\text{CH}}_2$(X)]: the latter is usually easy to obtain, but it is $F$[CH(X)] that is actually required in the 3-fragment problem K-[CH(X)]-L. Fragments of this form, already known for $\text{X}={\text{CH}}_3$, were successfully tested for $\text{X}={\text{C}}_2{\text{H}}_5\text{,}n-{\text{C}}_3{\text{H}}_7\text{,}{\text{C}}_6{\text{H}}_5\text{,CH}={\text{CH}}_2\text{and}\text{C}\left({\text{CH}}_3\right)\text{<mglyph></mglyph>}{\text{CH}}_2$; moreover, fragments of the form C(X,Y) with $\text{X}={\text{CH}}_3$ and $\text{Y}={\text{CH}}_3$, ${\text{C}}_2{\text{H}}_5$, $n\text{-}{\text{C}}_3{\text{H}}_7$ or were also shown to satisfy the master equation for$\Delta H_{\text{f}}^{\circ }$. The formula derived for CX bonds, ${\epsilon }_{\text{CX}}+\text{CNE}=F\left[\text{X}\right]+F\left[\text{CH(X)}\right]-F\left[\text{CHK(X)}\right]$ (where X = H, F, Cl, Br and I), has revealed its merits in tests made with K = H or ${\text{CH}}_3$. Finally, a brief inroad is made in the world of dissociation energies, also exemplifying the calculation of “difficult” bonds, like peroxydic O–O links or N–N bonds in hydrazines, which foreshadows new routes in quantitative bond-energy theory.