On quasi-transferable molecular fragments

The dissociation KL → K· + L· of molecules KL in their hypothetical vibrationless state at 0 K is described and a new formula is derived for their enthalpy of formation, ΔHf∘(KL). Each individual molecule is viewed as two electroneutral parts, K∘ and L∘, joined by a bond with intrinsic energy εkl (with k ∈ K, l ∈ L). K∘ and L∘ are the precursors of the radicals K· and L·, respectively, as they occur in symmetrical molecules KK and LL. (The K∘ of a methyl group, for example, is that found in ethane.) Each part is entirely defined by a number, F(K·), that can be evaluated from the energies of the bonds found in K∘. The formula is ΔHf∘(KL)=F(K·)+F(L·)+Z(KL)-εkl-CNE+EnbKL, where Z(KL) = ZPE + HT − H0 is the zero-point plus heat-content energy of KL. The “Charge Neutralization Energy”, CNE, accounts for any possible electronic charge transfer that may have occurred between K and L, and combines the energy increase of the donor with the energy lowering on the other side, which tends to keep CNE small to the point that CNE = 0 is an acceptable approximation for many pairs {K∘,L∘}. EnbKL stands for Coulomb non-bonded interactions between the atoms of K and those of L. Detailed examples, which include the calculation of the relevant bond energies, reveal the merits and the limitations of this extremely simple, though exact, formulation of ΔHf∘(KL).