On quasi-transferable molecular fragments. Part II: A generalization of their definition

Abstract

The merits and limitations of a new formula developed for the standard enthalpy of formation of perfect gas molecules, Δ H f ∘ = ∑ K F ( K ) + ZPE + ( H T - H 0 ) - ∑ k > l ε kl - ( CNE - E nb KL ) , are illustrated by numerous examples and comparisons with experimental results. F (K ) , F ( L ) , … are invariable parameters derived from the energies of the chemical bonds found in the hypothetical electroneutral precursors K ∘, L ∘ … of the free radicals K , L , … In no way do these F parameters depend on one another. ZPE + H T − H 0 is the familiar zero-point plus heat-content energy of the molecule; ε kl is the intrinsic bond energy connecting K and L in the ground-state molecule, to be calculated from the NMR chemical shifts of atoms k ∈ K ∘ and l ∈ L ∘, … etc. CNE—the so-called ‘Charge Neutralization Energy’—takes care of the fact that K and K (as well as L and L , etc.) are usually not isoelectronic. Finally, E nb KL stands for the sum of nonbonded interactions over all pairs of groups K and L, not counting similar interactions within the fragments K and L themselves. Considerable simplifications stem from the fact that in many situations CNE - E nb KL ≃ 0 is an acceptable approximation, greatly facilitating the calculation of Δ H f ∘ near experimental accuracy, even though detailed information regarding the electronic structure of large pluriatomic groups may be lacking.