In this research we consider some problems which arise in the application of ab-initio methods on isodesmic reactions to obtain enthalpies of formation. The fraction of the correlation energy which is not conserved in isodesmic reactions is evaluated at second order in perturbation theory. We found that this value may be well approximated with interorbital pair contributions calculated empirically as functionals of Hartree-Fock interorbital Coulomb interaction energies. The procedure proposed in this paper is based on a localized orbital picture and uses a few empirical parameters which are calculated from an orbital invariant formulation of Moller-Plesset perturbation theory. The final results agree well with experimental enthalpies of formation, the average absolute error being lower than 1.5 kcal/mol.