Attractive‐dominant and repulsive‐dominant hydrocarbon reactions

The difference between the expectation values of the total electronic kinetic energy operator (Δ E K ), and the operators accounting for the Coulombic interactions between the electrons and nuclei (Δ V en ), between all pairs of electrons (Δ V ee ), and between all pairs of nuclei (Δ V nn ) for the product and reactant species in a wide variety of hydrocarbon reactions are calculated using single determinant basis set data reported in the literature. Following Allen, their contributions to Δ E T , the difference between the corresponding total molecular energies and thus the reaction heat, are grouped together as a repulsion energy term, Δ E rep = Δ E K + Δ V ee + Δ V nn , and an attraction energy term Δ E attr = Δ V en . For all but 2 of the 71 individual reactions considered in this paper, the experimental reaction heat at 0°K corrected for zero‐point energy contributions, (Δ H   0 0 ) zpe , is the result of near compensation between far larger Δ E rep and Δ E attr terms, in sharp contrast to the much smaller Δ E rep and Δ E attr terms which are characteristic of many molecular rotation processes. By matching the sign of (Δ H   0 0 ) zpe with that of Δ E rep or Δ E attr , as the case may be, the reactions are classified as attractive‐dominant or repulsive‐dominant (46 in the former class and 23 in the latter), a property which is independent of the direction in which the reaction is written. The sign and magnitude of Δ V ee , Δ V nn , and Δ V en and reaction category are discussed in relation to the various kinds of structural change involved in going from reactants to products. For the vast majority of reactions, the numerical relationship Δ V ee ≈ Δ V nn has been found to hold to within a few percent.