Increase in Strain Energy during Conversion of [4.4.4.5]Fenestrane to [4.4.4.4]Fenestrane: a Method for Estimating the Heats of Formation of Hydrocarbons and Their Derivatives from Ab Initio Energies.

Our interest in fenestranes led us to wonder how large the change in strain energy with changes in the ring size might be. This led us to consider if satisfactory estimates of heat of formation could be easily derived from ab initio calculated energies. We started by examining 21 hydrocarbons having well-determined heats of formation via calculations of their enthalpies using W1BD, G4, CBS-APNO, CBS-QB3, and M062X. Making use of the molecular formula and an initial estimate of the energy of a hydrogen atom and of a carbon atom, along with the ab initio enthalpy, we calculated their heats of formation. The carbon energy parameter was adjusted slightly for each model to get the best fit between experiment and our estimate. This approach worked out very well giving an root mean square error of about 0.4 kcal/mol for most methods. This approach was also extended to a larger group of hydrocarbons, and the results were found to be generally useful. The extension to O and N compounds also was examined giving equally good results for the O-containing compounds but somewhat less satisfactory with N-containing compounds. In use, the procedure requires only the molecular formula, the calculated energy, and C, H, O, and N atomic constants given in the tables.