GROUND STATES OF CONJUGATED MOLECULES .11. IMPROVED TREATMENT OF HYDROCARBONS

Previous papers of this series1,4 have shown that the heats of formation of conjugated hydrocarbons, and of conjugated organic molecules containing nitrogen and/or oxygen, can be calculated with “chemical” accuracy by a semiempirical SCF-MO treatment based on the Pople5 method. The key feature of this is the use of a thermocycle6 to determine the one-electron core resonance integrals, previous workers having estimated these from spectroscopic data. In the previous papers,4 however, the bond energies of σ bonds were treated as independent parameters, rather than ascribed the values calculated in the thermocycle.6 This inconsistency has now been removed for calculations involving hydrocarbons, simultaneously reducing the number of parameters in the treatment, extending the calculations more reliably to compounds (e.g., polyenes) in which there is strong bond alternation, and increasing the accuracy of the results. The calculated heats of formation seem to agree with experiment to within the claimed limits of experimental error in almost every case, if allowance is made when necessary for ring strain. © 1969, American Chemical Society. All rights reserved.