Theoretical Studies in the Norbornadiene-Quadricyclane System

Ab initio molecular orbital theory is used to investigate interesting aspects of the chemistry of norbornadiene (N) and quadricyclane (Q). Effects of polarization functions and electron correlation are included in these calculations. The optimized structures of N and Q are found to be in excellent agreement with those obtained from electron diffraction experiments. N is calculated to be 27 kcal/mol more stable than Q, slightly higher than the experimental thermochemical measurements. Detailed structural information which is not available experimentally is provided for the radical cations of N and Q (N+ and Q+). N+ is calculated to be 11 kcal/mol more stable than Q+, in good agreement with the experimental value. The nature of the triplet state is investigated in detail. The structure of the triplet state is found to be distorted to a symmetry (Cs) lower than that of the parent compounds (C2v). An analysis of the orbital correlations for such a distorted triplet state reveals that conversion to Q on the singlet surface is more favorable than conversion to N. This explains the photochemical observations that reveal that the triplet state converts with a high yield (≃90%) to Q. Relative spin densities are calculated for both radical cations and are consistent with the measured nuclear spin polarization spectra.