VIBRATIONAL MODES, ORBITAL SYMMETRIES, AND UNIMOLECULAR REACTION PATHS

The conditions for favorable unimolecular reaction paths developed by Bader2 and one of the authors3 are used, together with a semilocalized molecular orbital model, to determine the allowed routes in the pyrolysis of cyclobutane and cyclohexene. It is found that the coplanar 2s + 2s decomposition of cyclobutane should proceed more favorably in two steps, with a predominant contribution from the Eu carbon-carbon stretching mode, than in the concerted B1g mode. On the other hand, 2s + 2s concerted decomposition by puckering in the B1u mode combined with (A1u + Eg) twisting is favored. These conclusions are in agreement with the predictions of Woodward and Hoffmann.4 The favored paths in cyclobutane are essentially determined by the harmonic terms in the potential energy surface and correspond to symmetry coordinates with low force constants. In the Diels-Alder retrogression of cyclohexene, however, the force constant for the allowed concerted motion is not much smaller than that for the forbidden process. The method should be useful for predicting the course of other unimolecular reactions. © 1969, American Chemical Society. All rights reserved.