RATES OF SPIN-FORBIDDEN ORGANIC-REACTIONS .3. EXTRUSION OF NITROGEN FROM METHYLENEPYRAZOLINE

The stereochemistry of the extrusion of molecular nitrogen from methylenepyrazoline is examined, with particular attention to the question whether the spin-forbidden production of triplet trimethylenemethane can be competitive in rate with the spin-conservative production of some singlet state of trimethylenemethane. The spin-extended orbital correspondence analysis in maximum symmetry (OCAMS) indicates that either a conrotation or a disrotation can produce triplet trimethylene directly upon N2 extrusion. Energetic considerations suggest that a conrotation enhances the probability of singlet-triplet crossover, while disrotation favors the formation of zwitterionic singlet products. OCAMS predicts substantial spin polarization in the product triplet trimethylenemethane. The qualitativeform of the potential energy surfaces is sketched by semiempirical MO computation, and a modified RRKM technique is employed to estimate the rate of triplet production relative to the rate of singlet production. Triplet production is apparently favored in spite of the spin flip required. Experimental data require a mixture of singlet and triplet production if reported energies and enthalpies are to be reproduced. The possibility that spin-forbidden processes may occur yet escape detection in routine tests is discussed. © 1979, American Chemical Society. All rights reserved.