Barriers to Decomposition of Ground and Excited State 1-Pyrazolines

Article abstract of DOI:10.1021/ja00370a041

3,3,5,5-Tetramethyl-4-isopropylidene-1-pyrazoline (2), 3,3,5,5-tetramethyl-1-pyrazoline (3), and 3-acetoxy-3,5,5-trimethyl-1-pyrazoline (4) exhibit barrers to triplet sensitized deazatization of about 7 kcal mol^-1.Because these barriers are in general higher than those found for direct irradiation, loss of nitrogen from the singlet state is more efficient than from the triplet state.Quantum yields and singlet liftimes determined here provide rate constants and partitioning ratios for both excited states; however, it is not yet clear whether radiationless decay of the singlet state proceeds via the triplet.The heat of formation of gaseous 2 has been measured as 18.2 +/- 0.6 kcal mol^-1.This figure is used to show that the high thermal stability of 2 is due to severe steric interactions which develop in the transition state for stepwise deazatization.