PHOTODISSOCIATION OF A BICYCLIC AZOALKANE - TIME-RESOLVED COHERENT ANTI-STOKES-RAMAN SPECTROSCOPY STUDIES OF VAPOR-PHASE 2,3-DIAZABICYCLO[2.2.1]HEPT-2-ENE

The photodissociative mechanism of vapor-phase 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) has been studied with nanosecond-regime transient spectroscopic methods. Following excitation of the vibrationless S1 level at 338.5 nm, data from time-resolved CARS (a vibrational spectroscopy) show the appearance rate for formation of N2 to be 4 ⨯ 107 s-1. This value is significantly slower than the 5 ⨯ 108 s-1 principal component observed in S1 fluorescence decay, establishing that the dissociating state is not Sv CARS measurements on the nascent N2 photofragments reveal a vibrational distribution (84% v = 0, 12% v = 1) very similar to that observed earlier for the nitrogen formed in the stepwise photodissociation of azomethane. This result and the low level of nascent rotational excitation suggest that dissociation into N2 plus 1,3-cyclopentanediyl biradical occurs from an excited state of the diazenyl biradical that has a linear CNN bond angle. Transient CARS probing has also revealed the subsequent appearance of bicyclo[2.1.0]pentane formed by ring closure of the 1,3-cyclopentanediyl biradicals. Formation kinetics of this ring closure product shows a single first-order component with a rate coefficient of approximately 5.1 ⨯ 106 s-1. This observation implies that S1 excitation of vapor-phase DBH produces 1,3-cyclopentanediyl biradicals only in their ground triplet state. Mechanistic differences between the gas-phase photochemistries of DBH and acyclic azoalkanes arc attributed to a low-lying excited state of the diazenyl biradical that becomes accessible in DBH through the release of ring strain energy. © 1990, American Chemical Society. All rights reserved.