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

Article abstract of DOI:10.1021/ja00181a013

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 S₁. 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 are attributed to a low-lying excited state of the diazenyl biradical that becomes accessible in DBH through the release of ring strain energy.