Photodissociation and photoisomerization pathways of the HNCN free radical

The photodissociation spectroscopy and dynamics of the HNCN free radical have been investigated by fast beam photofragment translational spectroscopy. Predissociative transitions for both the (B) over tilde (2)A'<--<(X)over tilde> (2)A " band and a higher-energy band system assigned to the (C) over tilde (2)A "<--<(X)over tilde> (2)A " band were observed. Photofragment mass distributions indicate that N-2 loss is the primary dissociation pathway. Translational energy distributions reveal a resolved vibrational structure of the N-2 fragment, suggesting that the HNCN radical first isomerizes to a cyclic HCN2 intermediate. A dissociation mechanism is proposed in which electronically excited HNCN undergoes internal conversion to the ground state, followed by isomerization to cyclic HCN2 and dissociation through a tight three-center transition state. The HNCN bond dissociation energy D-0 and heat of formation Delta H-f(0)(HNCN) were determined to be 2.80 +/-0.03 eV and 3.35 +/-0.03 eV, respectively. (C) 2001 American Institute of Physics.