The photodissociation spectroscopy and dynamics of the HNCN free radical ha
ve been investigated by fast beam photofragment translational spectroscopy.
Predissociative transitions for both the (B) over tilde (2)A'<--<(X)over t
ilde> (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 mas
s 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 e
lectronically excited HNCN undergoes internal conversion to the ground stat
e, followed by isomerization to cyclic HCN2 and dissociation through a tigh
t 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.