Competition between Photochemistry and Energy Transfer in Ultraviolet-Excited Diazabenzenes. 3. Photofragmentation and Collisional Quenching in Mixtures of 2-Methylpyrazine and Carbon Dioxide

The quantum yield for HCN formation via 248 and 266 nm photodissociation of methylpyrazine (C5N2H6) is determined by IR diode probing. HCN is produced at two different dissociation rates, one of which is extremely prompt. The total quantum yield is φ = 0.93 ± 0.08 for 248 nm and 0.35 ± 0.05 for 266 nm excitation. Analysis of the quenching data within the context of a gas kinetic, strong collision model allows an estimate of the rate constant for HCN production via “late” methylpyrazine photodissociation, k d1s = 6.4 × 104 s-1 and k d1s = 4.9 × 103 s-1 for 248 and 266 nm excitation, respectively. The rate constant for “prompt” dissociation is too large to be measured using this technique. After 266 nm excitation methylpyrazine lives more than an order of magnitude longer than after 248 nm excitation. Methylpyrazine also lives more than twice as long as pyrazine excited under identical conditions. Transient absorption measurements probing rotationally and translationally excited CO2 molecules produced following excitation of methylpyrazine are analyzed within the context of a kinetic scheme incorporating methylpyrazine photodissociation, as well as excitation of CO2 by both translationally hot HCN and vibrationally excited methylpyrazine. This analysis indicates that vibrationally hot methylpyrazine, which has sufficient energy to dissociate, is the source of excitation in collisions imparting large amounts of rotational and translational energy to CO2.