Kinetics and dynamics of azoalkane photofragmentation: Direct studies using transient CARS spectroscopy
Burton, Katherine Ann
Weisman, R. Bruce
Doctor of Philosophy
A fundamental and long-standing question surrounds the mechanism of primary bond cleavage in azoalkanes: do the two C-N bonds break in a synchronous or a stepwise manner? When a vapor phase azoalkane absorbs near ultraviolet light, it dissociates into two alkyl radicals and nitrogen. Transient CARS spectroscopy was used here as a time-resolved probe of the photoproducts formed from azoalkanes excited at 355 nm. In a detailed reinvestigation, azomethane was found to dissociate in a stepwise process involving a methyldiazenyl radical intermediate. The diazenyl intermediate was formed in less than 1 ns and lived for 5.3 $\pm$ 1 ns before fragmenting into a methyl radical and nitrogen. Kinetic studies on azoisopropane (AIP) also gave evidence for stepwise photodissociation with a similar diazenyl lifetime. The first methyl radical formed in azomethane photodissociation was found to have 0 to 4 quanta of $\nu\sb2$ excitation, whereas the second methyl radical was predominantly vibrationally unexcited. The nascent rotational temperature of N$\sb2$ from azomethane was found to be 2500 K, and its vibrational distribution was confirmed to be 84% in v = 0 and 16% in v = 1. These results seem consistent with predictions based on the transition state structure computed for methyldiazenyl dissociation. Internal energy distributions were also measured for the nitrogen formed from 3-(methylazo)-3-methyl-butene (MAMB), which was previously shown to undergo stepwise dissociation through a methyldiazenyl intermediate. The rotational and vibrational energy distribution from MAMB were almost identical to those from azomethane, consistent with a common dissociation mechanism. AIP also gave similar nitrogen rotational and vibrational distributions, suggesting that the dissociative transition state of isopropyldiazenyl is similar to that of methyldiazenyl. In summary, direct kinetic measurements have demonstrated stepwise gas phase photodissociation in acyclic azoalkanes. Related measurements of product internal energy distributions should form the basis for a detailed dynamical understanding.