Direct observation of non-RRKM behavior in femtosecond photophysically activated reactions

A nonstatistical behavior has been observed in the photodissociation of two acyl cyanide compounds, R-C(O)-CN, where R = methyl and tert-butyl groups. The initial activation was achieved by femtosecond two-photon excitation followed by ultrafast internal conversion.The excited-state dissociation dynamics was followed in real time by monitoring the temporal evolution of CN (X) products. The observed product appearance time constants are in picosecond time scales and are markedly similar for the two compounds studied. Our analyzes revealed that the excited-state dissociation rate does not decrease with molecular size as much as statistical rate theory predicts, suggesting a nonergodic system in which the available vibrational energy does not redistributed statistically among all modes before the bond breakage occurs.