Kinetics of C–C and C–H Bond Cleavage in Phenyl Alkane Radical Cations Generated by Photoinduced Electron Transfer

2014-11-26T00:00:00Z (GMT) by Douglas Cyr Paritosh Das
Employing nanosecond laser flash photolysis, we determined the relative importance of two fragmentation modes, namely, C–C bond cleavage and deprotonation, for the radical cation of 1,1,2,2-tetraphenylethane photogenerated by electron transfer to cyanoaromatic singlet excited states in acetonitrile at room temperature. Analysis of the kinetic data for this phenyl alkane suggests that the C–C bond cleavage dominates over the deprotonation by a ratio of about 2:1. In addition, the deprotonation kinetics of diphenylmethane, 1,1-diphenylethane, triphenylmethane, and several phenyl-substituted alcohols have been investigated. To aid identification and characterization, experiments based on two laser pulses in tandem (308 and 337.1 nm) were performed to probe fluorescence and photochemistry of the transient radicals formed as products of radical ion fragmentation. The first-order rate constants for growth of transient absorptions due to fragmentation-derived radicals were measured to be ≥1 × 106 s–1. Activation parameters, with activation enthalpies in the range 10–18 kJ/mol and activation entropies between −60 and −91 J/(mol.K), are also reported for fragmentation kinetics of radical cations of several systems under study.