Side-Chain Effects on the Electronic Relaxation of Radicals followed by Time-Resolved Pump−Probe Spectroscopy: 2,3-Dimethylbut-2-yl vs tert-Butyl

The excited-state lifetime of the 2,3-dimethylbut-2-yl (DMB) radical, a hexyl isomer, upon electronic excitation into the 3p Rydberg state at 265 nm, is measured by femtosecond time-resolved photoionization. It is shown that the 3p state deactivates in a two-step process, which is well described by two time constants of 25 and 400 fs. The results are compared to tert-butyl (t-C4H9), another tertiary radical investigated before. Time-dependent DFT calculations confirm the earlier suggestion that curve crossings along the C−C coordinate play an important role in the excited-state deactivation.