Observation of Fluorescence Excitation Spectra of tert-Pentoxy and 3-Pentoxy Radicals

Fluorescence excitation spectra of tert-pentoxy [CH3CH2C(O·)(CH3)2] and 3-pentoxy radicals were observed for the first time. The radicals were produced by laser photolysis of the corresponding pentyl nitrites at 355 nm and studied in the wavelength range 345−400 nm. For tert-pentoxy, 12 vibronic bands were labeled in three progressions. One dominant progression corresponds to the C−O stretch mode with initial vibrational interval 551 ± 10 cm-1. Two other unknown mode progressions have vibrational intervals of 587 ± 10 and 631 ± 10 cm-1. The transition origin was tentatively assigned at 25 491 ± 10 cm-1 (392.3 ± 0.1 nm). For 3-pentoxy, 15 vibronic bands were labeled in three progressions. The vibrational intervals in each progression are of consistent anharmonic tendency. On the basis of our assignments of the C−O stretch progression, the transition origin T 0, the C−O stretching vibration frequency νC - O, and the anharmonic constant χC - O are derived to be 26 437 ± 10, 578 ± 6, and −17 ± 2 cm-1, respectively. The initial vibrational intervals of two other unknown modes are 596 ± 10 and 590 ± 10 cm-1. The fluorescence lifetime of 3-pentoxy was determined to be about 150 ns. The short extent of the laser-induced fluorescence (LIF) excitation spectra of many C2−C5 alkoxy radicals is tentatively explained as arising from the opening of a predissociation channel at relatively low excitation energies. Results of efforts to obtain LIF excitation spectra for 10 additional large alkoxy radicals are reported.