Infrared spectroscopic signature of a hydroperoxyalkyl radical (center dot QOOH)

Infrared (IR) action spectroscopy is utilized to characterize a prototypical carbon-centered hydroperoxyalkyl radical (& BULL;QOOH) transiently formed in the oxidation of volatile organic compounds. The & BULL;QOOH radical formed in isobutane oxidation, 2-hydroperoxy-2-methylprop-1-yl, & BULL;CH2(CH3)(2)COOH, is generated in the laboratory by H-atom abstraction from tert-butyl hydroperoxide (TBHP). IR spectral features of jet-cooled and stabilized & BULL;QOOH radicals are observed from 2950 to 7050 cm(-1) at energies that lie below and above the transition state barrier leading to OH radical and cyclic ether products. The observed & BULL;QOOH features include overtone OH and CH stretch transitions, combination bands involving OH or CH stretch and a lower frequency mode, and fundamental OH and CH stretch transitions. Most features arise from a single vibrational transition with band contours well simulated at a rotational temperature of 10 K. In each case, the OH products resulting from unimolecular decay of vibrationally activated & BULL;QOOH are detected by UV laser-induced fluorescence. Assignments of observed & BULL;QOOH IR transitions are guided by anharmonic frequencies computed using second order vibrational perturbation theory, a 2 + 1 model that focuses on the coupling of the OH stretch with two low-frequency torsions, as well as recently predicted statistical & BULL;QOOH unimolecular decay rates that include heavy-atom tunneling. Most of the observed vibrational transitions of & BULL;QOOH are readily distinguished from those of the TBHP precursor. The distinctive IR transitions of & BULL;QOOH, including the strong fundamental OH stretch, provide a general means for detection of & BULL;QOOH under controlled laboratory and real-world conditions.