A search for mode-selective chemistry: The unimolecular dissociation of t-butyl hydroperoxide induced by vibrational overtone excitation

Abstract

The use of optoacoustic spectroscopy permits both the monitoring of the overtone excitation of t-butylhydroperoxide (t-BuOOH) and the in situ detection of the resulting reaction product t-butanol (t-BuOH). The sample is contained in a reaction cell, equipped with a microphone, in which all surfaces have been specially passivated. The cell is placed inside the cavity of a dye laser tuned to excite the 5-0 O-H stretch of the t-BuOOH at 619.0 nm. The dissociation process yields directly ·OH and t-BuO·, and the latter readily abstracts a hydrogen atom from a parent molecule to form t-butanol [t-BuOH). The appearance rate of t-BuOH is obtained by ratioing the area under the 5-0 O-H stretch of t-BuOH to that of a combination band of t-BuOOH. At low pressures, below 40 Torr, a plot of the reciprocal of the t-BuOH appearance rate versus total pressure shows near linear behavior. This linearlity can be well described by a statistical model (RRKM) when careful averaging of the dissociation rate over the thermal energy distribution of the photoactivated molecules is included. At pressures above 40 Torr, a marked deviation from linearity appears. This deviation is fit to a kinetic model in which the dissociation rate of an energy nonrandomized molecule competes with the rate of intramolecular energy relaxation. This places a lower bound of ≥ 5.0×1011 s -1 on the rate of energy randomization. A discussion of this model in the context of other possible kinetic schemes as well as other photoactivated and chemically activated systems is presented. © 1982 American Institute of Physics.