The use of infrared kinetic spectroscopy to study small free radicals

Abstract

The technique of infrared kinetic spectroscopy has been used to study the reaction of the propargyl radical (C3H3) with nitric oxide (NO). The overall rate constant at the high pressure limit for the reaction was determined to be 1.09 +/- .28 * 10-11 cc molec-1 s -1 at 296 K. The reaction has an apparent negative activation energy for temperatures between 195--473 K and its rate constant is pressure dependent at 195 K and 296 K. At these temperatures the mechanism is believed to involve adduct formation. The establishment of an adduct equilibrium is not observed at higher temperatures, however. Instead the reaction is thought to form final stable products. A search was carried out for several possible reaction products including HCN, HCCN, HCNO, HCCH, HNC and H2CN. Several of these species were observed and their yields determined. These species were at most produced in 5% yield and are therefore not a major product channel. An alternative sources of these species are reactions between NO and other photolysis products. A possible explanation for the failure to observe any smaller molecular products is that the adduct does not dissociate at high temperatures and instead, rearranges to a stable isomer of C3H 3NO. Transient IR laser spectroscopy near 3100 cm-1 was used to observe the nu1 and nu13 transitions of the allyl radical, with their band origins found to lie at 3113.9779(3) cm -1 and near 3110.5 cm-1, respectively. Detailed assignment and analysis of the nu1 transition identified over 1100 transitions, and yielded improved values for DeltaK and phiK ground state centrifugal distortion constants, as well as a new set of excited state (nu1 = 1) rotational and centrifugal distortion constants, in addition to determining the band origin. Transitions involving states with quantum number up to Ka = 11 and N = 27 were identified. Strong perturbations were found in the excited state, included global perturbations of the Ka' = 3 branch, extensive perturbations in the Ka' = 2 and 4 branches, and very localized perturbations in the Ka' = 6 branch. The K a' = 9 and 10 branches appear to be the most perturbed. Possible sources for these perturbations are discussed.