High-resolution infrared spectra of jet-cooled allyl radical (CH2–CH–CH2): ν2, ν3, and ν14 C–H stretch vibrations

Abstract

The high-resolution infrared spectrum of jet-cooled allyl radical (CH2–CH–CH2) in the region 3010–3045 cm−1 has been recorded in pulsed slit jet expansions using both modulated discharge and excimer laser flash photolysis methods to produce the radicals. Over 400 transitions are observed and assigned to the ν2, ν3, and ν14 C–H stretch vibrations. The band origins have been determined to be 3033.8745(6), 3023.4605(6), and 3020.32(1) cm−1, respectively. Spectral analysis indicates that all three upper states are perturbed, with the perturbations in the ν14 upper state so pervasive as to make least-squares analysis difficult. The gas phase frequencies reveal a somewhat unusual, in comparison with the other bands, matrix shift to the blue for ν2 of 18 cm−1.

Infrared laser spectroscopy of the helium-solvated allyl and allyl peroxy radicals

“…The bands centered at 3020.64, 3023.25, and 3034.28 cm −1 are very close to the origins of gas-phase bands previously assigned to the ν 14 (b 2 ), ν 3 (a 1 ), and ν 2 (a 1 ) fundamental bands associated with the excitation of the out-of-phase symmetric CH 2 , lone CH, and in-phase symmetric CH 2 stretching motions, respectively. 14 The symmetry types of these three bands in the droplet spectrum are completely consistent with an assignment that follows the frequency ordering determined previously from the gas-phase study, 14 and this again implies that the allyl radical band origins are shifted by less than 1 cm −1 upon helium-solvation. In addition to the five bands assigned to fundamentals in the gas-phase study, the other four bands in Figs.…”

“…These, however, were not assigned definitively to specific interactions, although it was noted that the ν 1 (a 1 ) and ν 13 (b 2 ) levels can interact via a c-axis Coriolis perturbation. 14,15,36 As a possible alternative assignment of the four previously unreported bands, we have considered the impurities that may arise due to allyl radical recombination reactions within the pyrolysis source. For example, under hightemperature conditions, Ellison and co-workers 13 found that the pyrolysis of allyl iodide produced weak features in the solid-Ar matrix IR spectrum that could be assigned to 1,5-hexadiene, allene, and propene.…”

“…13 High resolution jetcooled gas-phase IR spectra have been reported for the five C-H stretch fundamentals and the CH 2 symmetric wagging mode, providing accurate values for vibrational band origins and rotational constants. [14][15][16] From the C-H stretch gas-phase spectra, it was determined that the upper states are all perturbed, although no definitive explanation was provided as to the nature of the perturbing states. Curl and co-workers 14 noted that some of these perturbations are so global that the determination of rotational constants via least-squares fitting was significantly affected.…”

“…[14][15][16] From the C-H stretch gas-phase spectra, it was determined that the upper states are all perturbed, although no definitive explanation was provided as to the nature of the perturbing states. Curl and co-workers 14 noted that some of these perturbations are so global that the determination of rotational constants via least-squares fitting was significantly affected. The low temperature of helium droplets allows for a reinvestigation of the C-H stretch region with sufficient resolution and sensitivity to disentangle the spectral complexity that leads to the perturbations observed in the gas-phase spectra.…”

Observation of hot bands in the infrared spectrum of H2CO

“…The apparatus and procedures involved have been described previously in some detail [40,41]. The molecules were formed by the excimer flash photolysis (248 nm, 120 mJ) of methyl nitrite (CH 3 ONO) in the nozzle of a pulsed-valve slit-jet expansion.…”