Rovibronic bands of the Ã←X̃ transition of CH3OO and CD3OO detected with cavity ringdown absorption near 1.2–1.4μm

Abstract

We have recorded several rovibronic bands of CH3OO and CD3OO in their A<--X transitions in the range of 1.18-1.40 microm with the cavity ringdown technique. While the electronic origins for these species have been reported previously, many newly observed rovibronic bands are described here. The experimental vibrational frequencies (given as nu in the unit cm(-1) in this paper) for the COO bending (nu8) and COO symmetric stretching (nu7) modes in the A state are 378 and 887 cm(-1) for CH3OO, and 348 and 824 cm(-1) for CD3OO, respectively. In addition, two other vibrational frequencies were observed for the A state of CD3OO, namely, nu5 (954 cm(-1)) and nu6 (971 cm(-1)). These experimental vibrational frequencies for the A state of both CH3OO and CD3OO are in good agreement with predictions from quantum-chemical calculations at the UB3LYP/aug-cc-pVTZ level. The enhanced activity of the nu5 vibrational mode in CD3OO is rationalized by mode mixing with the nu7 mode, as supported by calculations of multidimensional Franck-Condon factors. In addition, many hot bands involving the methyl torsional mode (nu12) are observed for both normal and deuterated methyl peroxy. These bands include the "typical" sequence transitions and some "atypical" ones due to the nature of the eigenvalues and eigenfunctions which are a consequence of the low, but very different, torsional barriers in the X and A states. In addition, the 12(2)2 band in CH3OO and the 12(3)3 band in CD3OO show quite different structures than the origin bands, an effect which results from tunneling splittings comparable to the rotational contour.

Examining the ground and first excited states of methyl peroxy radical with high-level coupled-cluster theory

“…Beginning with the lowest reported frequency, ν 8 , we find excellent agreement (1 cm −1 ) between our computed value (379.4 cm −1 ) and the sole reported transition, that from Miller and co-workers (378 cm −1 ) [12]. For ν 7 , there are four reported values ranging from 887 [12] to 910 cm −1 [17] with an average of 898 cm −1 . Our prediction is 910.0 cm −1 .…”

“…These include cavity ring-down investigations by Miller and co-workers (2007) [12] and Hunziker and Wendt (1976) [11]; photoelectron spectroscopy from Lineberger and co-workers (2001) [17]; and a supersonic jet-expansion study by Fu, Hu, and Bernstein (2006) [16]. Beginning with the lowest reported frequency, ν 8 , we find excellent agreement (1 cm −1 ) between our computed value (379.4 cm −1 ) and the sole reported transition, that from Miller and co-workers (378 cm −1 ) [12]. For ν 7 , there are four reported values ranging from 887 [12] to 910 cm −1 [17] with an average of 898 cm −1 .…”

“…Although this is a relatively weak transition, advances in spectroscopic techniques -including cavity ring-down [12][13][14][15], IR/vacuum-UV ionisation [16], and negative-ion photoelectron spectroscopies [17] -have fostered interest in this absorbance as a tool for monitoring peroxy radicals. The abundance of information available from vibronic IR spectra even allows for the identification of individual conformers provided that the transitions are known a priori.…”

An intercomparison of CH3O2 measurements by Fluorescence Assay by Gas Expansion and Cavity Ring–Down Spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry)

“…The CRDS technique has been used under both ambient and jet-cooled conditions to provide insight into the molecular structure of CH3O2 and more complex RO2, and to selectively measure [RO2] in the laboratory (Sharp et al, 2008;Kline and Miller, 2014;Pushkarsky et al, 2000;Farago et al, 2013;Atkinson and Spillman, 2002;Sprague et al, 2013). Good agreement 10 has been found between the experimental spectrum of CH3O2 in the range between ~7200-8600 cm -1 (~1.18-1.40 m) measured using pulsed CRDS at typically 200 mbar of N2:O2 = 1.5:1.0 and theoretical predictions (Chung et al, 2007;Sharp et al, 2008). The origin band of the A  X transition has been located at 7382.8 cm -1 and a value of 2.7 × 10 -20 cm 2 molecule -1 has been estimated for the absorption cross section at this wavenumber (Pushkarsky et al, 2000;Chung et al, 2007).…”

Ab initio calculations and Franck–Condon analysis of photoelectron spectroscopy of CH3OO− and CD3OO−

“…In the past several decades, lots of theoretical and experimental studies have been carried out to enrich the information about CH 3 OO and CD 3 OO [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. The majority of the early gas phase studies of Methyl peroxy radical have concentrated on estimating the electron affinity, the absorption cross section and its reaction kinetics [27,[32][33][34][35], and theoretical and/or experimental studies about the spectroscopy and its structural parameters concerning the molecular ion CH 3 OO À /CD 3 OO À are fairly scarce.…”