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Cavity Ringdown Spectroscopy of the Hydroxy-Methyl-Peroxy Radical

Sprague, Matthew K. and Mertens, Laura A. and Widgren, Heather N. and Okumura, Mitchio and Sander, Stanley P. and McCoy, Anne B. (2013) Cavity Ringdown Spectroscopy of the Hydroxy-Methyl-Peroxy Radical. Journal of Physical Chemistry A, 117 (39). pp. 10006-10017. ISSN 1089-5639. doi:10.1021/jp400390y.

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We report vibrational and electronic spectra of the hydroxy-methyl-peroxy radical (HOCH_2OO^• or HMP), which was formed as the primary product of the reaction of the hydroperoxy radical, HO_2^•, and formaldehyde, HCHO. The ν_1 vibrational (OH stretch) spectrum and the à ← X electronic spectrum of HMP were detected by infrared cavity ringdown spectroscopy (IR-CRDS), and assignments were verified with density functional calculations. The HMP radical was generated in reactions of HCHO with HO_2^•. Free radical reactions were initiated by pulsed laser photolysis (PLP) of Cl_2 in the presence of HCHO and O_2 in a flow reactor at 300–330 Torr and 295 K. IR-CRDS spectra were measured in mid-IR and near-IR regions over the ranges 3525–3700 cm^(–1) (ν_1) and 7250–7800 cm^(–1) (à ← X) respectively, at a delay time 100 μs after photolysis. The ν_1 spectrum had an origin at 3622 cm^(–1) and exhibited partially resolved P- and R-branch contours and a small Q-branch. At these short delay times, spectral interference from HOOH and HCOOH was minimal and could be subtracted. From B3LYP/6-31+G(d,p) calculations, we found that the anharmonic vibrational frequency and band contour predicted for the lowest energy conformer, HMP-A, were in good agreement with the observed spectrum. In the near-IR, we observed four well spaced vibronic bands, each with partially resolved rotational contours. We assigned the apparent origin of the à ← X electronic spectrum of HMP at 7389 cm^(–1) and two bands to the blue to a progression in ν15′, the lowest torsional mode of the à state (ν_(15′) = 171 cm^(–1)). The band furthest to the red was assigned as a hot band in ν^(15″), leading to a ground state torsional frequency of (ν^(15″) = 122 cm^(–1)). We simulated the spectrum using second order vibrational perturbation theory (VPT2) with B3LYP/6-31+G(d,p) calculations at the minimum energy geometries of the HMP-A conformer on the X and à states. The predictions of the electronic origin frequency, torsional frequencies, anharmonicities, and rotational band contours matched the observed spectrum. We investigated the torsional modes more explicitly by computing potential energy surfaces of HMP as a function of the two dihedral angles τ_(HOCO) and τ_(OOCO). Wave functions and energy levels were calculated on the basis of this potential surface; these results were used to calculate the Franck–Condon factors, which reproduced the vibronic band intensities in the observed electronic spectrum. The transitions that we observed all involved states with wave functions localized on the minimum energy conformer, HMP-A. Our calculations indicated that the observed near-IR spectrum was that of the lowest energy X state conformer HMP-A, but that this conformer is not the lowest energy conformer in the à state, which remains unobserved. We estimated that the energy of this lowest conformer (HMP-B) of the à state is E_0 (Ã, HMP-B) ≈ 7200 cm^(–1), on the basis of the energy difference E_0(HMP-B) – E_0(HMP-A) on the à state computed at the B3LYP/6-31+G(d,p) level.

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URLURL TypeDescription DOIArticle
Okumura, Mitchio0000-0001-6874-1137
Sander, Stanley P.0000-0003-1424-3620
Additional Information:© 2013 American Chemical Society. Received: January 12, 2013. Revised: May 3, 2013. Publication Date (Web): May 3, 2013. Financial support was provided by the National Aeronautics and Space Administration (NASA) Upper Atmosphere Research Program (grants NNX09AE21G and NNX12AI01G), the National Science Foundation (NSF, Grant CHE-0957490 for experimental work at Caltech and Grant CHE-1213347 for computational work by ABM), and the NASA Tropospheric Chemistry Program. Part of this research was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA). We acknowledge support of a National Defense Science and Engineering Graduate Fellowship for M.K.S., an NSF Graduate Fellowship for L.A.M., and the Caltech Student-Faculty Programs office for H.N.W. through the Summer Undergraduate Research Fellowship program. We thank Dr. Andrew Mollner, who performed the initial setup of the experiment and the formaldehyde sampling system, Dr. Ralph Page for vital technical assistance and optimization of the spectrometer optics, Michael Roy for machining support, and Richard Gerhart for glassware construction and repair. We acknowledge the inspiration provided by Takeshi Oka for this work. The authors declare no competing financial interest. Note that there was a sign error in the last term in eq 2 of ref 48.
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National Defense Science and Engineering Graduate (NDSEG) FellowshipUNSPECIFIED
NSF Graduate Research FellowshipUNSPECIFIED
Caltech Summer Undergraduate Research Fellowship (SURF)UNSPECIFIED
Issue or Number:39
Record Number:CaltechAUTHORS:20131206-082335728
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Official Citation:Cavity Ringdown Spectroscopy of the Hydroxy-Methyl-Peroxy Radical Matthew K. Sprague, Laura A. Mertens, Heather N. Widgren, Mitchio Okumura, Stanley P. Sander, and Anne B. McCoy The Journal of Physical Chemistry A 2013 117 (39), 10006-10017
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42874
Deposited By: Ruth Sustaita
Deposited On:06 Dec 2013 17:20
Last Modified:10 Nov 2021 16:29

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