Cavity Ringdown Spectroscopy of the Hydroxy-Methyl-Peroxy Radical
journal contributionposted on 03.10.2013, 00:00 authored by Matthew K. Sprague, Laura A. Mertens, Heather N. Widgren, Mitchio Okumura, Stanley P. Sander, Anne B. McCoy
We report vibrational and electronic spectra of the hydroxy-methyl-peroxy radical (HOCH2OO• or HMP), which was formed as the primary product of the reaction of the hydroperoxy radical, HO2•, and formaldehyde, HCHO. The ν1 vibrational (OH stretch) spectrum and the Ã ← X̃ 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 HO2•. Free radical reactions were initiated by pulsed laser photolysis (PLP) of Cl2 in the presence of HCHO and O2 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 (ν1) and 7250–7800 cm–1 (Ã ← X̃) respectively, at a delay time 100 μs after photolysis. The ν1 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 Ã ← X̃ 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 Ã state (ν15′ = 171 cm–1). The band furthest to the red was assigned as a hot band in ν15″, leading to a ground state torsional frequency of (ν15″ = 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 X̃ and Ã 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 τHOCO and τOOCO. 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 X̃ state conformer HMP-A, but that this conformer is not the lowest energy conformer in the Ã state, which remains unobserved. We estimated that the energy of this lowest conformer (HMP-B) of the Ã state is E0 (Ã, HMP-B) ≈ 7200 cm–1, on the basis of the energy difference E0(HMP-B) – E0(HMP-A) on the Ã state computed at the B3LYP/6-31+G(d,p) level.
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delay time 100 μvibronic band intensitiesOHwave functionsHCOOHHCHOanharmonic vibrational frequencyCavity Ringdown Spectroscopyground state torsional frequencycontourHOCHOOCOHMPcm2OOorder vibrational perturbation theorycavity ringdown spectroscopydihedral angles τ HOCOPLPenergy conformerspectrumHOOHVPTB 3LYP calculationsB 3LYP level