Cavity Ringdown Spectroscopy of the Hydroxy-Methyl-Peroxy Radical

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 à ← 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 (à ← 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 à ← 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 à 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 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 <i>X̃</i> 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 <i>E</i>₀ (<i>Ã</i>, HMP-B) ≈ 7200 cm^–1, on the basis of the energy difference <i>E</i>₀(HMP-B) – <i>E</i>₀(HMP-A) on the à state computed at the B3LYP/6-31+G­(d,p) level