30495
title and abstract
Cavity Ringdown Spectroscopy of the Hydroxy-Methyl-Peroxy Radical.
We report vibrational and electronic spectra of the hydroxyl-methyl-peroxy radical (HOCH2OO, or HMP), the primary product of the reaction of the hydroperoxy radical, HO2, and formaldehyde, HCHO. The ν1 vibrational (OH stretch) spectrum and the Ã-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 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 295K. 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-X) (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/6-31G+(d,p) calculations, we found that the anharmonic vibrational frequency and band contour predicted for the lowest energy conformer, HMP-A, The calculated anharmonic vibrational frequency and band contour computed using B3LYP/63-1G(d,p) level 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 electronic spectrum of HMP at 7392 cm(-1) and two bands to the blue to a progression in ν15', the lowest torsional mode of the 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 X ̃ and A ̃ 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 τOCOH and τOOCO. Wave functions and energy levels were calculated based on 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 minimum energy conformer HMP-A, but that this conformer is not the lowest energy conformer in the state, which remains unobserved. We estimated that the energy of this lowest conformer (HMP-B) of the Ã state to be T0 (Ã) ≈ 7200 cm(-1), based on the energy difference E0(HMP-B) - E0(HMP-A) on the Ã state computed at the B3LYP/6-31+G(d,p) level.