4 research outputs found

    Cavity Ringdown Spectroscopy of the Hydroxy-Methyl-Peroxy Radical

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    We report vibrational and electronic spectra of the hydroxy-methyl-peroxy radical (HOCH<sub>2</sub>OO<sup>•</sup> or HMP), which was formed as the primary product of the reaction of the hydroperoxy radical, HO<sub>2</sub><sup>•</sup>, and formaldehyde, HCHO. The ν<sub>1</sub> 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 HO<sub>2</sub><sup>•</sup>. Free radical reactions were initiated by pulsed laser photolysis (PLP) of Cl<sub>2</sub> in the presence of HCHO and O<sub>2</sub> 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<sup>–1</sup> (ν<sub>1</sub>) and 7250–7800 cm<sup>–1</sup> (à ← X̃) respectively, at a delay time 100 μs after photolysis. The ν<sub>1</sub> spectrum had an origin at 3622 cm<sup>–1</sup> 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<sup>–1</sup> and two bands to the blue to a progression in ν<sub>15</sub>′, the lowest torsional mode of the à state (ν<sub>15</sub>′ = 171 cm<sup>–1</sup>). The band furthest to the red was assigned as a hot band in ν<sub>15</sub>″, leading to a ground state torsional frequency of (ν<sub>15</sub>″ = 122 cm<sup>–1</sup>). 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<sub>HOCO</sub> and τ<sub>OOCO</sub>. 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><sub>0</sub> (<i>Ã</i>, HMP-B) ≈ 7200 cm<sup>–1</sup>, on the basis of the energy difference <i>E</i><sub>0</sub>(HMP-B) – <i>E</i><sub>0</sub>(HMP-A) on the à state computed at the B3LYP/6-31+G­(d,p) level

    VUV Photoionization Cross Sections of HO<sub>2</sub>, H<sub>2</sub>O<sub>2</sub>, and H<sub>2</sub>CO

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    The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical (HO<sub>2</sub>), hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), and formaldehyde (H<sub>2</sub>CO) have been measured from their first ionization thresholds to 12.008 eV. HO<sub>2</sub>, H<sub>2</sub>O<sub>2</sub>, and H<sub>2</sub>CO were generated from the oxidation of methanol initiated by pulsed-laser-photolysis of Cl<sub>2</sub> in a low-pressure slow flow reactor. Reactants, intermediates, and products were detected by time-resolved multiplexed synchrotron photoionization mass spectrometry. Absolute concentrations were obtained from the time-dependent photoion signals by modeling the kinetics of the methanol oxidation chemistry. Photoionization cross sections were determined at several photon energies relative to the cross section of methanol, which was in turn determined relative to that of propene. These measurements were used to place relative photoionization spectra of HO<sub>2</sub>, H<sub>2</sub>O<sub>2</sub>, and H<sub>2</sub>CO on an absolute scale, resulting in absolute photoionization spectra

    Kinetics of <i>n</i>-Butoxy and 2-Pentoxy Isomerization and Detection of Primary Products by Infrared Cavity Ringdown Spectroscopy

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    The primary products of <i>n</i>-butoxy and 2-pentoxy isomerization in the presence and absence of O<sub>2</sub> have been detected using pulsed laser photolysis-cavity ringdown spectroscopy (PLP-CRDS). Alkoxy radicals <i>n</i>-butoxy and 2-pentoxy were generated by photolysis of alkyl nitrite precursors (<i>n</i>-butyl nitrite or 2-pentyl nitrite, respectively), and the isomerization products with and without O<sub>2</sub> were detected by infrared cavity ringdown spectroscopy 20 μs after the photolysis. We report the mid-IR OH stretch (ν<sub>1</sub>) absorption spectra for δ-HO-1-C<sub>4</sub>H<sub>8</sub>•, δ-HO-1-C<sub>4</sub>H<sub>8</sub>OO•, δ-HO-1-C<sub>5</sub>H<sub>10</sub>•, and δ-HO-1-C<sub>5</sub>H<sub>10</sub>OO•. The observed ν<sub>1</sub> bands are similar in position and shape to the related alcohols (<i>n</i>-butanol and 2-pentanol), although the HOROO• absorption is slightly stronger than the HOR• absorption. We determined the rate of isomerization relative to reaction with O<sub>2</sub> for the <i>n</i>-butoxy and 2-pentoxy radicals by measuring the relative ν<sub>1</sub> absorbance of HOROO• as a function of [O<sub>2</sub>]. At 295 K and 670 Torr of N<sub>2</sub> or N<sub>2</sub>/O<sub>2</sub>, we found rate constant ratios of <i>k</i><sub>isom</sub>/<i>k</i><sub>O<sub>2</sub></sub> = 1.7 (±0.1) × 10<sup>19</sup> cm<sup>–3</sup> for <i>n</i>-butoxy and <i>k</i><sub>isom</sub>/<i>k</i><sub>O<sub>2</sub></sub> = 3.4(±0.4) × 10<sup>19</sup> cm<sup>–3</sup> for 2-pentoxy (2σ uncertainty). Using currently known rate constants <i>k</i><sub>O<sub>2</sub></sub>, we estimate isomerization rates of <i>k</i><sub>isom</sub> = 2.4 (±1.2) × 10<sup>5</sup> s<sup>–1</sup> and <i>k</i><sub>isom</sub> ≈ 3 × 10<sup>5</sup> s<sup>–1</sup> for <i>n</i>-butoxy and 2-pentoxy radicals, respectively, where the uncertainties are primarily due to uncertainties in <i>k</i><sub>O<sub>2</sub></sub>. Because isomerization is predicted to be in the high pressure limit at 670 Torr, these relative rates are expected to be the same at atmospheric pressure. Our results include corrections for prompt isomerization of hot nascent alkoxy radicals as well as reaction with background NO and unimolecular alkoxy decomposition. We estimate prompt isomerization yields under our conditions of 4 ± 2% and 5 ± 2% for <i>n</i>-butoxy and 2-pentoxy formed from photolysis of the alkyl nitrites at 351 nm. Our measured relative rate values are in good agreement with and more precise than previous end-product analysis studies conducted on the <i>n</i>-butoxy and 2-pentoxy systems. We show that reactions typically neglected in the analysis of alkoxy relative kinetics (decomposition, recombination with NO, and prompt isomerization) may need to be included to obtain accurate values of <i>k</i><sub>isom</sub>/<i>k</i><sub>O<sub>2</sub></sub>

    O(<sup>3</sup><i>P</i>) + CO<sub>2</sub> Collisions at Hyperthermal Energies: Dynamics of Nonreactive Scattering, Oxygen Isotope Exchange, and Oxygen-Atom Abstraction

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    The dynamics of O(<sup>3</sup><i>P</i>) + CO<sub>2</sub> collisions at hyperthermal energies were investigated experimentally and theoretically. Crossed-molecular-beams experiments at ⟨<i>E</i><sub>coll</sub>⟩ = 98.8 kcal mol<sup>–1</sup> were performed with isotopically labeled <sup>12</sup>C<sup>18</sup>O<sub>2</sub> to distinguish products of nonreactive scattering from those of reactive scattering. The following product channels were observed: elastic and inelastic scattering (<sup>16</sup>O(<sup>3</sup><i>P</i>) + <sup>12</sup>C<sup>18</sup>O<sub>2</sub>), isotope exchange (<sup>18</sup>O + <sup>16</sup>O<sup>12</sup>C<sup>18</sup>O), and oxygen-atom abstraction (<sup>18</sup>O<sup>16</sup>O + <sup>12</sup>C<sup>18</sup>O). Stationary points on the two lowest triplet potential energy surfaces of the O(<sup>3</sup><i>P</i>) + CO<sub>2</sub> system were characterized at the CCSD(T)/aug-cc-pVTZ level of theory and by means of W4 theory, which represents an approximation to the relativistic basis set limit, full-configuration-interaction (FCI) energy. The calculations predict a planar CO<sub>3</sub>(<i>C</i><sub>2<i>v</i></sub>, <sup>3</sup>A″) intermediate that lies 16.3 kcal mol<sup>–1</sup> (W4 FCI excluding zero point energy) above reactants and is approached by a <i>C</i><sub>2<i>v</i></sub> transition state with energy 24.08 kcal mol<sup>–1</sup>. Quasi-classical trajectory (QCT) calculations with collision energies in the range 23–150 kcal mol<sup>–1</sup> were performed at the B3LYP/6-311G(d) and BMK/6-311G(d) levels. Both reactive channels observed in the experiment were predicted by these calculations. In the isotope exchange reaction, the experimental center-of-mass (c.m.) angular distribution, <i>T</i>(θ<sub>c.m.</sub>), of the <sup>16</sup>O<sup>12</sup>C<sup>18</sup>O products peaked along the initial CO<sub>2</sub> direction (backward relative to the direction of the reagent O atoms), with a smaller isotropic component. The product translational energy distribution, <i>P</i>(<i>E</i><sub>T</sub>), had a relatively low average of ⟨<i>E</i><sub>T</sub>⟩ = 35 kcal mol<sup>–1</sup>, indicating that the <sup>16</sup>O<sup>12</sup>C<sup>18</sup>O products were formed with substantial internal energy. The QCT calculations give c.m. <i>P</i>(<i>E</i><sub>T</sub>) and <i>T</i>(θ<sub>c.m.</sub>) distributions and a relative product yield that agree qualitatively with the experimental results, and the trajectories indicate that exchange occurs through a short-lived CO<sub>3</sub>* intermediate. A low yield for the abstraction reaction was seen in both the experiment and the theory. Experimentally, a fast and weak <sup>16</sup>O<sup>18</sup>O product signal from an abstraction reaction was observed, which could only be detected in the forward direction. A small number of QCT trajectories leading to abstraction were observed to occur primarily via a transient CO<sub>3</sub> intermediate, albeit only at high collision energies (149 kcal mol<sup>–1</sup>). The oxygen isotope exchange mechanism for CO<sub>2</sub> in collisions with ground state O atoms is a newly discovered pathway through which oxygen isotopes may be cycled in the upper atmosphere, where O(<sup>3</sup><i>P</i>) atoms with hyperthermal translational energies can be generated by photodissociation of O<sub>3</sub> and O<sub>2</sub>