Publications (89) View all
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Article: Direct Measurements of Conformer-Dependent Reactivity of the Criegee Intermediate CH3CHOO.
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ABSTRACT: Although carbonyl oxides, "Criegee intermediates," have long been implicated in tropospheric oxidation, there have been few direct measurements of their kinetics, and only for the simplest compound in the class, CH2OO. Here, we report production and reaction kinetics of the next larger Criegee intermediate, CH3CHOO. Moreover, we independently probed the two distinct CH3CHOO conformers, syn- and anti-, both of which react readily with SO2 and with NO2. We demonstrate that anti-CH3CHOO is substantially more reactive toward water and SO2 than is syn-CH3CHOO. Reaction with water may dominate tropospheric removal of Criegee intermediates and determine their atmospheric concentration. An upper limit is obtained for the reaction of syn-CH3CHOO with water, and the rate constant for reaction of anti-CH3CHOO with water is measured as 1.0 × 10(-14) ± 0.4 × 10(-14) centimeter(3) second(-1).Science 04/2013; 340(6129):177-80. · 31.20 Impact Factor -
Article: A general scaling rule for the collision energy dependence of a rotationally inelastic differential cross-section and its application to NO(X) + He.
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ABSTRACT: The quasi-quantum treatment (QQT) (Gijsbertsen et al., J. Am. Chem. Soc., 2006, 128, 8777) provides a physically compelling framework for the evaluation of rotationally inelastic scattering, including the differential cross sections (DCS). In this work the QQT framework is extended to treat the DCS in the classically forbidden region as well as the classically allowed region. Most importantly, the QQT is applied to the collision energy dependence of the angular distributions of these DCSs. This leads to an analytical formalism that reveals a scaling relationship between the DCS calculated at a particular collision energy and the DCS at other collision energies. This scaling is shown to be exact for QM calculated or experimental DCSs if the magnitude of the (kinematic apse frame) underlying scattering amplitude depends solely on the projection of the incoming momentum vector onto the kinematic apse vector. The QM DCSs of the NO(X)-He collision system were found to obey this scaling law nearly perfectly for energies above 63 meV. The mathematical derivation is accompanied by a mechanistic description of the Feynman paths that contribute to the scattering amplitude in the classically allowed and forbidden regions, and the nature of the momentum transfer during the collision process. This scaling relationship highlights the nature of (and limits to) the information that is obtainable from the collision-energy dependence of the DCS, and allows a description of the relevant angular range of the DCSs that embodies this information.Physical Chemistry Chemical Physics 03/2013; · 3.57 Impact Factor -
Article: Formation of dimethylketene and methacrolein by reaction of the CH radical with acetone.
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ABSTRACT: The reaction of the methylidyne radical (CH) with acetone ((CH(3))(2)C[double bond, length as m-dash]O) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of bromoform and react with acetone in an excess of helium and nitrogen gas flow. The main observed reaction exit channel is elimination of a hydrogen atom to form C(4)H(6)O isomers. Analysis of photoionization spectra identifies dimethylketene and methacrolein as the only H-elimination products. The best fit to the data gives branching ratios of 0.68 ± 0.14 for methacrolein and 0.32 ± 0.07 for dimethylketene. A methylketene spectrum measured here is used to reanalyze the photoionization spectrum obtained at m/z = 56 for the CH + acetaldehyde reaction, (Goulay et al., J. Phys. Chem. A, 2012, 116, 6091) yielding new H-loss branching ratios of 0.61 ± 0.12 for acrolein and 0.39 ± 0.08 for methylketene. The contribution from methyleneoxirane to the reaction product distribution is revised to be negligible. Coupled with additional product detection for the CD + acetone reaction, these observations pave the way for development of general set of reaction mechanisms for the addition of CH to compounds containing an acetyl subgroup.Physical Chemistry Chemical Physics 02/2013; 15(11):4049-58. · 3.57 Impact Factor -
Article: Spectroscopy of the Simplest Criegee Intermediate CH(2) OO: Simulation of the First Bands in Its Electronic and Photoelectron Spectra.
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ABSTRACT: CH(2) OO, the simplest Criegee intermediate, and ozone are isoelectronic. They both play very important roles in atmospheric chemistry. Whilst extensive experimental studies have been made on ozone, there were no direct gas-phase studies on CH(2) OO until very recently when its photoionization spectrum was recorded and kinetics studies were made of some reactions of CH(2) OO with a number of molecules of atmospheric importance, using photoionization mass spectrometry to monitor CH(2) OO. In order to encourage more direct studies on CH(2) OO and other Criegee intermediates, the electronic and photoelectron spectra of CH(2) OO have been simulated using high level electronic structure calculations and Franck-Condon factor calculations, and the results are presented here. Adiabatic and vertical excitation energies of CH(2) OO were calculated with TDDFT, EOM-CCSD, and CASSCF methods. Also, DFT, QCISD and CASSCF calculations were performed on neutral and low-lying ionic states, with single energy calculations being carried out at higher levels to obtain more reliable ionization energies. The results show that the most intense band in the electronic spectrum of CH(2) OO corresponds to the ${{\rm{\tilde B}}}$(1) A' ← ${{\rm{\tilde X}}}$(1) A' absorption. It is a broad band in the region 250-450 nm showing extensive structure in vibrational modes involving O-O stretching and C-O-O bending. Evidence is presented to show that the electronic absorption spectrum of CH(2) OO has probably been recorded in earlier work, albeit at low resolution. We suggest that CH(2) OO was prepared in this earlier work from the reaction of CH(2) I with O(2) and that the assignment of the observed spectrum solely to CH(2) IOO is incorrect. The low ionization energy region of the photoelectron spectrum of CH(2) OO consists of two overlapping vibrationally structured bands corresponding to one-electron ionizations from the highest two occupied molecular orbitals of the neutral molecule. In each case, the adiabatic component is the most intense and the adiabatic ionization energies of these bands are expected to be very close, at 9.971 and 9.974 eV at the highest level of theory used.Chemistry 08/2012; 18(39):12411-23. · 5.93 Impact Factor -
Article: New mechanistic insights to the O(3P) + propene reaction from multiplexed photoionization mass spectrometry.
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ABSTRACT: The reaction of O((3)P) with propene (C(3)H(6)) has been examined using tunable vacuum ultraviolet radiation and time-resolved multiplexed photoionization mass spectrometry at 4 Torr and 298 K. The temporal and isomeric resolution of these experiments allow the separation of primary from secondary reaction products and determination of branching ratios of 1.00, 0.91 ± 0.30, and 0.05 ± 0.04 for the primary product channels CH(3) + CH(2)CHO, C(2)H(5) + HCO, and H(2) + CH(3)CHCO, respectively. The H + CH(3)CHCHO product channel was not observable for technical reasons in these experiments, so literature values for the branching fraction of this channel were used to convert the measured product branching ratios to branching fractions. The results of the present study, in combination with past experimental and theoretical studies of O((3)P) + C(3)H(6), identify important pathways leading to products on the C(3)H(6)O potential energy surface (PES). The present results suggest that up to 40% of the total product yield may require intersystem crossing from the initial triplet C(3)H(6)O PES to the lower-lying singlet PES.Physical Chemistry Chemical Physics 06/2012; 14(30):10410-23. · 3.57 Impact Factor
