Co-reporter:Benjamin W. Toulson;Kara M. Kapnas;Dmitry A. Fishman
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 22) pp:14276-14288
Publication Date(Web):2017/06/07
DOI:10.1039/C7CP02573D
Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265–328 nm. Ion images recorded probing CH3 radicals with single-photon VUV ionization show different dissociation dynamics in three distinct wavelength regions. At the longest photolysis wavelengths, λ > 318 nm, CH3 radicals are formed over tens of nanoseconds with a speed distribution that is consistent with statistical unimolecular dissociation on the S0 surface following internal conversion. In the range 292 nm ≤ λ ≤ 318 nm, dissociation occurs almost exclusively on the T1 surface following intersystem crossing and passage over a barrier, leading to the available energy being partitioned primarily into photofragment recoil. The CH3 speed distributions become bimodal at λ < 292 nm. In addition to the translationally fast T1 products, a new translationally slow, but non-statistical, component appears and grows in importance as the photolysis wavelength is decreased. Photofragment excitation (PHOFEX) spectra of CH3CHO obtained probing CH3 and HCO products are identical across the absorption band, indicating that three-body fragmentation is not responsible for the non-statistical slow component. Rather, translationally slow products are attributed to dissociation on S0, accessed via a conical intersection between the S1 and S0 surfaces at extended C–C distances. Time-resolved ion images of CH3 radicals measured using a picosecond laser operating at a photolysis wavelength of 266 nm show that product formation on T1 and S0via the conical intersection occurs with time constants of 240 ps and 560 ps, respectively.
Co-reporter:Kara M. Kapnas;Benjamin W. Toulson;Elizabeth S. Foreman;Sarah A. Block;J. Grant Hill
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 46) pp:31039-31053
Publication Date(Web):2017/11/29
DOI:10.1039/C7CP06532A
Photolysis of geminal diiodoalkanes in the presence of molecular oxygen has become an established route to the laboratory production of several Criegee intermediates, and such compounds also have marine sources. Here, we explore the role that the trihaloalkane, chlorodiiodomethane (CHI2Cl), may play as a photolytic precursor for the chlorinated Criegee intermediate ClCHOO. CHI2Cl has been synthesized and its UV absorption spectrum measured; relative to that of CH2I2 the spectrum is shifted to longer wavelength and the photolysis lifetime is calculated to be less than two minutes. The photodissociation dynamics have been investigated using DC slice imaging, probing ground state I and spin–orbit excited I* atoms with 2 + 1 REMPI and single-photon VUV ionization. Total translational energy distributions are bimodal for I atoms and unimodal for I*, with around 72% of the available energy partitioned in to the internal degrees of freedom of the CHICl radical product, independent of photolysis wavelength. A bond dissociation energy of D0 = 1.73 ± 0.11 eV is inferred from the wavelength dependence of the translational energy release, which is slightly weaker than typical C–I bonds. Analysis of the photofragment angular distributions indicate dissociation is prompt and occurs primarily via transitions to states of A′′ symmetry. Complementary high-level MRCI calculations, including spin–orbit coupling, have been performed to characterize the excited states and confirm that states of A′′ symmetry with highly mixed singlet and triplet character are predominantly responsible for the absorption spectrum. Transient absorption spectroscopy has been used to measure the absorption spectrum of ClCHOO produced from the reaction of CHICl with O2 over the range 345–440 nm. The absorption spectrum, tentatively assigned to the syn conformer, is at shorter wavelengths relative to that of CH2OO and shows far weaker vibrational structure.
Co-reporter:Benjamin W. Toulson, Jonathan P. Alaniz, J. Grant Hill and Craig Murray
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 16) pp:11091-11103
Publication Date(Web):06 Apr 2016
DOI:10.1039/C6CP01063F
The near-UV photodissociation dynamics of CH2I2 has been investigated using a combination of velocity-map (slice) ion imaging and ab initio calculations characterizing the excited states. Ground state I(2P3/2) and spin–orbit excited I*(2P1/2) atoms were probed using 2 + 1 resonance-enhanced multiphoton ionization (REMPI) or with single-photon VUV ionization. Two-color ion images were recorded at pump wavelengths of 355 nm, 266 nm and 248 nm, and one-color ion images at the REMPI wavelengths of ∼304 nm and ∼280 nm. Analysis of the ion images shows that, regardless of iodine spin–orbit state, ∼20% of the available energy is partitioned into translation ET at all excitation wavelengths indicating that the CH2I co-fragment is formed highly internally excited. The translational energy distributions comprise a slow, “statistical” component that peaks near zero and faster components that peak away from zero. The slow component makes an increasingly large contribution to the distribution as the excitation wavelength is decreased. The C–I bond dissociation energy of D0 = 2.155 ± 0.008 eV is obtained from the trend in the ET release of the faster components with increasing excitation energy. The I and I* ion images are anisotropic, indicating prompt dissociation, and are characterized by β parameters that become increasingly positive with increasing ET. The decrease in β at lower translational energies can be attributed to deviation from axial recoil. MRCI calculations including spin–orbit coupling have been performed to identify the overlapping features in the absorption spectrum and characterize one-dimensional cuts through the electronically excited potential energy surfaces. The excited states are of significantly mixed singlet and triplet character. At longer wavelengths, excitation directly accesses repulsive states primarily of B1 symmetry, consistent with the observed 〈β〉, while shorter wavelengths accesses bound states, also of B1 symmetry that are crossed by repulsive states.
Co-reporter:Benjamin W. Toulson and Craig Murray
The Journal of Physical Chemistry A 2016 Volume 120(Issue 34) pp:6745-6752
Publication Date(Web):August 23, 2016
DOI:10.1021/acs.jpca.6b06060
Photofragment excitation spectra of carbonyl sulfide (OCS) have been recorded from 212–260 nm by state-selectively probing either electronically excited S(1D) or ground state S(3P) photolysis products via 2 + 1 resonance-enhanced multiphoton ionization. Probing the major S(1D) product results in a broad, unstructured action spectrum that reproduces the overall shape of the first absorption band. In contrast, spectra obtained probing S(3P) products display prominent resonances superimposed on a broad continuum; the resonances correspond to the diffuse vibrational structure observed in the conventional absorption spectrum. The vibrational structure is assigned to four progressions, each dominated by the C–S stretch, ν1, following direct excitation to quasi-bound singlet and triplet states. The S(3PJ) products are formed with a near-statistical population distribution over the J = 2, 1, and 0 spin–orbit levels across the wavelength range investigated. Although a minor contributor to the S atom yield near the peak of the absorption cross section, the relative yield of S(3P) increases significantly at longer wavelengths. The experimental measurements validate recent theoretical work characterizing the electronic states responsible for the first absorption band by Schmidt and co-workers.
Co-reporter:Elizabeth S. Foreman;Kara M. Kapnas ;Dr. Craig Murray
Angewandte Chemie International Edition 2016 Volume 55( Issue 35) pp:10419-10422
Publication Date(Web):
DOI:10.1002/anie.201604662
Abstract
Criegee intermediates (CIs) are a class of reactive radicals that are thought to play a key role in atmospheric chemistry through reactions with trace species that can lead to aerosol particle formation. Recent work has suggested that water vapor is likely to be the dominant sink for some CIs, although reactions with trace species that are sufficiently rapid can be locally competitive. Herein, we use broadband transient absorption spectroscopy to measure rate constants for the reactions of the simplest CI, CH2OO, with two inorganic acids, HCl and HNO3, both of which are present in polluted urban atmospheres. Both reactions are fast; at 295 K, the reactions of CH2OO with HCl and HNO3 have rate constants of 4.6×10−11 cm3 s−1 and 5.4×10−10 cm3 s−1, respectively. Complementary quantum-chemical calculations show that these reactions form substituted hydroperoxides with no energy barrier. The results suggest that reactions of CIs with HNO3 in particular are likely to be competitive with those with water vapor in polluted urban areas under conditions of modest relative humidity.
Co-reporter:Elizabeth S. Foreman;Kara M. Kapnas ;Dr. Craig Murray
Angewandte Chemie 2016 Volume 128( Issue 35) pp:10575-10578
Publication Date(Web):
DOI:10.1002/ange.201604662
Abstract
Criegee intermediates (CIs) are a class of reactive radicals that are thought to play a key role in atmospheric chemistry through reactions with trace species that can lead to aerosol particle formation. Recent work has suggested that water vapor is likely to be the dominant sink for some CIs, although reactions with trace species that are sufficiently rapid can be locally competitive. Herein, we use broadband transient absorption spectroscopy to measure rate constants for the reactions of the simplest CI, CH2OO, with two inorganic acids, HCl and HNO3, both of which are present in polluted urban atmospheres. Both reactions are fast; at 295 K, the reactions of CH2OO with HCl and HNO3 have rate constants of 4.6×10−11 cm3 s−1 and 5.4×10−10 cm3 s−1, respectively. Complementary quantum-chemical calculations show that these reactions form substituted hydroperoxides with no energy barrier. The results suggest that reactions of CIs with HNO3 in particular are likely to be competitive with those with water vapor in polluted urban areas under conditions of modest relative humidity.
Co-reporter:Elizabeth S. Foreman, Kara M. Kapnas, YiTien Jou, Jarosław Kalinowski, David Feng, R. Benny Gerber and Craig Murray
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 48) pp:32539-32546
Publication Date(Web):23 Nov 2015
DOI:10.1039/C5CP04977F
Carbonyl oxides, or Criegee intermediates, are formed from the gas phase ozonolysis of alkenes and play a pivotal role in night-time and urban area atmospheric chemistry. Significant discrepancies exist among measurements of the strong 1A′–1A′ electronic transition of the simplest Criegee intermediate, CH2OO in the visible/near-UV. We report room temperature spectra of the 1A′–1A′ electronic absorption band of CH2OO acquired at higher resolution using both single-pass broadband absorption and cavity ring-down spectroscopy. The new absorption spectra confirm the vibrational structure on the red edge of the band that is absent from ionization depletion measurements. The absolute absorption cross sections over the 362–470 nm range are in good agreement with those reported by Ting et al. Broadband absorption spectra recorded over the temperature range of 276–357 K were identical within their mutual uncertainties, confirming that the vibrational structure is not due to hot bands.
Co-reporter:Elizabeth S. Foreman and Craig Murray
The Journal of Physical Chemistry A 2015 Volume 119(Issue 34) pp:8981-8990
Publication Date(Web):August 12, 2015
DOI:10.1021/acs.jpca.5b05058
Cavity ring-down spectroscopy was used to study the kinetics of formation of IO radicals in the reaction of CH2I + O2 in a flow cell at 52 ± 3 Torr total pressure of N2 diluent and a temperature of 295 K. CH2I was produced by photolysis of CH2I2 at 355 nm and IO probed on the A2Π3/2–X2Π3/2 (3,0) and (3,1) bands at 435.70 and 448.86 nm, respectively. The rates of formation of IO(v″ = 0) and IO(v″ = 1) were measured as a function of O2 number density using either conventional transient absorption or the simultaneous kinetic and ring-down technique, respectively. IO(v″ = 1) was found to be formed with a significantly larger rate constant, but reached far smaller peak concentrations than IO(v″ = 0). Kinetic modeling supports the conclusion that IO(v″ = 0) is produced both directly and through secondary chemistry, most probably involving the initial formation of the Criegee intermediate CH2OO and subsequent reaction with I atoms, while IO(v″ = 1) is produced exclusively via a direct mechanism. We propose that the reaction mechanism (direct or indirect) depends upon the degree of initial excitation of the photolytically produced CH2I reagent.
Co-reporter:James O. Thomas, Katherine E. Lower, and Craig Murray
The Journal of Physical Chemistry A 2014 Volume 118(Issue 42) pp:9844-9852
Publication Date(Web):September 24, 2014
DOI:10.1021/jp508562w
The photochemistry of methylamine has been investigated following state-specific excitation of the S1 state. 2 + 1 resonance-enhanced multiphoton ionization was used to detect nascent methyl radical products via the 3p2A2″–X̃2A2″ electronic transition. Methyl radicals were formed at all photolysis wavelengths used over the range of 222–240 nm. The nascent products showed significant rotational excitation and several quanta of vibrational excitation in ν3, the degenerate C–H stretch. The partially deuterated methyl-d3-amine isotopologue yielded methyl-d3 fragments with vibrational distributions entirely consistent with those measured for the fully protiated species; no mixed isotopologues were detected. Energetic constraints require that the vibrationally excited methyl radicals be produced in conjunction with electronic ground-state NH2 X̃2B1 radicals on the S0 surface, negating the previous interpretation that dissociation occurs on the upper adiabat. New ab initio calculations characterizing the C–N bond cleavage coordinate confirm the presence of a barrier to dissociation on S1 that is insurmountable at the photolysis wavelengths used in this work. We propose a “semi-direct” mechanism in which frustrated aminyl H atom loss on the upper adiabatic potential energy surface leads to internal conversion at the exit channel conical intersection at an extended N–H distance on its return. It is proposed that C–N bond cleavage then occurs promptly and nonstatistically on the S0 surface.
Co-reporter:Elizabeth S. Foreman, Kara M. Kapnas, YiTien Jou, Jarosław Kalinowski, David Feng, R. Benny Gerber and Craig Murray
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 48) pp:NaN32546-32546
Publication Date(Web):2015/11/23
DOI:10.1039/C5CP04977F
Carbonyl oxides, or Criegee intermediates, are formed from the gas phase ozonolysis of alkenes and play a pivotal role in night-time and urban area atmospheric chemistry. Significant discrepancies exist among measurements of the strong 1A′–1A′ electronic transition of the simplest Criegee intermediate, CH2OO in the visible/near-UV. We report room temperature spectra of the 1A′–1A′ electronic absorption band of CH2OO acquired at higher resolution using both single-pass broadband absorption and cavity ring-down spectroscopy. The new absorption spectra confirm the vibrational structure on the red edge of the band that is absent from ionization depletion measurements. The absolute absorption cross sections over the 362–470 nm range are in good agreement with those reported by Ting et al. Broadband absorption spectra recorded over the temperature range of 276–357 K were identical within their mutual uncertainties, confirming that the vibrational structure is not due to hot bands.
Co-reporter:Benjamin W. Toulson, Jonathan P. Alaniz, J. Grant Hill and Craig Murray
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 16) pp:NaN11103-11103
Publication Date(Web):2016/04/06
DOI:10.1039/C6CP01063F
The near-UV photodissociation dynamics of CH2I2 has been investigated using a combination of velocity-map (slice) ion imaging and ab initio calculations characterizing the excited states. Ground state I(2P3/2) and spin–orbit excited I*(2P1/2) atoms were probed using 2 + 1 resonance-enhanced multiphoton ionization (REMPI) or with single-photon VUV ionization. Two-color ion images were recorded at pump wavelengths of 355 nm, 266 nm and 248 nm, and one-color ion images at the REMPI wavelengths of ∼304 nm and ∼280 nm. Analysis of the ion images shows that, regardless of iodine spin–orbit state, ∼20% of the available energy is partitioned into translation ET at all excitation wavelengths indicating that the CH2I co-fragment is formed highly internally excited. The translational energy distributions comprise a slow, “statistical” component that peaks near zero and faster components that peak away from zero. The slow component makes an increasingly large contribution to the distribution as the excitation wavelength is decreased. The C–I bond dissociation energy of D0 = 2.155 ± 0.008 eV is obtained from the trend in the ET release of the faster components with increasing excitation energy. The I and I* ion images are anisotropic, indicating prompt dissociation, and are characterized by β parameters that become increasingly positive with increasing ET. The decrease in β at lower translational energies can be attributed to deviation from axial recoil. MRCI calculations including spin–orbit coupling have been performed to identify the overlapping features in the absorption spectrum and characterize one-dimensional cuts through the electronically excited potential energy surfaces. The excited states are of significantly mixed singlet and triplet character. At longer wavelengths, excitation directly accesses repulsive states primarily of B1 symmetry, consistent with the observed 〈β〉, while shorter wavelengths accesses bound states, also of B1 symmetry that are crossed by repulsive states.
Co-reporter:Benjamin W. Toulson, Kara M. Kapnas, Dmitry A. Fishman and Craig Murray
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 22) pp:NaN14288-14288
Publication Date(Web):2017/05/24
DOI:10.1039/C7CP02573D
Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265–328 nm. Ion images recorded probing CH3 radicals with single-photon VUV ionization show different dissociation dynamics in three distinct wavelength regions. At the longest photolysis wavelengths, λ > 318 nm, CH3 radicals are formed over tens of nanoseconds with a speed distribution that is consistent with statistical unimolecular dissociation on the S0 surface following internal conversion. In the range 292 nm ≤ λ ≤ 318 nm, dissociation occurs almost exclusively on the T1 surface following intersystem crossing and passage over a barrier, leading to the available energy being partitioned primarily into photofragment recoil. The CH3 speed distributions become bimodal at λ < 292 nm. In addition to the translationally fast T1 products, a new translationally slow, but non-statistical, component appears and grows in importance as the photolysis wavelength is decreased. Photofragment excitation (PHOFEX) spectra of CH3CHO obtained probing CH3 and HCO products are identical across the absorption band, indicating that three-body fragmentation is not responsible for the non-statistical slow component. Rather, translationally slow products are attributed to dissociation on S0, accessed via a conical intersection between the S1 and S0 surfaces at extended C–C distances. Time-resolved ion images of CH3 radicals measured using a picosecond laser operating at a photolysis wavelength of 266 nm show that product formation on T1 and S0via the conical intersection occurs with time constants of 240 ps and 560 ps, respectively.
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