Crossed molecular beam study of the reaction O(3<i>P</i>)+C2H4

Schmoltner, Anne-Marie; Chu, Pamela M.; Brudzynski, Richard J.; Lee, Yuan T.

doi:10.1063/1.457309

Cited by 114 publications

(106 citation statements)

References 58 publications

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“…In this study, the product fragmentation patterns and Jacobian factors were explicitly included in the analysis. The electron ionization cross-sections were estimated for each product from additivity rules based on atomic polarizabilities (16,34).…”

Section: Resultsmentioning

confidence: 99%

“…1, and important dynamical details of product angular and translational energy distributions have been measured in recent state-ofthe-art CMB experiments (7). These, together with more recent CMB studies with VUV synchrotron radiation detection (9) where BRs could not be derived, have shed new light on the detailed dynamics of the title reaction with respect to the results obtained in earlier CMB work (15,16). These experiments provide an important test ground for theoretical dynamics methods that can be used widely to study ISC dynamics.…”

mentioning

confidence: 92%

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Intersystem crossing and dynamics in O( ³ P) + C ₂ H ₄ multichannel reaction: Experiment validates theory

Han

Bowman

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

111

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The Oð 3 PÞ þ C 2 H 4 reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet-and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H þ CH 2 CHO, H þ CH 3 CO, H 2 þ CH 2 CO, CH 3 þ HCO, CH 2 þ CH 2 O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization mass-spectrometric detection at a collision energy of 8.4 kcal∕mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential crosssections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is it to consider the ISC as one of the nonadiabatic effects for this and similar systems involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH 3 þ HCO and H þ CH 2 CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.oxygen atom reactions | polyatomic reaction dynamics | quasiclassical trajectory surface-hopping calculations | crossed beam reactive scattering C omparisons between experimental scattering cross-sections and theoretical predictions, by both quantum-mechanical (QM) and quasiclassical trajectory (QCT) methods on accurate ab initio potential energy surfaces (PESs) for benchmark threeatom (1-4), four-atom (5), and recently noncomplex forming fiveatom (e.g., Cl þ CH 4 ; ref. 6) reactions have greatly advanced our understanding of chemical reactivity over the last decade.Nonetheless, experimental and theoretical investigations of the dynamics of more complex polyatomic reactions, with numerous competing product channels (e.g., the title reaction) still represent a major challenge for both experiment and theory. Experimentally, a major challenge is to study all the open channels with the same degree of accuracy and under the same experimental conditions. This is a prerequisite to identify the primary reaction products and determine their relative importance (branching ratios, BRs). A "universal" detection method to interrogate all product channels on the same footing, such as mass-spectrometry (MS), is required, and this can be best pursued in crossed molecular beam (CMB) experiment...

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Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 92%

Intersystem crossing and dynamics in O( ³ P) + C ₂ H ₄ multichannel reaction: Experiment validates theory

Han

Bowman

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

111

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“…Upon electron bombardment detection, vinoxy is known to undergo dissociative ionization to m/e = 42 and m/e = 29 with high probability. 27,28 There is a broad underlying signal apparent in both our m/e = 42 TOF spectrum in Figure 4 and our m/e = 29 TOF spectrum in Figure 10, though the major signal in both spectra is from the dissociative ionization of the stable BrCH 2 CH 2 O radicals. (The magnitude of the underlying signal at m/e = 29 is much larger compared to the underlying signal at m/e = 42 than one would expect from the published mass spectrum.)…”

Section: The Journal Of Physical Chemistry Amentioning

confidence: 99%

“…The fraction f of cations at the detected mass is estimated from the literature mass spectrum of vinoxy radicals for m/e = 42. 27 (Note that the ratio of signals we attributed to vinoxy at m/e = 42 to that at m/e = 29 is smaller than one would expect from ref 27, but we used the mass fragmentation pattern in ref 27 to substitute into eq 5 as that paper includes the entire mass spectrum.) The value of f used for HBr is taken from the literature mass spectrum of HBr, 31 in which the ratio of Br + and HBr + is 39:100 and also includes the fact that the signal at m/e = 82 is detecting only H 81 Br products.…”

Section: The Journal Of Physical Chemistry Amentioning

confidence: 99%

Imaging and Scattering Studies of the Unimolecular Dissociation of the BrCH₂CH₂O Radical from BrCH₂CH₂ONO Photolysis at 351 nm

et al. 2014

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We report a study of the unimolecular dissociation of BrCH 2 CH 2 O radicals produced from the photodissociation of BrCH 2 CH 2 ONO at 351/355 nm. Using both a crossed laser-molecular beam scattering apparatus with electron bombardment detection and a velocity map imaging apparatus with tunable VUV photoionization detection, we investigate the initial photodissociation channels of the BrCH 2 CH 2 ONO precursor and the subsequent dissociation of the vibrationally excited BrCH 2 CH 2 O radicals. The only photodissociation channel of the precursor we detected upon photodissociation at 351 nm was O−NO bond fission. C−Br photofission and HBr photoelimination do not compete significantly with O−NO photofission at this excitation wavelength. The measured O−NO photofission recoil kinetic energy distribution peaks near 14 kcal/mol and extends from 5 to 24 kcal/mol. There is also a small signal from lower kinetic energy NO product (it would be 6% of the total if it were also from O−NO photofission). We use the O−NO photofission P(E T ) peaking near 14 kcal/mol to help characterize the internal energy distribution in the nascent ground electronic state BrCH 2 CH 2 O radicals. At 351 nm, some but not all of the BrCH 2 CH 2 O radicals are formed with enough internal energy to unimolecularly dissociate to CH 2 Br + H 2 CO. Although the signal at m/e = 93 (CH 2 Br + ) obtained with electron bombardment detection includes signal both from the CH 2 Br product and from dissociative ionization of the energetically stable BrCH 2 CH 2 O radicals, we were able to isolate the signal from CH 2 Br product alone using tunable VUV photoionization detection at 8.78 eV. We also sought to investigate the source of vinoxy radicals detected in spectroscopic experiments by Miller and co-workers (J. Phys. Chem. A 2012, 116, 12032) from the photodissociation of BrCH 2 CH 2 ONO at 351 nm. Using velocity map imaging and photodissociating the precursor at 355 nm, we detected a tiny signal at m/e = 43 and a larger signal at m/e = 15 that we tentatively assign to vinoxy. An underlying signal in the time-of-flight spectra at m/e = 29 and m/e = 42, the two strongest peaks in the literature electron bombardment mass spectrum of vinoxy, is also apparent. Comparison of those signal strengths with the signal at HBr + , however, shows that the vinoxy product does not have HBr as a cofragment, so the prior suggestion by Miller and co-workers that the vinoxy might result from a roaming mechanism is contraindicated.

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“…For 3: excitation of both isotopes, the product state distribution is described by an adiabatic picture in which the product excitation slowly increases with increasing photon energy. Excitation of 3",6:,in CH30 may result in more rcitational excitation of the products, consistent with sampling of nonC3v geometries in the 2 state. Excitation of 3;2: in CD30 shows a surprising dynamical effect: the product state distribution of the C D 3 fragment is bimodal, with excitation of UL = 0 and UL = 2 always greater than that for UL = 1.…”

Section: Vl Conclusionmentioning

confidence: 79%

Photodissociation dynamics and spectroscopy of free radical combustion intermediates

Osborn

1996

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The photodissociation spectroscopy and dynamics of free radicals is studied by the technique of fast beam photofragment translational spectroscopy. Photodetachment of internally cold, mass-selected negative ions produces a clean source of radicals, which are subsequently dissociated and detected. The photofragment yield as a function of photon energy is obtained, mapping out the dissociative and predissociative electronic states of the radical. In addition, the photodissociation dynamics, product branching ratios, and bond energies are probed at fned photon energies by measuring the translational energy, ET), and angular distribution of the recoiling fragments using a time-and positionsensitive detector. Ab initio calculations are combined with dynamical and statistical models to interpret the observed data.The photodissociation of three prototypical hydrocarbon combustion intermediates forms the core of this work. The methoxy radical (CH30), the vinoxy radical (CH2CHO) 2 and the ketenyl radical (HCCO), are representative examples of alkanoxy, alkenoxy, and alkynoxy radicals, respectively. The dominant channel in methoxy dissociation following ultraviolet excitation is CH30 + CH3 + 0. Vibrational structure is resolved in the P(&) distribution, and a new heat of formation is obtained for CH30. Two primary channels are active in vinoxy photodissociation: CH2CHO + CH3 + CO, and CH2CHO + CH2CO + H. The dissociation dynamics in vinoxy occur by internal conversion to the ground state followed by dissociation over a barrier to products. The ketenyl radical is observed for the first time in the ultraviolet, dissociating via two distinct channels: HCCO + CH X(213) + CO, and HCCO -+ CH u (~C -) + CO. The dynamics in this case occur by internal conversion and intersystem crossing, followed by dissociation on potential surfaces with negligible barriers to products. The branching ratio is a function of photon energy, and a heat of formation is determined for ketenyl.In addition, the photodissociation of a negative ion radical, N202-, is also explored.This ion photodissociates via two channels: N20F + 0-+ N20, and N20; + NO-+ NO.Product vibrational structure is observed in the N20 bending mode, and a heat of formation is obtained for N202-. However, even in the relatively well-defined limit of a crossed molecular beam experiment, in which the reactants approach and the products are detected along defined trajectories, the results are inherently averaged over a range of impact parameters describing the difference between head-on and glancing collisions of the reactants. One way to avoid averaging over the impact parameter in the bimolecular "full collision" described above is to simpllfv the system by studying the unimolecular "half collision" of Is there more than one product channel participating?What is the bond strength, of AHm for each channel?How fast ,does dissociation take place?How is the excess energy distributed among the products?Can we determine the dissociation mechanism from this information?For the s...

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Crossed molecular beam study of the reaction O(3P)+C2H4

Cited by 114 publications

References 58 publications

Intersystem crossing and dynamics in O( ³ P) + C ₂ H ₄ multichannel reaction: Experiment validates theory

Intersystem crossing and dynamics in O( ³ P) + C ₂ H ₄ multichannel reaction: Experiment validates theory

Imaging and Scattering Studies of the Unimolecular Dissociation of the BrCH₂CH₂O Radical from BrCH₂CH₂ONO Photolysis at 351 nm

Photodissociation dynamics and spectroscopy of free radical combustion intermediates

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Crossed molecular beam study of the reaction O(3P)+C2H4

Cited by 114 publications

References 58 publications

Intersystem crossing and dynamics in O( 3 P) + C 2 H 4 multichannel reaction: Experiment validates theory

Intersystem crossing and dynamics in O( 3 P) + C 2 H 4 multichannel reaction: Experiment validates theory

Imaging and Scattering Studies of the Unimolecular Dissociation of the BrCH2CH2O Radical from BrCH2CH2ONO Photolysis at 351 nm

Photodissociation dynamics and spectroscopy of free radical combustion intermediates

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Intersystem crossing and dynamics in O( ³ P) + C ₂ H ₄ multichannel reaction: Experiment validates theory

Intersystem crossing and dynamics in O( ³ P) + C ₂ H ₄ multichannel reaction: Experiment validates theory

Imaging and Scattering Studies of the Unimolecular Dissociation of the BrCH₂CH₂O Radical from BrCH₂CH₂ONO Photolysis at 351 nm