Dissociation Channels of the 1-Buten-2-yl Radical and Its Photolytic Precursor 2-Bromo-1-butene

Miller, Johanna L.; Krisch, Maria J.; Butler, Laurie J.; Shu, Jinian

doi:10.1021/jp0460137

Cited by 14 publications

(23 citation statements)

References 25 publications

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“…We have explicitly tested the predictions of such RRKM calculations against our experimental results for a related system, the 1-buten-2-yl radical 17 to determine the change in product branching with internal energy in the radical. Both the experimental data and RRKM results indicate that the methyl + allene channel is dominant for dissociation of 1-buten-2-yl.…”

Section: Results and Analysismentioning

confidence: 99%

A Study of the Unimolecular Dissociation of the 2-Buten-2-yl Radical via the 193 nm Photodissociation of 2-Chloro-2-butene

et al. 2005

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This work investigates the unimolecular dissociation of the 2-buten-2-yl radical. This radical has three potentially competing reaction pathways: C-C fission to form CH 3 + propyne, C-H fission to form H + 1,2-butadiene, and C-H fission to produce H + 2-butyne. The experiments were designed to probe the branching to the three unimolecular dissociation pathways of the radical and to test theoretical predictions of the relevant dissociation barriers. Our crossed laser-molecular beam studies show that 193 nm photolysis of 2-chloro-2-butene produces 2-buten-2-yl in the initial photolytic step. A minor C-Cl bond fission channel forms electronically excited 2-buten-2-yl radicals and the dominant C-Cl bond fission channel produces groundstate 2-buten-2-yl radicals with a range of internal energies that spans the barriers to dissociation of the radical. Detection of the stable 2-buten-2-yl radicals allows a determination of the translational, and therefore internal, energy that marks the onset of dissociation of the radical. The experimental determination of the lowest-energy dissociation barrier gave 31 ( 2 kcal/mol, in agreement with the 32.8 ( 2 kcal/mol barrier to C-C fission at the G3//B3LYP level of theory. Our experiments detected products of all three dissociation channels of unstable 2-buten-2-yl as well as a competing HCl elimination channel in the photolysis of 2-chloro-2-butene. The results allow us to benchmark electronic structure calculations on the unimolecular dissociation reactions of the 2-buten-2-yl radical as well as the CH 3 + propyne and H + 1,2-butadiene bimolecular reactions. They also allow us to critique prior experimental work on the H + 1,2-butadiene reaction.

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Section: Results and Analysismentioning

confidence: 99%

A Study of the Unimolecular Dissociation of the 2-Buten-2-yl Radical via the 193 nm Photodissociation of 2-Chloro-2-butene

et al. 2005

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“…There is much less data available to empirically correct or benchmark predictions for barrier heights, particularly in reactions involving radical species. 20 A recent work in our group has provided such data for the barriers associated with bimolecular and unimolecular reactions involving 2-propenyl, 21 1-propenyl, 22 1-buten-2-y1, 23 2-buten-2-y1, 24 propionyl, 25 and vinoxy 26 radical intermediates. Currently, the Gaussian-3 ͑G3͒ theoretical procedure and its variants [27][28][29] are among the most popular quantum-chemical computation schemes for energetic calculations because of their high efficiency and accuracy.…”

Section: Introductionmentioning

confidence: 99%

Probing the barrier for CH2CHCO→CH2CH+CO by the velocity map imaging method

Lau

Liu

Butler

2005

The Journal of Chemical Physics

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This work determines the dissociation barrier height for CH2CHCO --> CH2CH + CO using two-dimensional product velocity map imaging. The CH2CHCO radical is prepared under collision-free conditions from C-Cl bond fission in the photodissociation of acryloyl chloride at 235 nm. The nascent CH2CHCO radicals that do not dissociate to CH2CH + CO, about 73% of all the radicals produced, are detected using 157-nm photoionization. The Cl(2P(3/2)) and Cl(2P(1/2)) atomic fragments, momentum matched to both the stable and unstable radicals, are detected state selectively by resonance-enhanced multiphoton ionization at 235 nm. By comparing the total translational energy release distribution P(E(T)) derived from the measured recoil velocities of the Cl atoms with that derived from the momentum-matched radical cophotofragments which do not dissociate, the energy threshold at which the CH2CHCO radicals begin to dissociate is determined. Based on this energy threshold and conservation of energy, and using calculated C-Cl bond energies for the precursor to produce CH2CHC*O or C*H2CHCO, respectively, we have determined the forward dissociation barriers for the radical to dissociate to vinyl + CO. The experimentally determined barrier for CH2CHC*O --> CH2CH + CO is 21+/-2 kcal mol(-1), and the computed energy difference between the CH2CHC*O and the C*H2CHCO forms of the radical gives the corresponding barrier for C*H2CHCO --> CH2CH + CO to be 23+/-2 kcal mol(-1). This experimental determination is compared with predictions from electronic structure methods, including coupled-cluster, density-functional, and composite Gaussian-3-based methods. The comparison shows that density-functional theory predicts too low an energy for the C*H2CHCO radical, and thus too high a barrier energy, whereas both the Gaussian-3 and the coupled-cluster methods yield predictions in good agreement with experiment. The experiment also shows that acryloyl chloride can be used as a photolytic precursor at 235 nm of thermodynamically stable CH2CHC*O radicals, most with an internal energy distribution ranging from approximately 3 to approximately 21 kcal mol(-1). We discuss the results with respect to the prior work on the O(3P) + propargyl reaction and the analogous O(3P) + allyl system.

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“…The dynamics of the C-Br bond cleavage in alkyl-substituted vinyl bromides and the subsequent dissociation of internally hot radical co-fragments were the subject of prior investigations by Butler and coworkers. 50,51 The initially excited p CC p CC * state and the nearby p CC s CBr *, n X s CBr * states do not asymptotically correlate to HBr in its electronic ground state. 22 However, HBr elimination can take place from the ground state of each of the parent molecules and is associated with a large reaction barrier.…”

Section: Resultsmentioning

confidence: 98%

Direct comparison of 3-centre and 4-centre HBr elimination pathways in methyl-substituted vinyl bromides

Pandit

Hornung

Orr-Ewing

2016

Phys. Chem. Chem. Phys.

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Elimination of HBr from UV-photoexcited vinyl bromides can occur through both 3-centre and 4-centre transition states (TSs). The competition between these pathways is examined using velocity map imaging of HBr (v = 0-2, J) photofragments. The three vinyl bromides chosen for study have methyl substituents that block either the 3-centre or the 4-centre TS, or leave both pathways open. The kinetic energy distributions extracted from velocity map images of HBr from 193 nm photolysis of the three vinyl bromide compounds are approximately described by a statistical model of energy disposal among the degrees of freedom of the photoproducts, and are attributed to dissociation on the lowest electronic state of the molecule after internal conversion. Dissociation via the 4-centre TS gives greater average kinetic energy release than for the 3-centre TS pathway. The resonance enhanced multi-photon ionization (REMPI) schemes used to detect HBr restrict measurements to J ≤ 7 for v = 2 and J ≤ 15 for v = 0. Within this spectroscopic range, the HBr rotational temperature is colder for the 4-centre than for the 3-centre elimination pathway. Calculations of the intrinsic reaction coordinates and RRKM calculations of HBr elimination rate coefficients provide mechanistic insights into the competition between the pathways.

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Dissociation Channels of the 1-Buten-2-yl Radical and Its Photolytic Precursor 2-Bromo-1-butene

Cited by 14 publications

References 25 publications

A Study of the Unimolecular Dissociation of the 2-Buten-2-yl Radical via the 193 nm Photodissociation of 2-Chloro-2-butene

A Study of the Unimolecular Dissociation of the 2-Buten-2-yl Radical via the 193 nm Photodissociation of 2-Chloro-2-butene

Probing the barrier for CH2CHCO→CH2CH+CO by the velocity map imaging method

Direct comparison of 3-centre and 4-centre HBr elimination pathways in methyl-substituted vinyl bromides

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