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

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• 作者：Lei Wang ; Chow-Shing Lam ; Rabi Chhantyal-Pun ; Matthew D. Brynteson ; Laurie J. Butler ; Terry A. Miller
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：January 16, 2014
• 年：2014
• 卷：118
• 期：2
• 页码：404-416
• 全文大小：594K
• ISSN：1520-5215

文摘

We report a study of the unimolecular dissociation of BrCH₂CH₂O radicals produced from the photodissociation of BrCH₂CH₂ONO 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 BrCH₂CH₂ONO precursor and the subsequent dissociation of the vibrationally excited BrCH₂CH₂O radicals. The only photodissociation channel of the precursor we detected upon photodissociation at 351 nm was O鈥揘O bond fission. C鈥揃r photofission and HBr photoelimination do not compete significantly with O鈥揘O photofission at this excitation wavelength. The measured O鈥揘O 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鈥揘O photofission). We use the O鈥揘O photofission P(E_T) peaking near 14 kcal/mol to help characterize the internal energy distribution in the nascent ground electronic state BrCH₂CH₂O radicals. At 351 nm, some but not all of the BrCH₂CH₂O radicals are formed with enough internal energy to unimolecularly dissociate to CH₂Br + H₂CO. Although the signal at m/e = 93 (CH₂Br⁺) obtained with electron bombardment detection includes signal both from the CH₂Br product and from dissociative ionization of the energetically stable BrCH₂CH₂O radicals, we were able to isolate the signal from CH₂Br 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 BrCH₂CH₂ONO 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.