Dynamics of H and D abstraction in the reaction of Cl atom with butane-1,1,1,4,4,4-d6

Abstract

We report the primary (D-atom) and secondary (H-atom) abstraction dynamics of chlorine atom reaction with butane-1,1,1,4,4,4-d(6). The H- and D-atom abstraction channels were studied over a range of collision energies: 10.4 kcal mol(-1) and 12.9 kcal mol(-1); 5.2 kcal mol(-1) to 12.8 kcal mol(-1), respectively, using crossed molecular beam dc slice ion imaging techniques. Single photon ionization at 157 nm was used to probe the butyl radical products resulting from the H- and D-atom abstraction reactions. These two channels manifest distinct dynamics principally in the translational energy distributions, while the angular distributions are remarkably similar. The reduced translational energy distribution for the primary abstraction showed marked variation with collision energy in the backward direction, while the secondary abstraction showed this variation in the forward direction.

Primary vs. secondary H-atom abstraction in the Cl-atom reaction with n-pentane

“…The TKER distribution is shifted upwards in energy for the Cl + n-pentane reaction when compared to our measured distribution for Cl + ethane, but both peak at the same value of TKER / Ec = 0.72. The experimental data in isolation point to both mechanisms (1a) and (1b) being active, consistent with the room-temperature studies of Cl + propane and Cl + n-butane reactions by Tyndall et al 36 and the CMB experiments for Cl + CD3CH2CH2CD3 by Estillore et al 39 However, the relative contributions cannot be extracted from our experimental measurements and are better estimated from our QCT calculations (see Section 4.6).…”

“…One possible conclusion is that primary H-atom abstraction dynamics dominate the reaction of Cl atoms with n-pentane at our superthermal collision energies. More likely is that both pathways contribute, as suggested by previous studies of ratios of reaction at primary and secondary sites in alkanes, 34,[36][37][38][39] in which case we deduce similar dynamics of HCl production at the two abstraction sites despite the difference in rH values.…”

“…Selective deuteration of primary or secondary sites provides some further evidence for a preference for abstraction from secondary (or tertiary) sites by Cl atoms at super-thermal collision energies, 34,37,38 although the isotopic substitution may itself modify the reactivity. However, Estillore et al 39 studied the Cl atom reaction with CD3CH2CH2CD3 and concluded that the angular scattering distributions for the channels forming HCl and DCl were identical for chosen collision energies in excess of 20 kJ mol -1 . Suits and coworkers also explored the dependence of the scattering dynamics on alkane isomer for the examples of n-butane versus isobutene, 40 and n-pentane versus isopentane and neopentane, 19 in all cases by detection of the radical products.…”

Dynamics of Cl + propane, butanes revisited: a crossed beam slice imaging study

“…That is, no primary abstraction product is detected for the selectively deuterated propane systems. This is an indirect consequence of the radical source improvement that makes possible the use of an unfocused probe laser beam: although we could have access to primary abstraction products as already shown by multiphoton ionization with a focused VUV probe, 15,21 we believe our results here are site-selective even in the case of non-labeled butanes, given the similarity in the vertical IEs of propane and butane. We will therefore discuss all velocity-flux maps as measured for secondary H(D) abstractions in propane and n-butane, and solely tertiary H abstraction in isobutane.…”

“…The only existing direct measurement of DCSs associated to primary and secondary abstraction dynamics was undertaken in our group on selectively deuterated n-butane. 21 No significant differences were however observed in that study.…”