Stereodynamics of the reactions of O(3P) with saturated hydrocarbons: The dependences on the collision energy and the structural features of hydrocarbons

Abstract

State-selected differential cross sections (DCSs) have been measured for the OH radicals produced from the reactions of O(3P) with saturated hydrocarbons by utilizing Doppler-resolved polarization spectroscopy. Stereodynamics in the reactions of secondary (c-C6H12) and tertiary (i-C4H10) hydrogen atoms are discussed based on the dependences of the DCSs on the collision energy and the structure of these hydrocarbons. For the c-C6H12 reaction, the DCS of the OH(2Π3/2,v′=1,j′=3.5,A′) shows predominant intensities in the backward hemisphere with reference to the incident O(3P) atom at a mean collision energy of 〈Ecoll〉=12 kJ/mol. When the collision energy is raised to 〈Ecoll〉=33 kJ/mol, the OH radicals scattered in the forward hemisphere grow almost to match those in the backward hemisphere. The observed increase in the forward scattering implies that the collision energy makes the large impact parameter collisions contribute to the reactive scattering. At a similar collision energy of 〈Ecoll〉=31 kJ/mol the forward scattering component in the DCS of the i-C4H10 reaction does not exceed that of the c-C6H12. This shows that the cone of acceptance is not enlarged in the i-C4H10 reaction from that in the c-C6H12 reaction, as opposed to the expectation based on the height of activation barrier. The absence of the enlargement of the cone of acceptance can be attributed to a large steric hindrance caused by the three bulky methyl groups surrounding the reactive tertiary C–H bond of i-C4H10. The difference in the steric hindrance can explain the difference in the temperature-dependent pre-exponential factors of the macroscopic reaction rates between the abstraction of the secondary and tertiary C–H bonds. The collision energy dependence of the DCS as well as the internal excitation of alkyl radical products reveal that the O(3P)+alkane reactions are not always dominated by the simple rebound mechanism, which has long been believed.

Imaging O(3P)+alkane reactions in crossed molecular beams: Vertical versus adiabatic H abstraction dynamics

“…This modest correction will clearly not have a significant impact on the conclusions of these studies. Tsurumaki and co-workers provided evidence for internal energy in the alkyl fragment in their Doppler studies discussed above, 12 and noted that the simple triatomic picture of the reaction could not account for this. Unfortunately, their report only concerned the products formed in conjunction with OH (vϭ1) so that it was difficult to generalize this aspect of their conclusions to the global dynamics.…”

“…12 Their angular distributions show backscattering at their lowest collision energy, but their fitted distributions become nearly forwardbackward symmetric at high-collision energy, and they have proposed the onset of a stripping pathway to explain this phenomenon. This conclusion is contradictory to our analysis of the present ion imaging measurements, and we have attempted to understand the source of the discrepancy.…”

Imaging the dynamics of gas phase reactions

“…Such a biasing has been suggested to account for the discrepancy between the results obtained using universal ionisation and Doppler-resolved polarised LIF of the OH products from the reaction of O( 3 P) atoms with isobutane and cyclohexane. 173 Several limitations of universal detection schemes involving direct one-photon ionisation have been mentioned above: the lack of state-to-state information, background due to parent photodissociation, and possible biasing in favour of products with greater levels of internal excitation. Its main advantage is that it allows study of a very wide variety of reactions, with high sensitivity.…”

The dynamics of the Cl+C2H6→HCl(v′,j′)+C2H5 reaction at 0.24 eV: Is ethyl a spectator?

“…Experiments of this type have fallen into two distinct classes, those performed under single molecular beam ͑coexpansion͒ conditions, [7][8][9] and those performed under room temperature bulb conditions. 6,[10][11][12][13][14][15][16] An important feature of the PHOTOLOC technique is that it allows product state-resolved CM angular distributions to be obtained from bulb or single beam ͑coexpansion͒ experiments, which provide much higher product number densities than can be achieved in crossed molecular beams. A second advantage is that the transformation between density and flux is comparatively straightforward, provided that the pump-probe delay times employed are sufficiently short to avoid problems with fly-out.…”