Probing the dynamics of radical reactions with polyatomic hydrocarbons by crossed-beam dc slice imaging

This work presents results of crossed molecular beam imaging studies on the reaction of radicals (Cl and CN) with polyatomic hydrocarbons of different functionalities such as pentane isomers, deuterated alkanes, alkenes, and alcohols. The reactively scattered alkyl radicals are probed as a function of collision energy using single photon ionization. The scattering results for pentane are quite similar for all reactants, suggesting that the nature of the abstraction site has surprisingly little influence on the dynamics studied at ~5 and ~9 kcal/mol. The angular distributions are broad with a backscattered peak at low collision energy and a sharp forward peak at high collision energy. The similarity of the angular distributions was observed in the Cl reactions with deuterated butane albeit studied at considerably high collision energy. The presence of conformers in this target molecule likely play a major role. The reduced translational energy distributions manifested distinct dynamics showing marked variation with collision energy in the backward direction and variation in the forward direction for primary versus secondary abstraction, respectively. For alkenes, an isotropic component was observed in the angular distributions at low collision energy suggesting complex formation that survive for few rotational period, followed by HCl elimination. At increased collision energy, the distributions show a sharp forward peak superimposed on the isotropic component accounting for ~13% of the product flux. The forward translational energy distributions changed dramatically with collision energy. A sharp forward peak at ~80% of the collision energy appears at higher energy, similar to that of pentane isomers. The butanol isomers exhibit similar dynamics with that of saturated hydrocarbons although the HCl product distributions for these two systems are different. The angular distributions showed direct reaction with backscattering at low collision energy and enhanced forward scattering with respect to the alcohol beam with increased energy. This confirms that the well present in the potential energy surface is shallow to cause long-lived complexes to exist. The product translational energy distributions further support the similarity of these reactions. At high collision energy, a sharp peak of ~80% of the collision energy is seen in the forward scattered products. The sideways-scattered product showed the lowest fraction of energy appearing in translation. Hydrogen abstraction of CN radical with alkanes indicate direct reaction with the products largely backscattered and that most of the available energy (~80% - 85%) goes into internal energy the recoiling products. In the 1-pentene system, the results demonstrate the presence of H-atom abstraction channel yielding a resonantly-stabilized C5H9 radical. The results have implications for hydrocarbon growth and nitrile incorporation in formation of haze particles on Saturn\u27s moon, Titan