H abstraction dynamics in crossed molecular beams : Cl + ROH reactions

ion reactions of hydrogen atoms from hydrocarbons are of great importance in combustion and atmospheric chemistry. This is reÑected in numerous publications where the kinetics of these processes have been probed in detail. While rate information provides some insight into the mechanisms of these reactions, it is the dynamics that provides a deeper understanding. For example, free radical abstraction of hydrogen atoms from saturated hydrocarbons and the di†ering propensities for reaction of primary, secondary, or tertiary H atoms, as well as di†ering dynamics underlying these pathways, are important for a detailed understanding of combustion chemistry. With the advent of laser technology and development of new techniques in studying such processes, there has been an explosion of studies, particularly for the reactions of O and Cl with saturated hydrocarbons. The earliest study of the dynamics of H abstraction from saturated hydrocarbons can be traced back to two seminal papers by Andresen and Luntz,1,2 wherein they probed the dynamics of O(3P) with a variety of saturated hydrocarbons (neopentane, cyclohexane, and isobutane) using laser induced Ñuorescence (LIF) in conjunction with crossed molecular beams. They found that the resultant OH rotational state distributions were nearly identical for all the hydrocarbons and decrease rapidly from a peak at the lowest rotational level, suggesting strong dynamical constraints favoring collinearity in the critical region along the reaction pathway. The trends that they observed for the OH rotational distribution have been conÐrmed in all subsequent studies of H abstraction dynamicsÈthat the newly formed hydride products, regardless of the identity of the hydrocarbon or the attacking atom, are formed rotationally cold. Furthermore they found that the vibrational state distribution of OH depends markedly on the type of hydrogen abstracted, with vibrational excitation increasing dramatically across the series primary to secondary to tertiary, correlating with the increasing exothermicity of the reaction. They interpreted this dynamic behavior as a shift from a repulsive toward a more attractive surface across the series of hydrocarbons studied. Subsequently the reactions of O(3P) with a number of organic compounds were studied using similar techniques by a number of groupsÈnotably Kleinermanns and Luntz3h5 and Whitehead and co-workers.6,7 Very recently McKendrick and co-workers8,9 have reinvestigated the reactions of oxygen with a series of hydrocarbons. They conÐrmed the previous results of Andresen and Luntz, and went on to investigate the reaction of O(3P) with and They found similar 4 26 . low levels of rotational energy release, suggesting that the previously proposed strong collinear constraint also applied to the smaller hydrocarbons. The abstraction reaction