Product State Resolved Study of the Cl + (CH 3 ) 3 CD Reaction: Comparison of the Dynamics of Abstraction of Primary versus Tertiary Hydrogens David F. Varley † and Paul J. Dagdigian* Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218-2685 ReceiVed: October 24, 1995; In Final Form: December 13, 1995 X The reaction of Cl atoms with the selectively deuterated isobutane, 2-methylpropane-2-d 1 , has been investigated under single-collision conditions using state-selective detection of the products by resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The reaction was initiated in a crossed, pulsed flow of the reagents by 355 nm photolysis of Cl 2 precursor. The internal state distributions of HCl and DCl products, formed by abstraction of primary and tertiary hydrogen atoms, respectively, from the hydrocarbon reagent are reported. The degree of rotational excitation of both products was found to be very low. By comparison of the intensities of the HCl and DCl REMPI signals, the ratio of the yield of HCl to DCl product was found to be 3.3 ( 0.4. With the known speed and angular distribution of the Cl reagent, it was possible to obtain information on the product center-of-mass angular distributions from measurement of the masses 37 and 38 time of arrival profiles. The DCl product is found to be mainly backward scattered with respect to the incoming Cl atom, while the HCl product is sideways peaked. Inferences on the dynamics of the two possible abstraction pathways from these results are discussed. 1. Introduction Recent experiments from several laboratories have begun to provide detailed information on the dynamics of the reactions of chlorine atoms with hydrocarbons. Park et al. 1 have employed high-resolution infrared absorption spectroscopy to interrogate the internal and translational excitation of the DCl product from the reaction of Cl atoms with c-C 6 D 12 . Simpson et al. 2 have utilized resonance enhanced multiphoton ionization (REMPI) detection in a time-of-flight mass spectrometer (TOFMS) to observe HCl product in its V) 1 vibrational level from the reaction of Cl atoms with vibrationally excited CH 4 - (ν 3 )1). An especially novel aspect of this study was the extraction 3 of the product center-of-mass (c.m.) angular distribu- tion from analysis of the projection of the HCl product laboratory velocity distribution along the TOFMS axis. This reaction is one of a growing number of examples of photoini- tiated reactions and energy-transfer processes for which vector correlations have been derived. 4-11 Yen et al. 12, 13 have also employed REMPI detection in order to investigate site specific- ity in the reaction of Cl atoms with selectively deuterated propane and butane isotopomers. In all these experiments, the reaction was initiated by photolysis of a Cl atom precursor. We have recently carried out a study 14 of the photoinitiated reaction of Cl atoms with methane, propane, and isobutane, all in their ground vibrational states. In that investigation, we reported product HCl ro-vibrational state distributions and a preliminary product c.m. angular distribution for the Cl + isobutane reaction. The angular distribution was derived from analysis of the projection of the HCl(V)0) product laboratory velocity distribution along the TOFMS axis with a procedure similar to those employed by Zare, Hall, and co-workers. 2, 3, 9 A notable feature of our experiment was the use of crossed pulsed flows of Cl 2 , the Cl atom photolytic precursor, and the hydrocarbon reagent, in order to avoid prereaction between these reagents in premixed flows. As in the Cl + hydrocarbon reactions studied by Park et al. 1 and Simpson et al., 2 we found little rotational excitation of the HCl product in the Cl + CH 4 ,C 3 H 8 , and i-C 4 H 10 reactions. 14 These observations are consistent with our initial expectations for the dynamics of these reactions since the geometry of the transition state for the Cl + CH 4 reaction has been found in ab initio calculations 15-17 to have a linear Cl-H-CH 3 structure. With this transition state geometry, there will be a bias toward linear Cl-H‚‚‚R recoil, and hence little torque will be exerted on the departing HCl product. Park et al. 18 were able to rationalize the low product DCl vibrational excitation in the Cl + c-C 6 D 12 reaction with classical trajectory calculations on an empirical three-body (Cl-D-C 6 D 11 ) potential energy surface, consistent with a direct abstraction mechanism through a collinear Cl-D‚‚‚R recoil geometry. We have also found that the HCl(V)0) product from the Cl + isobutane reaction was primarily backward scattered with respect to the incoming Cl atom. 14 Simpson et al. 2 find the particularly surprising result that the HCl(V)1) product from Cl + CH 4 (ν 3 )1) was scattered mainly in the forward direction with respect to the incoming Cl reagent. This suggests that the dynamics of reactions of Cl atoms with hydrocarbons, particu- larly in vibrationally excited states, is richer than we might have anticipated just from knowledge of the transition state geometry. In contrast to the prototype Cl + CH 4 reaction, the reaction of Cl atoms with larger hydrocarbons is complicated by the availability of several reaction pathways, involving the abstrac- tion of a primary, secondary, or tertiary hydrogen. Hence, the HCl product detected in a our previous study 14 of the reactions of Cl atoms with propane and isobutane could have been produced by abstraction of several types of hydrogen atoms from the hydrocarbon. This ambiguity significantly hampered our analysis of the dynamics of these reactions. To gain more insight into the dynamics of these two reaction pathways, we have carried out a study of the reaction of chlorine atoms with selectively deuterated isobutane reagent in order to distinguish these two pathways isotopically: † Present address: Stanford Research Systems, 1290-D Reamwood Ave., Sunnyvale, CA 94089. X Abstract published in AdVance ACS Abstracts, February 15, 1996. 4365 J. Phys. Chem. 1996, 100, 4365-4374 0022-3654/96/20100-4365$12.00/0 © 1996 American Chemical Society