Reactions occurring at a carbon center are one of the most important and useful classes of reactions in chemistry. The simplest reaction at a carbon atom with a tetrahedral environment is that of an H atom with methane, and understanding this prototypical reaction has implications for a number of fields ranging from organic and combustion chemistry to fundamental reaction dynamics. Consequently, it has been the subject of numerous experimental studies [1] exploring the kinetics and isotope effects of both the forward and reverse reactions. Recent theoretical work includes new potential energy surface calculations, [2][3][4][5][6] direct dynamics studies, [7] calculation of isotope effects, [8] and new quantum scattering methods [9] that have been reviewed by Althorpe and Clary.[10]Herein we report the first study of the nascent CD 3 products from the H + CD 4 reaction. This isotope combination was chosen for experimental reasons because this arrangement allows for the detection of all possible reaction products: CD 3 , HD and D. We also examined the CH 3 products from the H + CH 4 reaction and find that it shows very similar behavior [11] to that reported herein for the H + CD 4 reaction. By using the well-established photoloc technique, [12] we find that at a collision energy of 1.95 AE 0.05 eV the CD 3 products are produced in their ground vibrational state or have one quantum of excitation in the low-frequency umbrella-bending mode (n 2 ). In addition, the CD 3 products are sideways/backward scattered [hcos qi = À0.20 AE 0.09] with respect to the incident H-atom direction. This result stands in stark contrast with the benchmark H + D 2 !HD + D bimolecular exchange reaction in which the D atom is scattered in the same direction as the incoming H atom, thus indicating a rebound mechanism.[13] The H + D 2 reaction is a logical choice for comparison with the H + CD 4 abstraction reaction because they are both nearly thermoneutral and have similar classical barriers. [14,15] We propose two possible explanations for the unusual angular distribution of the CD 3 products: (1) a stripping mechanism is more important at this energy than a rebound mechanism, and (2) a competition between abstraction and exchange diminishes the probability for abstraction at small impact parameters.Hot-atom chemistry has established the existence of the abstraction and exchange channels by using T atoms from nuclear recoil experiments [16] and photolytic sources.[17] Equation (1) and Equation (2) are close to thermoneutralbut have very different classical barrier heights (0.65 and 1.65 eV respectively). [14] Since the early hot-atom studies, experimental probes of the dynamics have been rather sparse. Valentini and coworkers [18] examined the abstraction channel by measuring the rovibrational state distributions of the HD product from H + CD 4 at 1.5 eV by using coherent antiStokes Raman spectroscopy (CARS). They found that the total cross section for reaction is 0.14 AE 0.03 2 , from which it is concluded that the maximum impact paramete...
