Dynamics of the Simplest Reaction of a Carbon Atom in a Tetrahedral Environment

Abstract

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...

Constructing Potential Energy Surfaces for Polyatomic Systems: Recent Progress and New Problems

“…At higher collision energies, as they increase from 1.06 to 1.99 eV, Zhang et al [66] found a shift of the product angular distribution from backwards to sideways. This behaviour agrees qualitatively with the previous observations of Camden et al [63][64][65], although these latter workers found a clear sideways distribution at 1.21 and 1.95 eV, with practical extinction of the backward signal. This may have been due to the use of the Photoloc technique, which neglects the internal energy contribution of the HD co-product, and, as the authors themselves recognized in 2006, "clearly a more detailed picture of the differential cross section is desirable but it will have to await more experimental work."…”

“…Camden et al [63] reported the first study of the stateto-state dynamics differential cross-section at high energies (1.95 eV) for the H + CD 4 gas-phase reaction using the Photoloc technique. They found that the CD 3 products are sideways/forward scattered with respect to the incident CD 4 , suggesting a stripping mechanism (note that in the original papers [63][64][65] the CD 3 product is measured with respect to the incident H). Later, this same laboratory [64,65] reported new experimental studies, also at high energy (1.2 eV), finding the same experimental behaviour.…”

“…However, they contradicted the experimental information available at the time the study was carried out-in 2006. In addition, since direct-dynamics QCT calculations at the B3LYP/631G(d,p) level showed sideways scattering, reproducing the experimental measurements [63][64][65], the validity of the analytical PES-2002 was questioned. The recent experiments of Zhang et al [66], however, changed the whole picture and now the direct-dynamics QCT calculations at low collision energies are questioned because of, as noted above, the presence of reactive trajectories with erroneously large impact parameters due to the underestimation of the barrier height.…”

Quasiclassical trajectory study of H+SiH 4 reactions in full-dimensionality reveals atomic-level mechanisms

“…For example, a major reaction path, which does not follow the intrinsic reaction coordinate, has recently been found by Hase et al (6) in their quasiclassical trajectory (QCT) study on the F Ϫ ϩCH 3 OOH reaction. Another example is the reaction of HϩCH 4 (and its isotopic variants), for which recent investigations (7) have focused on the new stripping mechanism observed by Zare and coworkers (8)(9). The rebound mechanism is well-known for the HϩD 2 reaction (10-11), and a lot of polyatomic H abstraction reactions, including HϩCD 4 , have long been considered to proceed through this mechanism.…”

Bimolecular Reaction Dynamics in the Phenyl - Silane System: Exploring the Prototype of a Radical Substitution Mechanism

“…4,6 Although these substitution reactions play a pivotal role in chemical synthesis and in physical organic chemistry -especially for interchanging functional groups and carbon-carbon bond coupling -a radical-type substitution reaction at tetra-coordinated carbon atoms in aliphatic molecules in the gas phase such as the reaction of atomic deuterium (D·) with methane (CH4), [1a], has so far received little attention. 7 Here, both the hydrogen abstraction channel [1b] and the deuteriumhydrogen exchange [1a] are close to thermoneutral, but have significantly different barriers of 63 kJ mol -1 versus 159 kJ mol -1 , respectively. 7 Therefore, in radical reactions involving molecules with sp 3 hybridized carbon atoms such as in methane, the hydrogen abstraction pathway is clearly preferred [8][9] .…”