Reaction on the O−+CH4 potential energy surface: Dependence on translational and internal energy and on isotopic composition, 93–1313 K

Abstract

Rate constants have been measured for the reactions of O Ϫ with CH 4 , CH 2 D 2 , and CD 4 as a function of ion-neutral average center-of-mass kinetic energy, ͗KE cm ͘, at several temperatures over the range 93 K-565 K using a selected ion flow drift tube apparatus. For the CH 4 reaction we also report measurements made using a high-temperature flowing afterglow ͑HTFA͒ instrument over the temperature range 300 K-1313 K. The rate constants are found to have a very large isotope effect, with the CH 4 rate constant a factor of 15 higher than the CD 4 rate constant at 93 K. The rate constants generally have a minimum with respect to temperature and ͗KE cm ͘, except for the higher-temperature data for CD 4 where the rate constants show only an increase with increasing kinetic energy. The data indicate that increasing rotational temperature decreases the rate constants and that rotational energy behaves similarly to translational energy. Single excitations of bending and twisting vibrations have a negligible effect on the rate constant. Either the stretching vibrations or overtones of the bending vibrations increase the rate constants. If the stretches are responsible for the increase in the rate constants, the derived rate constant for a single quantum of stretch excitation (vϭ1) is 5ϫ10 Ϫ10 cm 3 s Ϫ1 , a factor of 6 larger than the rate constant for vϭ0. The CH 2 D 2 rate constants are approximately equal to the averages of the rate constants for the pure isotopes. The product branching ratio (OH Ϫ /OD Ϫ ) shows no dependence on CH 2 D 2 rotational temperature or low-frequency CH 2 D 2 vibrations. A theoretical study of the minimum energy reaction path was performed to help elucidate the reaction dynamics. The minimum energy reaction surface was characteristic of the standard double minimum pathway for ion molecule reactions. The height of the central barrier was found to be close to the energy of the reactants and varied with isotopic substitution. Conformationally different transition states are found for these isotopic reactions. Theoretical studies at the QCISD͑T͒ level of theory find distinct transition states corresponding to O Ϫ ϩCH 4 , O Ϫ ϩH-CHD 2 , O Ϫ ϩD-CH 2 D, O Ϫ ϩCD 4 . The transition state barriers increase in the order O Ϫ ϩCH 4 , O Ϫ ϩH-CHD 2 , O Ϫ ϩD-CH 2 D, and O Ϫ ϩCD 4 , in agreement with experimental reaction rates. The main features of the reactivity are explained by the characteristics of the reaction surface.

Ab Initio Molecular Dynamics Simulation Study on the Stereo Reactions between Atomic Oxygen Anion and Methane

“…Firstly, the thermochemistry calculations were carried out for reaction (2) at the second-order Møller-Plesset (MP2) [ 26 , 27 , 28 , 29 , 30 ] perturbation method with 6 − 31 + G(d) basis set. The energetic release Δ E of −0.23 eV is in good agreement with the previous values [ 19 , 20 , 21 , 23 , 24 ], suggesting that the MP2/6 − 31 + G(d) level is reliable in elaborating the reaction process. Excluding the influence of any electrostatic interactions between O − and CH 4 at the initial step in the dynamic simulations, we performed the interaction energy calculations for the three attack modes at the MP2/6 − 31 + G(d) level.…”

“…For this reaction mechanism, Comer and Schulz reported an associative-detachment channel [ 18 ], O − + CH 4 → CH 3 OH + e − and it was proved to be an exothermic process (∆ H = −2.43 eV [ 9 ]). In the subsequent experimental studies [ 19 , 20 , 21 , 22 , 23 ], the other reaction pathway [ 19 , 20 , 23 , 24 ], O − + CH 4 → OH − + CH 3 (∆ H = −0.26 eV) was characterized as an exothermic process, in which OH − was formed via direct H-abstraction via the transition-state complex [O-H-CH 3 ] − [ 21 ]. On the basis of the measurements of energy and angular distributions of the product OH − , Carpenter and Farr proposed two mechanisms for the above reaction, namely, one is the collisions of O − with H atom oriented essentially along the relative velocity vector and the other one is the collisions of O − with one of the three off-axis hydrogen atoms [ 23 ].…”

“…However, the spatial properties of this reaction remain unclear to our best knowledge although the stereodynamics is one of the essential issues of molecular reactions. Theoretically, the kinetics of the reactions of O − with CH 4 , CH 2 D 2 , and CD 4 has been explored by using the Gaussian-1, Gaussian-2, and the complete basis set extrapolation method [ 21 ]. The minimum energy reaction path for the above reaction (2) was constructed and was characteristic of the standard double minimum pathway for ion molecule reactions.…”

“…As the doorways of the reaction, these three typical O − attack directions may lead to the OH − product with the different momentum distributions. Moreover, besides reaction (2) which was believed as a single channel reaction between O − and CH 4 [ 19 , 20 , 21 , 22 , 23 ], it is still unknown about the existence of other possible reaction channels. To fully understand the reaction mechanism in detail, information concerning the possible reaction pathways for the reaction is indispensable.…”

Thermal hydrogen-atom transfer from methane: A mechanistic exercise

“…[15,19,20,32,34,35], there is compelling evidence that, if these species are generated, their reactions with hydrocarbons including methane will be facile even under ambient conditions (for an alternative view see, however, [13]f). This has already been demonstrated as early as 1969 by Bohme and Fehsenfeld [41] and later confirmed by Viggiano et al [42], when they reported that atomic [O] •− brings about efficient room-temperature HAT from various alkanes, Eq. (8), in the gas phase.…”