Quantum Mechanical Capture/Phase Space Theory Calculation of the Rate Constants for the Complex-Forming CH + H₂ Reaction

Abstract

Six-dimensional wave packet calculations on an accurate potential energy surface are used to obtain the quantum mechanical capture (QM C) probabilities for CH + H(2) corresponding to a variety of total angular momenta and internal reactant states. Rate constant calculations are made feasible by employing a Monte Carlo based sampling procedure. The QM C probabilities alone are also used to estimate the high pressure CH + H(2) rate constants corresponding to stabilization or CH(3) formation. The rate constants for CH + H(2) --> CH(2) + H reaction in the low pressure limit are obtained by combining the QM C probabilities with a phase space theory (PST) approximation for product formation from the complex. Our results are compared with the experimental results of Brownsword et al. (J. Chem. Phys. 1997, 106, 7662), as well as with purely classical PST calculations. The QM C probabilities are shown to be highly dependent on the initial rotational states of the reactants corresponding to orientational restrictions on complex formation. Consistent with this, our QM C high pressure rate constants for CH(3) formation are lower than the purely classical PST rate constants. These QM C rate constants also are in reasonable accord with experiment. A similar but somewhat more subtle picture emerges regarding the QM C/PST rate constants for CH(2) + H formation.

Capture and dissociation in the complex-forming CH+H2→ CH2+H, CH+H2 reactions

“…Capture and reaction rate coefficients were calculated for temperatures ranging from 200 to 800 K, i.e., including the interval of temperatures covered by experiments 16,17 and QM calculations. 1,27 The number of integrated trajectories ranges from 5 Â 10 5 at high temperatures to 6 Â 10 7 at low temperatures. The use of such a huge number of integrated trajectories is motivated by the need to achieve satisfactory statistics when considering the constraints of zero point energy.…”

“…Capture rate coefficients Fig. 1 shows the QCT capture rate coefficients as a function of temperature, as well as the QM 27 and experimental 16,17 rate coefficients. It is worth noting the narrow range of k c values shown here.…”

“…1 This reaction is important in combustion processes 2,3 and in astrochemistry, [4][5][6][7] and, consequently, there are numerous experimental and theoretical studies about it. 1,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Experimental kinetics studies provide both the capture and the abstraction rate coefficients of the title reaction for a wide range of temperatures, [8][9][10][11][12][13][14][15][16][17] including also some isotope variants. Moreover, the D atom exchange reaction, CH+D 2 -CD+HD, was studied using a crossed molecular beam technique.…”

“…On the theoretical side, attention has been devoted to the potential energy surface (PES), the kinetics and the dynamics. 1,[20][21][22][23][24][25][26][27] The most recent global PES (MHG PES), 26 which is based on high level ab initio data, indicates that the CH+H 2 -CH 2 +H reaction is barrierless, the deep CH 3 intermediate being in the minimum energy path (MEP). However, the MEP is somewhat unusual.…”

“…1 and ref. 27 by means of a real wavepacket method 28 along with a diatom-diatom Jacobi coordinates four-atom representation. 29 QM dynamic calculations at both J = 0 1 and J > 0 27 total angular momentum quantum numbers allowed the calculation of quantum capture probabilities. Abstraction rate coefficients were then estimated using a QM-C/PST model, where the reaction probability is calculated as a product of a quantum capture (QM-C) probability and a phase space theory (PST) probability for product formation.…”