Full-dimensional quantum dynamics calculations for the H+CH(4)-->H(2)+CH(3) reaction using curvilinear coordinates are presented. A curvilinear coordinate system to describe reactions of the type X+YCH(3)-->XY+CH(3) is developed which facilitates efficient calculations using the multiconfigurational time-dependent Hartree (MCTDH) approach. To describe the bending motion of the X and Y atoms relative to the axis defined by the CH(3) fragment, coordinates based on stereographic projection are introduced. These coordinates yield a kinetic energy operator free of singularities within the dynamically relevant region. Employing this curvilinear coordinate system, full-dimensional and reduced dimensional MCTDH calculations study the cumulative reaction probability (for J=0) and the thermal rate constant for the H+CH(4) reaction on the Jordan-Gilbert potential energy surface [J. Chem. Phys. 102, 5669 (1995)]. The full-dimensional results agree very well with previous full-dimensional MCTDH results which used transition state based normal coordinates. The results of our eight-dimensional (8D) calculations are in reasonable agreement with the full-dimensional ones. They deviate significantly from older 8D results of Zhang et al. [J. Chem. Phys. 127, 234213 (2007)] but agree well with more recent results from the same group.

Initial state-selected reaction probabilities for the H+CH(4)→H(2)+CH(3) reaction are computed for vanishing total angular momentum by full-dimensional calculations employing the multiconfigurational time-dependent Hartree approach. An ensemble of wave packets completely describing reactivity for total energies up to 0.58 eV is constructed in the transition state region by diagonalization of the thermal flux operator. These wave packets are then propagated into the reactant asymptotic region to obtain the initial state-selected reaction probabilities. Reaction probabilities for reactants in all rotational states of the vibrational 1A(1), 1F(2), and 1E levels of methane are presented. Vibrational excitation is found to decrease reactivity when reaction probabilities at equivalent total energies are compared but to increase reaction probabilities when the comparison is done at the basis of equivalent collision energies. Only a fraction of the initial vibrational energy can be utilized to promote the reaction. The effect of rotational excitation on the reactivity differs depending on the initial vibrational state of methane. For the 1A(1) and 1F(2) vibrational states of methane, rotational excitation decreases the reaction probability even when comparing reaction probabilities at equivalent collision energies. In contrast, rotational energy is even more efficient than translational energy in increasing the reaction probability when the reaction starts from the 1E vibrational state of methane. All findings can be explained employing a transition state based interpretation of the reaction process.

Communication: Ro-vibrational control of chemical reactivity in H+CH 4 → H 2 +CH 3 : Full-dimensional quantum dynamics calculations and a sudden model

The thermal rate constant of the H + CH(4) → H(2) + CH(3) reaction on the recent potential energy surface (PES) of Corchado et al (CBE) (Corchado, J. C.; Bravo, J. L.; Espinosa-Garcia, J. J. Chem. Phys. 2009, 130, 184314) is computed employing the state-averaged multiconfigurational time-dependent Hartree approach in the temperature range 250-500 K. The CBE PES is global and is based on an analytic functional form that is fast to evaluate. This surface is therefore potentially useful for testing new quantum dynamics methods, and reference rate constants for this purpose are presented here. The thermal rate constants obtained are compared to results previously obtained on an accurate PES particularly designed for thermal rate constant calculations (Wu, T.; Werner, H.; Manthe, U. Science 2004, 306, 2227). The two sets of rate constants are found to agree almost perfectly.

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