The role of rotation in the vibrational relaxation of water by hydrogen molecules

Bending Relaxation of H2 O by Collision with Para- and Ortho-H2

We extend our recent theoretical work on the bending relaxation of H2 O in collisions with H2 by including the three water modes of vibration coupled with rotation, as well as the rotation of H2 . Our full quantum close-coupling method (excluding the H2 vibration) is combined with a high-accuracy nine-dimensional potential energy surface. The collisions of para-H2 O and ortho-H2 O with the two spin modifications of H2 are considered and compared for several initial states of H2 O. The convergence of the results as a function of the size of the rotational basis set of the two colliders is discussed. In particular, near-resonant energy transfer between H2 O and H2 is found to control the vibrational relaxation process, with a dominant contribution of transitions withΔ j 2 = j 2 f - j 2 i ${{\rm{\Delta }}j_2 = j_2^f - j_2^i }$ =+ 2 , + 4 ${ + 2, + 4}$ ,j 2 i ${j_2^i }$ andj 2 f ${j_2^f }$ being respectively the H2 initial and final rotational quantum numbers. Finally, the calculated value of the H2 O bending relaxation rate coefficient at 295 K is found to be in excellent agreement with its experimental estimate.

A New Full-Dimensional Ab Initio Intermolecular Potential Energy Surface and Rovibrational Energies of the H2O-H2 Complex

H2O-H2 is a prototypical five-atom van der Waals system, and the interaction between H2O and H2 plays an important role in many physical and chemical environments. However, previous full-dimensional intermolecular potential energy surfaces (IPESs) cannot accurately describe the H2O-H2 interaction in the repulsive or van der Waals minimum region. In this work, we constructed a full-dimensional IPES for the title system with a small root-mean-square error of 0.252 cm-1 by using the permutation invariant polynomial neural network method. The ab initio calculations were performed by employing the explicitly corrected coupled cluster [CCSD(T)-F12a] method with the augmented correlation-consistent polarized valence quintuple-ζ basis set. Based on the newly developed IPES, the bound states of the H2O-H2 complex were calculated within the rigid-rotor approximation. The transition frequencies and band origins agreed well with the experimental values [Weida, M. J.; Nesbitt, D. J. J. Chem. Phys. 1999, 110, 156-167] with errors less than 0.1 cm-1 for most transitions. Those results demonstrate the high accuracy of our new IPES, which would build a solid foundation for the collisional dynamics of H2O-H2 at low temperatures.

Adiabatic Trajectory Approximation: A New General Method in the Toolbox of Mixed Quantum/Classical Theory for Collisional Energy Transfer

A new version of the MQCT program is presented, which includes an important addition, adiabatic trajectory approximation (AT-MQCT), in which the equations of motion for the classical and quantum parts of the system are decoupled. This method is much faster, which permits calculations for larger molecular systems and at higher collision energies than was possible before. AT-MQCT is general and can be applied to any molecule + molecule inelastic scattering problem. A benchmark study is presented for H₂O + H₂O rotational energy transfer, an important asymmetric-top rotor + asymmetric-top rotor collision process, a very difficult problem unamenable to the treatment by other codes that exist in the community. Our results indicate that AT-MQCT represents a reliable computational tool for prediction of collisional energy transfer between the individual rotational states of two molecules, and this is valid for all combinations of state symmetries (such as para and ortho states of each collision partner).

Rotational (de)-excitation of C5 by collision with He at low temperature

An appropriate estimation of the abundance of the observed C₅ radical in the interstellar medium requires accurate radiative and collisional rate coefficients. We present the first two-dimensional potential energy surface (2D-PES) for the ground electronic state of the C₅(X¹Σ⁺)-He(X¹S) van der Waals system, obtained using an explicitly correlated coupled-cluster method with single, double, and perturbative triple excitations (RCCSD(T)-F12). This PES is subsequently used in quantum close-coupling (CC) scattering calculations. Collisional excitation cross-sections of the rotational levels of C₅ by He were calculated for energies up to 1500 cm^-1 using the standard (CC) method. The thermal dependence of the corresponding rate coefficients is given for the low and moderate temperature T ≤ 300 K regime of interstellar molecular clouds. This is the first study on the collisional rate coefficients for this system and may have important implications for the astrophysical detection of C₅(X¹Σ⁺) and modeling of carbon-rich media.

Quantum nature of molecular vibrational quenching: Water-molecular hydrogen collisions

Rates of conversions of molecular internal energy to and from kinetic energy by means of molecular collision allow us to compute collisional line shapes and transport properties of gases. Knowledge of ro-vibrational quenching rates is necessary to connect spectral observations to physical properties of warm astrophysical gasses, including exo-atmospheres. For a system of paramount importance in this context, the vibrational bending mode quenching of H₂O by H₂, we show here that the exchange of vibrational to rotational and kinetic energy remains a quantum process, despite the large numbers of quantum levels involved and the large vibrational energy transfer. The excitation of the quantized rotor of the projectile is by far the most effective ro-vibrational quenching path of water. To do so, we use a fully quantum first-principles computation, potential and dynamics, converging it at all stages, in a full coupled channel formalism. We present here rates for the quenching of the first bending mode of ortho-H₂O by ortho-H₂, up to 500 K, in a fully converged coupled channel formalism.

A close coupling study of the bending relaxation of H2O by collision with He

We present a close coupling study of the bending relaxation of H₂O by collision with He, taking explicitly into account the bending-rotation coupling within the rigid-bender close-coupling method. A 4D potential energy surface is developed based on a large grid of ab initio points calculated at the coupled-cluster single double triple level of theory. The bound states energies of the He-H₂O complex are computed and found to be in excellent agreement with previous theoretical calculations. The dynamics results also compare very well with the rigid-rotor results available in the Basecol database and with experimental data for both rotational transitions and bending relaxation. The bending-rotation coupling is also demonstrated to be very efficient in increasing bending relaxation when the rotational excitation of H₂O increases.

Rotational quenching of an interstellar gas thermometer: CH3CNHe collisions

Among all the molecular species found in the interstellar medium, molecules with threefold symmetry axes play a special role, as their rotational spectroscopy allows them to act as practical gas thermometers. Methyl-cyanide (CH3CN) is the second most abundant of those (after ammonia). We compute in this paper the collisional dynamics of methyl-cyanide in collision with helium, for both the A- and the E-symmetries of CH3CN. The potential energy surface is determined using the CCSD(T)-F12b formalism and fit with convenient analytic functions. We compute the rotationally inelastic cross sections for all levels up to 510 cm-1 of collision energy, employing at low energy exact Coupled Channels methods, and at higher energies, approximate Coupled States methods. For temperatures from 7 K up to 300 K, rates of quenching are computed and most are found to differ from those reported earlier (up to a factor of a thousand), calling for a possible reexamination of the temperatures assigned to low density gasses.

A general formulation of the quasiclassical trajectory method for reduced-dimensionality reaction dynamics calculations

Dimension reduction by freezing the unimportant coordinates is widely used in intramolecular and reaction dynamics calculations when the solution of the accurate full-dimensional nuclear Schrödinger equation is not feasible. In this paper we report on a novel form of the exact classical internal-coordinate Hamiltonian for full and reduced-dimensional vibrational motion of polyatomic molecules with the purpose of using it in quasiclassical trajectory (QCT) calculations. The derivation is based on the internal to body-fixed frame transformation, as in the t-vector formalism, however it does not require the introduction of rotational variables to allow cancellation of non-physical rotations within the body-fixed frame. The formulas needed for QCT calculations: normal mode analysis and state sampling as well as for following the dynamics and normal-mode quantum number assignment at instantaneous states are presented. The procedure is demonstrated on the CH4, CD4, CH3D and CHD3 isotopologs of methane using three reduced-dimensional models, which were previously used in quantum reactive scattering studies of the CH4 + X → CH3 + HX type reactions. The reduced-dimensional QCT methodology formulated this way combined with full-dimensional QCT calculations makes possible the classical validation of reduced-dimensional models that are used in the quantum mechanical description of the nuclear dynamics in reactive systems [A. Vikár et al., J. Phys. Chem. A, 2016, 120, 5083-5093].

Semiclassical extension of the Landau-Teller theory of collisional energy transfer

A semiclassical version of the quantum coupled-states approximation for the vibrational relaxation of diatomic molecules in collisions with monatomic bath gases is presented. It is based on the effective mass approximation and a recovery of the semiclassical Landau exponent from the classical Landau-Teller collision time. For an interaction with small anisotropy, the Landau exponent includes first order corrections with respect to the orientational dependence of the collision time and the effective mass. The relaxation N(2)(v=1)-->N(2)(v=0) in He is discussed as an example. Employing the available vibrationally elastic potential, the semiclassical approach describes the temperature dependence of the rate constant k(10)(T) over seven orders of magnitude across the temperature range of 70-3000 K in agreement with experimental data and quantum coupled-states calculations. For this system, the hierarchy of corrections to the Landau-Teller conventional treatment in the order of importance is the following: quantum effects in the energy release, dynamical contributions of the rotation of N(2) to the vibrational transition, and deviations of the interaction potential from a purely repulsive form. The described treatment provides significant simplifications over complete coupled-states calculations such that applications to more complex situations appear promising.

Rotational excitation of water by hydrogen molecules: Comparison of results from classical and quantum mechanics

Quasiclassical trajectory calculations are carried out for rotational excitation of water by hydrogen molecules. State-to-state rate coefficients are determined at 100 K and are compared to available quantum results. A good agreement between classical and quantum rates is observed for downward transitions, with an average accuracy of classical results better than a factor of 2. It is thus found that the ambiguities described by Faure and Wiesenfeld [J. Chem. Phys. 121, 6771 (2004)] can be solved in the particular case of waterlike asymmetric-top molecules.