Rotational excitation of HCN by para- and ortho-H2

Interaction of methanol with molecular hydrogen: Ab initio potential energy surface and scattering calculations

The potential energy surface (PES) describing the interaction of the methanol molecule with molecular hydrogen has been calculated by the use of the explicitly correlated coupled cluster method, including single, double, and (perturbative) triple excitations [CCSD(T)-F12a] and a correlation-consistent aug-cc-pVTZ basis, with the assumption of fixed molecular geometries. The computed points were fit to a functional form appropriate for time-independent quantum scattering calculations of rotationally inelastic cross sections and rate coefficients. Stationary points on the PES were located, and the global minimum was found to have an energy equal to -254.7 cm-1 relative to the energy of the separated molecules. This PES was used in time-independent close coupling quantum scattering calculations to determine state-to-state cross sections and rate coefficients for rotational transitions within the A- and E-type nuclear spin torsional ground states.

Collisional excitation of HCNH+ by He and H2: New potential energy surfaces and inelastic rate coefficients

Protonated molecules have been increasingly detected in the interstellar medium (ISM), and usually astrochemical models fail at reproducing the abundances derived from observational spectra. Rigorous interpretation of the detected interstellar emission lines requires prior calculations of collisional rate coefficients with H₂ and He, i.e., the most abundant species in the ISM. In this work, we focus on the excitation of HCNH⁺ induced by collision with H₂ and He. Therefore, we first calculate ab initio potential energy surfaces (PESs) using the explicitly correlated and standard coupled cluster method with single, double, and non-iterative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta basis set. Both the HCNH⁺-H₂ and HCNH⁺-He potentials are characterized by deep global minima of 1426.60 and 271.72 cm^-1, respectively, and large anisotropies. From these PESs, we derive state-to-state inelastic cross sections for the 16 low-lying rotational energy levels of HCNH⁺ using the quantum mechanical close-coupling approach. The differences between cross sections due to ortho- and para-H₂ impacts turn out to be minor. Using a thermal average of these data, we retrieve downward rate coefficients for kinetic temperatures of up to 100 K. As it could be anticipated, differences of up to two orders of magnitude exist between the rate coefficients induced by H₂ and He collisions. We expect that our new collision data will help to improve the disagreement between abundances retrieved from observational spectra and astrochemical models.

Quantum Dynamics of Rotational Energy Transfer Processes for N2-HF and N2-DF Systems

The rate coefficients of rotationally inelastic collision processes for N₂-HF as well as N₂-DF systems were calculated by applying the recently developed coupled-states approximation including the nearest neighbor Coriolis couplings approach, based on the full-dimensional ab initio intermolecular potential energy surface. It was found that the energy gap law governs these energy transfer processes. For rotational quenching of N₂ (j₁ = 2-10) by the ground rotational state of HF, j₁ = 6 and 5 have the maximum quenching rate for ortho-N₂ and para-N₂, respectively. Quenching rate coefficients for initially excited HF and DF (j₂ = 1) in collisions with N₂ were also reported, where N₂-DF has a larger quenching rate than N₂-HF due to larger density of states of the N₂-DF system.

Interaction of the HCO radical with molecular hydrogen: Ab initio potential energy surface and scattering calculations

The potential energy surface describing the interaction of the HCO radical with molecular hydrogen has been computed through explicitly correlated coupled cluster calculations including single, double, and (perturbative) triple excitations [RCCSD(T)-F12a], with the assumption of fixed molecular geometries. The computed points were fit to an analytical form suitable for time-independent quantum scattering calculations of rotationally inelastic cross sections and rate coefficients. Since the spin-rotation splittings in HCO are small, cross sections for fine-structure resolved transitions are computed with electron-spin free T matrix elements through the recoupling technique usually employed to determine hyperfine-resolved cross sections. Both spin-free and fine-structure resolved state-to-state cross sections for rotationally inelastic transitions are presented and discussed.

Interaction of the H2S molecule with molecular hydrogen: Ab initio potential energy surface and scattering calculations

The potential energy surface describing the interaction of H₂S with molecular hydrogen has been computed through explicitly correlated coupled cluster calculations including single, double, and (perturbative) triple excitations [CCSD(T)-F12a], with the assumption of fixed molecular geometries. The computed points were fit to an analytical form suitable for time-independent quantum scattering calculations of rotationally inelastic cross sections and rate coefficients. Representative sets of energy dependent state-to-state rotationally inelastic cross sections and rate coefficients are presented and discussed.

Interaction of C2H with molecular hydrogen: Ab initio potential energy surface and scattering calculations

The potential energy surface (PES) describing the interaction of the ethynyl (C₂H) radical in its ground X̃²Σ⁺ electronic state with molecular hydrogen has been computed through restricted coupled cluster calculations including single, double, and (perturbative) triple excitations [RCCSD(T)], with the assumption of fixed molecular geometries. The computed points were fit to an analytical form suitable for time-independent quantum scattering calculations of rotationally inelastic cross sections and rate constants. A representative set of energy dependent state-to-state cross sections is presented and discussed. The PES and cross sections for collisions of H₂(j = 0) are compared with a previous study [F. Najar et al., Chem. Phys. Lett. 614, 251 (2014)] of collisions of C₂H with H₂ treated as a spherical collision partner. Good agreement is found between the two sets of calculations when the H₂ molecule in the present calculation is spherically averaged.

Inelastic Scattering of Identical Molecules within Framework of the Mixed Quantum/Classical Theory: Application to Rotational Excitations in H2 + H2

Theoretical foundation is laid out for description of permutation symmetry in the inelastic scattering processes that involve collisions of two identical molecules, within the framework of the mixed quantum/classical theory (MQCT). In this approach, the rotational (and vibrational) states of two molecules are treated quantum-mechanically, whereas their translational motion (responsible for scattering) is treated classically. This theory is applied to H2 + H2 system, and the state-to-state transition cross sections are compared versus those obtained from the full-quantum calculations and experimental results from the literature. Good agreement is found in all cases. It is also found that results of MQCT, where the Coriolis coupling is included classically, are somewhat closer to exact full-quantum results than results of the other approximate quantum methods, where those coupling terms are neglected. These new developments allow applications of MQCT to a broad variety of molecular systems and processes.