Exact quantum scattering calculations of transport properties: CH2($\tilde{X}^3$X̃3B1, $\tilde{a}^1$ã1A1)–helium

Abstract

Transport properties for collisions of methylene, in both its ground X̃(3)B1 and low-lying ã(1)A1 electronic states, with helium have been computed using recently computed high-quality ab initio potential energy surfaces (PESs). Because of the difference in the orbital occupancy of the two electronic states, the anisotropies of the PESs are quite different. The CH2(ã)-He PES is very anisotropic because of the strong interaction of the electrons on the helium atom with the unoccupied CH2 orbital perpendicular to the molecular plane, while the anisotropy of the CH2(X̃)-He PES is significantly less since this orbital is singly occupied in this case. To investigate the importance of the anisotropy on the transport properties, calculations were performed with the full potential and with the spherical average of the potential for both electronic states. Significant differences (over 20% for the ã state at the highest temperatures considered) in the computed transport properties were found.

First-principles binary diffusion coefficients for H, H2, and four normal alkanes + N2

“…Semiclassical and quantal diffusion coefficients (which are inversely related to the collision integral (1,1) ) have been calculated using accurate full-dimensional potentials for systems with 2-4 atoms (e.g., Refs. [13][14][15][16][17][18][19]. One notable result from these studies is that the neglect of anisotropy in the intermolecular potential has only a small effect on the computed diffusion coefficients.…”

Quantum Scattering Calculations of Transport Properties for the H–N2and H–CH4Collision Pairs

“…For the vast majority of collision pairs for which we have investigated transport properties computed in QS calculations with ab initio PESs, − we found that the QS calculations yielded diffusion coefficients larger than the LJ estimates, for some systems, for example, H–O 2 , H–CO, H–CO 2 , and the present systems, significantly larger. At least part of the difference between the results from QS calculations and the LJ estimates can be traced to the fact that the repulsive wall of the LJ 12–6 potential is steeper than that of a typical ab initio potential .…”

“…We see in Figure for H–N 2 and H–CH 4 that the state-specific collision integrals are reproduced very well by calculations using isotropic potentials. A major exception for the use of an isotropic potential are the CH 2 ( X̃ , ã )–He collision pairs . For these systems, significant error was found in employing isotropic potentials.…”

“…The state-specific cross sections Q i → f ( n ) ( E ) in eq are weighted angle averages of the corresponding differential cross sections: where d R̂ is the orientation of the Jacobi vector R . Explicit formulas for the weighting factors Φ n ( E ), as well as expressions for the state-to-state transport cross sections have been given previously. ,, …”

“…where dR ̂is the orientation of the Jacobi vector R. Explicit formulas for the weighting factors Φ n (E), as well as expressions for the state-to-state transport cross sections have been given previously. 20,39,41 The transport properties needed for combustion simulations are related in the following way to the collision integrals. The binary diffusion coefficient D equals…”