Electronic nonadiabatic effects in low temperature radical-radical reactions. I. C(3P) + OH(2Π)

Abstract

The formation of collision complexes, as a first step towards reaction, in collisions between two open-electronic shell radicals is treated within an adiabatic channel approach. Adiabatic channel potentials are constructed on the basis of asymptotic electrostatic, induction, dispersion, and exchange interactions, accounting for spin-orbit coupling within the multitude of electronic states arising from the separated reactants. Suitable coupling schemes (such as rotational + electronic) are designed to secure maximum adiabaticity of the channels. The reaction between C((3)P) and OH((2)Π) is treated as a representative example. The results show that the low temperature association rate coefficients in general cannot be represented by results obtained with a single (generally the lowest) potential energy surface of the adduct, asymptotically reaching the lowest fine-structure states of the reactants, and a factor accounting for the thermal population of the latter states. Instead, the influence of non-Born-Oppenheimer couplings within the multitude of electronic states arising during the encounter markedly increases the capture rates. This effect extends up to temperatures of several hundred K.

The vibrational relaxation of NO in Ar: tunneling in a curve-crossing mechanism

“…In our paper, which addresses a more modest aim to only estimate the tunneling correction without full calculation of the rate coefficient, we have identified the ''reaction coordinate'' of the transition state approach by considering the shape of the crossing line found from the asymptotic form of the exchange interaction between the 2p 00 electron of MO and electrons of the Ar atom, the approach which has already been introduced quite some time ago 3,4 and used recently again. 16 In this way, it became possible to derive a simplified expression for the tunneling correction coefficient that depends on two dimensionless parameters, d and W, which in turn are expressed through the conventional physical parameters, the energy E a at the crossing point, the steepness of the potentials a at the crossing point and the effective mass m eff for the motion along the ''reaction coordinate''. Once the latter is identified, E a , a, and m eff can be determined for any crossing PES, including those calculated ab initio in different approximations.…”

“…This scheme of transformation of asymptotic states into the states with strong interfragment interaction is discussed in detail in ref. 16. Within the transition state approach, the initial canonical distribution over the states of collision partners becomes a canonical distribution over the states of the activated complex.…”

Reactions of N+(3P) ions with H2and HD molecules at low temperatures

“…The neglect of fine structure effects in the NH + (X 2 Π) product molecule is certainly a simplification, but it should be satisfactory since we are only interested in the sum over all final fine structure states of the products. If information is required on the distribution of the fine structure states of the reaction products, a more complete treatment, such as the approach of Maergoiz et al (2014), can be used.…”

Statistical Theory of Low-Energy Reactive Collisions of N+ Ions with H2, D2, and HD Molecules

“…Recently, A. Maergoiz et al, 12 in their study of adduct formation in collisions of C( 3 P) + OH( 2 Π), developed a more advanced approach in which the FS states of both open-shell reactants have been treated on the same footing as rotations. Our simpler model will be applied here to reactions of N + ions with H 2 , D 2 , and HD molecules and the calculated cross sections compared with experimental results of Sunderlin where j a = 0−2 are the FS quantum numbers of the initial state of the N + ion, (ν, j) are the initial vibrational and rotational quantum numbers of H 2 , and j a0 and (ν k ,j k ), k = 0,1 are the corresponding quantum numbers in nonreactive (k = 0) and reactive (k = 1) channels.…”