Mutual neutralization of H+ and D+ with the atomic halide anions Cl-,Br-, and I. J Chem Phys 2018;149:044303. [PMID: 30068160 DOI: 10.1063/1.5036522]

Neutral gas pressure dependence of ion-ion mutual neutralization rate constants using Landau-Zener theory coupled with trajectory simulations

In this computational study, we describe a self-consistent trajectory simulation approach to capture the effect of neutral gas pressure on ion-ion mutual neutralization (MN) reactions. The electron transfer probability estimated using Landau-Zener (LZ) transition state theory is incorporated into classical trajectory simulations to elicit predictions of MN cross sections in vacuum and rate constants at finite neutral gas pressures. Electronic structure calculations with multireference configuration interaction and large correlation consistent basis sets are used to derive inputs to the LZ theory. The key advance of our trajectory simulation approach is the inclusion of the effect of ion-neutral interactions on MN using a Langevin representation of the effect of background gas on ion transport. For H+ - H- and Li+ - H(D)-, our approach quantitatively agrees with measured speed-dependent cross sections for up to ∼105 m/s. For the ion pair Ne+ - Cl-, our predictions of the MN rate constant at ∼1 Torr are a factor of ∼2 to 3 higher than the experimentally measured value. Similarly, for Xe+ - F- in the pressure range of ∼20 000-80 000 Pa, our predictions of the MN rate constant are ∼20% lower but are in excellent qualitative agreement with experimental data. The paradigm of using trajectory simulations to self-consistently capture the effect of gas pressure on MN reactions advanced here provides avenues for the inclusion of additional nonclassical effects in future work.

Reactions of C+ + Cl-, Br-, and I--A comparison of theory and experiment

Rate constants for the reactions of C⁺ + Cl^-, Br^-, and I^- were measured at 300 K using the variable electron and neutral density electron attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. Upper bounds of <10^-8 cm³ s^-1 were found for the reaction of C⁺ with Br^- and I^-, and a rate constant of 4.2 ± 1.1 × 10^-9 cm³ s^-1 was measured for the reaction with Cl^-. The C⁺ + Cl^- mutual neutralization reaction was studied theoretically from first principles, and a rate constant of 3.9 × 10^-10 cm³ s^-1, an order of magnitude smaller than experiment, was obtained with spin-orbit interactions included using a semiempirical model. The discrepancy between the measured and calculated rate constants could be explained by the fact that in the experiment, the total loss of C⁺ ions was measured, while the theoretical treatment did not include the associative ionization channel. The charge transfer was found to take place at small internuclear distances, and the spin-orbit interaction was found to have a minor effect on the rate constant.

Mutual neutralization in collisions of H+ with Cl. J Chem Phys 2019;151:214305. [PMID: 31822073 DOI: 10.1063/1.5128357]

The cross section and final state distribution for mutual neutralization in collisions of H⁺ with Cl^- have been calculated using an ab initio quantum mechanical approach. It is based on potential energy curves and nonadiabatic coupling elements for the six lowest ¹Σ⁺ states of HCl computed with the multireference configuration interaction method. The reaction is found to be driven by nonadiabatic interactions occurring at relatively small internuclear distances (R < 6 a₀). Effects on the mutual neutralization cross section with respect to the asymptotic form of the potential energy curves, inclusion of closed channels, as well as isotopic substitution are investigated. The effect of spin-orbit interaction is investigated using a semiempirical model and found to be small. A simple two-state Landau-Zener calculation fails to predict the cross section.