Dynamics of the isotope exchange reaction of D with H3+, H2D+, and D2H+

Abstract

We have measured the merged-beams rate coefficient for the titular isotope exchange reactions as a function of the relative collision energy in the range of ∼3 meV-10 eV. The results appear to scale with the number of available sites for deuteration. We have performed extensive theoretical calculations to characterize the zero-point energy corrected reaction path. Vibrationally adiabatic minimum energy paths were obtained using a combination of unrestricted quadratic configuration interaction of single and double excitations and internally contracted multireference configuration interaction calculations. The resulting barrier height, ranging from 68 meV to 89 meV, together with the various asymptotes that may be reached in the collision, was used in a classical over-the-barrier model. All competing endoergic reaction channels were taken into account using a flux reduction factor. This model reproduces all three experimental sets quite satisfactorily. In order to generate thermal rate coefficients down to 10 K, the internal excitation energy distribution of each H + 3 isotopologue is evaluated level by level using available line lists and accurate spectroscopic parameters. Tunneling is accounted for by a direct inclusion of the exact quantum tunneling probability in the evaluation of the cross section. We derive a thermal rate coefficient of < 1 × 10 −12 cm 3 s −1 for temperatures below 44 K, 86 K, and 139 K for the reaction of D with H + 3 , H 2 D + , and D 2 H + , respectively, with tunneling effects included. The derived thermal rate coefficients exceed the ring polymer molecular dynamics prediction of Bulut et al. [J. Phys. Chem. A 123, 8766 (2019)] at all temperatures.

Low-temperature reaction dynamics of paramagnetic species in the gas phase

“…This unique merged beam set-up has enabled, for example, rate coefficients for isotopic exchange in the collisions of D atoms merged with beams of H 3 + , H 2 D + or D 2 H + to be recorded over a large range of collision energies (3 meV–10 eV). 80–82 The measured reaction rate coefficients were found to be consistent with the number of available deuteration sites in the different isotopic exchange systems studied. 80 Interestingly, the experimentally derived rate coefficients exceeded those predicted by quasi-classical trajectory calculations and ring polymer molecular dynamics methods (with the calculations performed on a full-dimensional PES)—at all temperatures considered, and for all three reactions.…”

“…80–82 The measured reaction rate coefficients were found to be consistent with the number of available deuteration sites in the different isotopic exchange systems studied. 80 Interestingly, the experimentally derived rate coefficients exceeded those predicted by quasi-classical trajectory calculations and ring polymer molecular dynamics methods (with the calculations performed on a full-dimensional PES)—at all temperatures considered, and for all three reactions. 83 Again, these findings highlight the importance of detailed experimental measurements over a range of conditions, even for processes as simple as H/D isotopic exchange.…”

Capturing quantum effects with quasi-classical trajectories in the D + H+3 → H2D+ + H reaction

“…The D + H + 3 reaction has been studied experimentally very recently. 10,11 These studies derived a rate constant that decreases with decreasing temperature, becoming quite small below 100 K. These results are in distinct contrast with previous quasi-classical trajectory (QCT) calculations by Moyano et al, 12 who found a slowly decreasing rate constant with increasing temperature from 100 K to 1500 K. By 100 K, the QCT rate constant is B3 orders of magnitude larger than the data of ref. 11.…”

“…10,11 These studies derived a rate constant that decreases with decreasing temperature, becoming quite small below 100 K. These results are in distinct contrast with previous quasi-classical trajectory (QCT) calculations by Moyano et al, 12 who found a slowly decreasing rate constant with increasing temperature from 100 K to 1500 K. By 100 K, the QCT rate constant is B3 orders of magnitude larger than the data of ref. 11. Because ZPE is not constrained during QCT trajectories, the difference may be due in part to the high ZPE of the H + 3 reactant unphysically flowing over the exchange reaction barrier of E3.32 kcal mol À1 in such a way that the vibrational energy of the product ion H 2 D + is lower than its ZPE.…”

“…1 shows the intrinsic reaction coordinate (IRC) path, adapted from ref. 11. The ball and stick diagrams show the important geometries, with the red ball denoting the Deuterium atom.…”