Single-molecule cold chemistry: Isotopic branching ratios of the H2O+ + HD reaction at cryogenic temperatures
Abstract
Isotopic branching in ion-molecule reactions influences interstellar deuterium fractionation, yet its dynamics remain poorly understood for many important systems at astrophysically relevant temperatures. Here, we investigate H- and D-atom transfer in the H2O+ + HD reaction using single sympathetically cooled water ions in a cryogenic radiofrequency trap. Nondestructive identification of the products from 441 individual reaction events at a mean collision energy of 7 K reveals an H3O+/H2DO+ product branching ratio of 1.50 ± 0.15 demonstrating a preferential H-atom transfer. The experiments are rationalized using quasi-classical-trajectory calculations which indicate a transition from direct dynamics at higher energies to a complex-mediated reaction under the present experimental conditions and point to an important role of intramolecular vibrational redistribution affecting the isotopic branching in this regime. The present results extend measurements into an astrochemically relevant energy regime and establish cryogenic single-ion chemistry as a new platform for resolving competing reaction pathways.
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BibTeX
@misc{kumar2026singlemolecule,
title = {{Single-molecule cold chemistry: Isotopic branching ratios of the $\mathrmH_2O^+ + HD$ reaction at cryogenic temperatures}},
author = {Nanditha Sunil Kumar and Prerna Paliwal and Mikhail Popov and Xueyi Li and Anyang Li and Hua Guo and Stefan Willitsch},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.32904},
eprint = {2610.32904},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.32904},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Single-molecule cold chemistry: Isotopic branching ratios of the H_2O^+ + HD reaction at cryogenic temperatures
AU - Nanditha Sunil Kumar
AU - Prerna Paliwal
AU - Mikhail Popov
AU - Xueyi Li
AU - Anyang Li
AU - Hua Guo
AU - Stefan Willitsch
PY - 2026
DA - 2026/10/23
PB - arXiv
DO - 10.48550/arxiv.2610.32904
UR - https://arxiv.org/abs/2610.32904
AB - Isotopic branching in ion-molecule reactions influences interstellar deuterium fractionation, yet its dynamics remain poorly understood for many important systems at astrophysically relevant temperatures. Here, we investigate H- and D-atom transfer in the H_2O^+ + HD reaction using single sympathetically cooled water ions in a cryogenic radiofrequency trap. Nondestructive identification of the products from 441 individual reaction events at a mean collision energy of 7 K reveals an H_3O^+/H_2DO^+ product branching ratio of 1.50 ± 0.15 demonstrating a preferential H-atom transfer. The experiments are rationalized using quasi-classical-trajectory calculations which indicate a transition from direct dynamics at higher energies to a complex-mediated reaction under the present experimental conditions and point to an important role of intramolecular vibrational redistribution affecting the isotopic branching in this regime. The present results extend measurements into an astrochemically relevant energy regime and establish cryogenic single-ion chemistry as a new platform for resolving competing reaction pathways.
ER -