Ionic Reactant Orientation Inhibits Ion-Neutral Reactions at Low Temperatures.

IF 8.7 Q1 CHEMISTRY, MULTIDISCIPLINARY
JACS Au Pub Date : 2025-08-21 eCollection Date: 2025-09-22 DOI:10.1021/jacsau.5c00613
Yongxu Peng, Junlong Li, Zongao Song, Mengyang Li, Yue Xiao, Xin Wang, Tao Wang, Yurun Xie, Jun Li, Tiangang Yang
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Abstract

Barrierless ion-molecule reactions are critical to interstellar chemistry, particularly at low temperatures. Their rate coefficients are often estimated using classical capture theory, which neglects the spatial orientation of molecular ions. Here, we investigate the reaction between sympathetically cooled BeD+ and O2 using a linear quadrupole ion trap combined with a high-resolution time-of-flight mass spectrometer. Isotopic substitution confirms BeOD+ and O were the only products. The measured rate coefficient, k = (5.4 ± 1.2) × 10-11 cm3/s at the collision energy of 97 K (E/kB), is approximately 15 times lower than that predicted by the classical capture model. Master equation modeling based on electronic structure calculations up to CCSDT-(Q) accurately reproduces the experiment. Unlike the point-charge approximation in capture theory, our results show that reactivity is strongly constrained by the steric requirements of the ionic reactant: only a small subset of orientations between reactants leads to a low-energy barrier that allows the reaction to proceed at low temperatures. These findings underscore the necessity to include steric effects in capture models for accurate predictions.

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离子反应物取向在低温下抑制离子中性反应。
无障碍离子-分子反应对星际化学至关重要,尤其是在低温下。它们的速率系数通常是用经典的俘获理论来估计的,该理论忽略了分子离子的空间取向。在这里,我们使用线性四极离子阱结合高分辨率飞行时间质谱仪研究了共感冷却的BeD+和O2之间的反应。同位素取代证实BeOD+和O是唯一的产物。在碰撞能量为97 k (E/kB)时,测量到的速率系数k =(5.4±1.2)× 10-11 cm3/s,比经典俘获模型预测的速率系数低约15倍。基于CCSDT-(Q)电子结构计算的主方程模型准确地再现了实验。与俘获理论中的点电荷近似不同,我们的结果表明,反应性受到离子反应物的空间要求的强烈约束:只有反应物之间的一小部分取向导致低能量势垒,从而允许反应在低温下进行。这些发现强调了在捕获模型中包含空间效应以进行准确预测的必要性。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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CiteScore
9.10
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