We study the rotational and translational dynamics of three small important linear molecules, namely, carbon monoxide (CO), nitric oxide (NO), and cyanide ion (CN⁻) in water–ethanol mixtures, at different compositions. Here, we report a detailed study of the dynamics of these diatomics in water–ethanol binary mixtures for the first time. We find multiple anomalous results, namely, (i) faster rotational motion of CO and NO than CN⁻, (ii) larger translational diffusion of CO and NO in pure ethanol than in water but the reverse for CN⁻, (iii) a pronounced anomaly in the composition dependence of translational–rotational dynamics at low ethanol composition, and (iv) a re-entrant type behavior in the viscosity dependence of orientational relaxation. We compare our simulation result-based observations with the existing experimental results wherever available and find that the simulation results are in reasonably good agreement with the experiments. We implement, for the first time, a calculation of the rotational binary friction following the sophisticated scheme of Evans and co-workers. We also calculate the hydrodynamic predictions for the solute molecules. On comparison with the simulation and experimental results, we find that neither the rotational binary friction nor the hydrodynamic expressions (both stick and slip) can reproduce the results. To rectify the situation, we develop a detailed mode-coupling theory and suggest that such an approach if completely implemented can provide a more reliable description than the hydrodynamic approach. Many of our results could be tested in experimental studies because these linear molecules are amenable to spectroscopic studies, such as 2D-IR.

中文翻译：

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水-乙醇混合物中小双原子的旋转：流体动力学预测的多次分解。

我们研究三个小重要线性分子，即一氧化碳（CO），一氧化氮（NO），和氰化物离子（CN的旋转和平移的动力学^- ）在水-乙醇混合物中，在不同的组合物。在这里，我们首次报道了这些双原子分子在水-乙醇二元混合物中的动力学的详细研究。我们发现多个异常的结果，即，（i）的CO和NO比CN的更快旋转运动^- ，（ⅱ）在纯乙醇的CO和NO的较大的平移扩散比在水中但CN反向^-，（iii）在低乙醇含量下，平移-旋转动力学的成分依赖性存在明显异常，（iv）在取向松弛的粘度依赖性中存在折返型行为。我们将基于模拟结果的观察结果与现有的实验结果进行了比较（如果有），发现模拟结果与实验相当吻合。我们首次按照Evans和同事的复杂方案对旋转二元摩擦进行了计算。我们还计算了溶质分子的流体动力学预测。与仿真和实验结果进行比较，我们发现旋转二元摩擦和流体动力表达式（粘滞和滑移）均不能重现结果。为了纠正这种情况，我们开发了一种详细的模式耦合理论，并建议这种方法（如果完全实施）可以提供比流体力学方法更可靠的描述。我们的许多结果都可以在实验研究中进行测试，因为这些线性分子适合光谱研究，例如2D-IR。