Despite apparent similarity, two isotopomers of acetonitrile (ACN-h₃ and ACN-d₃) show unexpected differences in their structural and dynamic properties. These differences cannot be fully understood within the usual isotope effect and therefore called as unresolved casus. Sensing the solvent properties, the H-bond dynamics of picolinic acid N-oxide (PANO) in ACN was studied using ¹H and ¹⁷O NMR. Several overlapping factors, like the presence of micro-traces of water, liquid-liquid equilibrium, effect of ionic ingredient, etc., which may cause the observed anomalies in the dependences of PANO chemical shift and signal width on temperature, were analyzed. The deuterium isotope effect on the phase separation in organic solute/water mixtures was deduced to be the crucial factor inducing those surprising differences between ACN-h₃ and ACN-d₃. Thus the casus could be resolved within the second-order phase transition model, in which the vibrational contributions to the intermolecular interaction energies would be properly taken into account.

尽管有明显的相似性，但乙腈的两种同功异构体（ACN- h ₃和ACN- d ₃）在结构和动力学性质方面显示出乎意料的差异。这些差异在通常的同位素效应下无法完全理解，因此被称为未解决的因果关系。通过¹ H和^17来研究ACN中的吡啶甲酸N-氧化物（PANO）的H键动力学NMR。分析了一些重叠的因素，例如水的微量元素的存在，液-液平衡，离子成分的影响等，这些因素可能导致所观察到的PANO化学位移和信号宽度对温度的依赖性异常。推断氘同位素对有机溶质/水混合物中相分离的影响是引起ACN- h ₃和ACN- d ₃之间令人惊讶的差异的关键因素。因此，可以在二阶相变模型中解决因果关系，在该模型中，将适当考虑对分子间相互作用能的振动贡献。