Intermolecular coupling of dipole moments is studied for a model system consisting of two diatomic molecules (AB monomers) arranged co-linearly and which can form non-covalently bound dimers. The dipolar coupling is a function of the bond length in each molecule as well as of the distance between the centers-of-mass of the two molecules. The calculations show that intermolecular coupling of the vibrations results in an isotope-dependent modification of the AB-AB intermolecular potential. This in turn alters the energies of the low-lying bound states of the dimers, producing isotope-dependent changes in the AB-AB dimer partition function. Explicit inclusion of intermolecular vibrational coupling then changes the predicted gas-dimer isotopic fractionation. In addition, a mass dependence in the intermolecular potential can also result in changes in the number of bound dimer states in an equilibrium mixture. This in turn leads to a significant dimer population shift in the model monomer-dimer equilibrium system considered here. The results suggest that intermolecular coupling terms should be considered when probing the origins of isotopic fractionation.

中文翻译：

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分子间振动耦合在同位素分馏中的意义的检验

研究了由两个共线排列并且可以形成非共价结合的二聚体的两个双原子分子（AB单体）组成的模型系统的偶极矩的分子间偶合。偶极偶合是每个分子中键长以及两个分子质心之间距离的函数。计算表明，振动的分子间耦合导致AB-AB分子间电势的同位素依赖性修饰。这进而改变了二聚体的低价键合态的能量，从而在AB-AB二聚体分配功能中产生了同位素依赖性的变化。明确包括分子间振动耦合，然后更改预测的气体二聚体同位素分馏。此外，分子间电势的质量依赖性还可以导致平衡混合物中结合的二聚体状态数的变化。这继而导致此处考虑的模型单体-二聚体平衡系统中的显着二聚体迁移。结果表明，在探查同位素分馏的起源时应考虑分子间偶联条件。