Department of Chemistry and Chemical Engineering, California Institute of Technology , Pasadena, California 91125, United States.
J Phys Chem A. 2014 Jan 16;118(2):467-74. doi: 10.1021/jp411134v. Epub 2014 Jan 3.
We combine path-integral Monte Carlo methods with high-quality potential energy surfaces to compute equilibrium isotope effects in a variety of systems relevant to 'clumped' isotope analysis and isotope geochemistry, including CO2, N2O, methane, and propane. Through a systematic study of heavy-atom isotope-exchange reactions, we quantify and analyze errors that arise in the widely used Urey model for predicting equilibrium constants of isotope-exchange reactions using reduced partition function ratios. These results illustrate that the Urey model relies on a nontrivial cancellation of errors that can shift the apparent equilibrium temperature by as much as 35 K for a given distribution of isotopologues. The calculations reported here provide the same level of precision as the best existing analytical instrumentation, resolving the relative enrichment of certain isotopologues to as little as 0.01‰. These findings demonstrate path-integral methods to be a rigorous and viable alternative to more approximate methods for heavy-atom geochemical applications.
我们将路径积分蒙特卡罗方法与高质量的势能面相结合，计算了与“聚集”同位素分析和同位素地球化学相关的各种系统中的平衡同位素效应，包括 CO2、N2O、甲烷和丙烷。通过对重原子同位素交换反应的系统研究，我们量化并分析了在广泛使用的 Urey 模型中预测同位素交换反应平衡常数时，使用约化配分函数比所产生的误差。这些结果表明，Urey 模型依赖于一种重要的误差抵消，对于给定的同位素异构体分布，这种误差抵消可以使表观平衡温度偏移多达 35 K。这里报告的计算结果与最好的现有分析仪器一样精确，可以解析某些同位素异构体的相对富集度，低至 0.01‰。这些发现表明，路径积分方法是重原子地球化学应用中更近似方法的一种严格而可行的替代方法。
