Position-specific and clumped isotope compositions that reveal intramolecular isotope distributions can offer novel insights into the physical and chemical properties of substances. In particular, the intramolecular isotope effects observed in propane have demonstrated significant potential for constraining the formation and evolution of hydrocarbons. To calibrate measurements and interpret observations, a comprehensive understanding of equilibrium isotope fractionation within propane is required. However, previous studies yielded inconsistent calculations of position-specific isotope equilibria, and lacked any prediction of clumped isotope equilibria in the methyl group of propane. Here, we employ quantum chemical calculations beyond the harmonic approximation and the Born-Oppenheimer approximation to study the intramolecular isotope equilibria in propane. Benchmark coupled cluster calculations (CCSD(T)) were used to produce reliable frequencies of propane isotopologues. By integrating the CCSD(T) outcomes with effects beyond the harmonic and Born-Oppenheimer approximations, we present high-accuracy equilibrium carbon and hydrogen isotope fractionation between the methylene and methyl sites, and report for the first time the equilibrium ΔCHD and ΔCHD signatures of the methyl group in propane. Our results of position-specific isotope equilibria can serve as a reference frame for calibrating both theoretical calculations and experimental measurements. The equilibrium ΔCHD and ΔCHD values provide a theoretical framework for further studies of CD and DD clumping in methyl groups of alkanes, and are valuable for tracing the sources and sinks of methyl groups as well as processes related to methylation.

中文翻译：

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丙烷中的位置特异性和聚集同位素平衡：超越谐波和玻恩-奥本海默近似的从头计算

揭示分子内同位素分布的位置特异性和聚集同位素组成可以为物质的物理和化学性质提供新的见解。特别是，在丙烷中观察到的分子内同位素效应已证明具有限制碳氢化合物形成和演化的巨大潜力。为了校准测量和解释观察结果，需要全面了解丙烷内的平衡同位素分馏。然而，以前的研究对特定位置同位素平衡的计算结果不一致，并且缺乏对丙烷甲基中聚集同位素平衡的任何预测。在这里，我们采用谐波近似和玻恩-奥本海默近似之外的量子化学计算来研究丙烷中的分子内同位素平衡。使用基准耦合簇计算 (CCSD(T)) 来产生丙烷同位素体的可靠频率。通过将 CCSD(T) 结果与谐波和玻恩-奥本海默近似之外的影响相结合，我们提出了亚甲基和甲基位点之间的高精度平衡碳和氢同位素分馏，并首次报告了平衡 ΔCHD 和 ΔCHD 特征丙烷中的甲基。我们的特定位置同位素平衡结果可以作为校准理论计算和实验测量的参考框架。平衡ΔCHD和ΔCHD值为进一步研究烷烃甲基中CD和DD的聚集提供了理论框架，对于追踪甲基的来源和汇以及甲基化相关过程具有重要价值。