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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Structure of neutral molecules and monoanions of selected oxopurines in aqueous solutions as studied by NMR spectroscopy and theoretical calculations
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Structure of neutral molecules and monoanions of selected oxopurines in aqueous solutions as studied by NMR spectroscopy and theoretical calculations

机译：核磁共振波谱法和理论计算研究水溶液中选定氧代嘌呤的中性分子和单阴离子的结构

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摘要

A methodology enabling investigation of a multicomponent tautomeric and acid-base equilibria by 13C NMR spectroscopy supported by theoretical calculations has been proposed. The effectiveness of this method has been illustrated in a study of 2-oxopurine, 6-oxopurine (hypoxanthine), 8-oxopurine, and 2,6-dioxopurine (xanthine) in neutral and alkaline aqueous solutions. For each compound a series of 13C NMR spectra were recorded at pH ranges in which neutral molecules, monoanions and/or dianions occurred in dynamic equilibrium. The carbon chemical shifts for these three forms of the investigated compounds were retrieved from the analysis of pH-dependence of the measured, dynamically averaged values of these parameters. The structures of several stable tautomers of the neutral and monoanionic oxopurine forms were predicted from theoretical calculations and nuclear magnetic shielding constants for 13C nuclei in these tautomers were calculated. At both calculation steps (molecular geometry optimization and calculation of NMR parameters) the PBE1PBE/6-311++G(2d,p) level of theory was used. The populations of the most stable tautomers were determined from the experimental data analysis exploiting the fact that they were population-weighted averages of the chemical shifts of particular tautomers. It has been shown that only the oxo forms of the investigated oxopurines are present in aqueous solutions and that the determined populations in most cases remain in a qualitative agreement with the calculated free energies of the appropriate tautomers. The obtained results are in general agreement with other literature reports on oxopurine tautomerism and confirm importance of the hydration phenomena for the investigated systems. The data analysis has shown that the best compliance between theory and experiment is obtained when the hydration phenomenon is modeled by discrete hydration augmented by PCM (polarizable continuum solvation model).
机译：提出了一种能够通过13C NMR光谱研究理论计算支持的多组分互变异构和酸碱平衡的方法。在中性和碱性水溶液中对2-氧代嘌呤，6-氧代嘌呤（次黄嘌呤），8-氧代嘌呤和2,6-二氧代嘌呤（黄嘌呤）的研究中证明了该方法的有效性。对于每种化合物，在pH范围内记录了一系列13C NMR光谱，在该pH范围内，中性分子，一价阴离子和/或二价阴离子动态平衡。这三种形式的被研究化合物的碳化学位移是通过对这些参数的动态平均值进行pH依赖性分析来获得的。通过理论计算预测了几种稳定的中性和单阴离子氧嘌呤形式的稳定互变异构体的结构，并计算了这些互变异构体中13 C核的核磁屏蔽常数。在两个计算步骤（分子几何优化和NMR参数的计算）中，使用了理论级的PBE1PBE / 6-311 ++ G（2d，p）。最稳定的互变异构体的种群是根据实验数据分析确定的，这些事实是它们是特定互变异构体化学位移的种群加权平均值。已经表明，水溶液中仅存在所研究的氧代嘌呤的氧代形式，并且在大多数情况下，所确定的种群与合适的互变异构体的计算出的自由能在质量上保持一致。所得结果与关于氧嘌呤互变异构的其他文献报道大体上一致，并证实了水合现象对于所研究系统的重要性。数据分析表明，当水化现象通过用PCM（可极化连续介质模型）增强的离散水化进行建模时，可获得最佳的理论与实验一致性。

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