Siskos Michael G, Choudhary M Iqbal, Gerothanassis Ioannis P
Section of Organic Chemistry and Biochemistry, Department of Chemistry, University of Ioannina, Ioannina, GR 45110, Greece.
Org Biomol Chem. 2017 May 31;15(21):4655-4666. doi: 10.1039/c7ob01019b.
Numerous gas phase electron diffraction, ultra-fast electron diffraction, X-ray and neutron diffraction experiments on β-dicarbonyl compounds exhibiting enol-enol tautomeric equilibrium, with emphasis on acetylacetone and dibenzoylmethane, have so far been reported with conflicting results on the structural details of the O-HO intramolecular hydrogen bond and resulted in alternative hypotheses on the intramolecular hydrogen bond potential function either a double minimum potential corresponding to two tautomeric forms in equilibrium or a single symmetrical one. We demonstrate herein, firstly, that the DFT calculated OH H NMR chemical shifts of acetylacetone and dibenzoylmethane exhibit a strong linear dependence on the computed OO hydrogen bond length of ∼-50 ppm Å and as a function of the O-HO bond angle of ∼1 ppm per degree, upon the transfer of the hydrogen atom from the ground state toward the transition state. Secondly, the refinement of labile hydrogen atomic positions in intramolecular hydrogen bonds based on the root-mean-square deviation between experimentally determined and DFT calculated H NMR chemical shifts in solution can provide high resolution structures of O-H and O(H)O bond lengths and O-HO bond angles with an accuracy of ∼10 Å and ∼0.5°, respectively. Thirdly, the calculated H NMR chemical shifts in solution of the two ground state tautomers in equilibrium of acetylacetone and dibenzoylmethane are in excellent agreement with the experimental value, even for moderate basis sets for energy minimization. In contrast, the single symmetrical structure in a strongly delocalized system is a transition state with calculated H NMR chemical shifts which strongly deviate from the experimental value. Fourth, the DFT calculated ground state O-H bond lengths of acetylacetone and dibenzoylmethane are in quantitative agreement with the literature data which take into account the effect of quantum nuclear motion. The DFT structural results are critically discussed with respect to the state-of-the-art variable temperature X-ray and neutron diffraction methods.
迄今为止，已经报道了许多关于呈现烯醇 - 烯醇互变异构平衡的β - 二羰基化合物的气相电子衍射、超快电子衍射、X射线和中子衍射实验，重点是乙酰丙酮和二苯甲酰甲烷，但关于O - HO分子内氢键结构细节的结果相互矛盾，并导致了关于分子内氢键势函数的替代假设，即对应于平衡中两种互变异构形式的双势阱势或单一对称势。我们在此证明，首先，在氢原子从基态向过渡态转移时，乙酰丙酮和二苯甲酰甲烷的密度泛函理论（DFT）计算的OH H NMR化学位移与计算出的OO氢键长度呈现出约 - 50 ppm Å的强线性依赖关系，并作为O - HO键角的函数，每度约为1 ppm。其次，基于溶液中实验测定的和DFT计算的H NMR化学位移之间的均方根偏差对分子内氢键中不稳定氢原子位置进行精修，可以分别以约10 Å和约0.5°的精度提供O - H和O(H)O键长以及O - HO键角的高分辨率结构。第三，即使对于用于能量最小化的中等基组，乙酰丙酮和二苯甲酰甲烷平衡态下两种基态互变异构体在溶液中的计算H NMR化学位移与实验值也非常吻合。相比之下，强离域体系中的单一对称结构是一种过渡态，其计算的H NMR化学位移与实验值有很大偏差。第四，乙酰丙酮和二苯甲酰甲烷的DFT计算的基态O - H键长与考虑了量子核运动影响的文献数据在定量上一致。针对最新的变温X射线和中子衍射方法，对DFT结构结果进行了批判性讨论。
