Sanz Pablo, Mó Otilia, Yañez Manuel, Elguero José
Departamento de Química, C-9, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain.
J Phys Chem A. 2007 May 10;111(18):3585-91. doi: 10.1021/jp067514q. Epub 2007 Apr 13.
The characteristics of the intramolecular hydrogen bond (IMHB) for a series of 40 different enols of beta-diketones and their nitrogen counterparts have been systematically analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In some cases, two tautomers may exist which are interconnected by a hydrogen shift through the IMHB. In tautomer a the HB donor group (YH) is attached to the six-membered ring, while in tautomer b the HB acceptor (X) is the one that is attached to the six-membered ring. We found that changing an O to a N favors the a tautomer when the atom is endo and the contrary when it is exo, while the presence of a double bond favors the a tautomers. As expected, the OH group behaves as a better HB donor than the NH2 group and the C=NH group as a better HB acceptor than the C=O group, although the first effect clearly dominates. Accordingly, the expected IMHB strength follows the [donor, acceptor] trend: [OH, C=NH] > [OH, C=O] > [NH2, C=NH] > [NH2, C=O]. For all those compounds in which the functionality exhibiting the IMHB is unsaturated (I-type), the IMHB is much stronger than in their saturated counterparts (II-type). However, when the systems of the II-type subset, which are saturated, are constrained to have the HB donor and the HB acceptor lying in the same plane and at the same distance as in the corresponding unsaturated analogue, the IMHB is of similar or even larger strength. Hence, we conclude that, at least for this series of unsaturated compounds, the resonance-assisted hydrogen bond effect is not the primary reason behind the strength of their IMHBs, which is simply a consequence of the structure of the sigma-skeleton of the system that keeps the HB donor and the HB acceptor coplanar and closer to each other.
在B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p)理论水平下，对一系列40种不同的β-二酮烯醇及其含氮类似物的分子内氢键（IMHB）特征进行了系统分析。在某些情况下，可能存在两种互变异构体，它们通过分子内氢键的氢转移相互连接。在互变异构体a中，氢键供体基团（YH）连接到六元环上，而在互变异构体b中，氢键受体（X）连接到六元环上。我们发现，当原子为内型时，将O换成N有利于互变异构体a，而当原子为外型时则相反，而双键的存在有利于互变异构体a。正如预期的那样，OH基团作为氢键供体比NH2基团表现更好，C=NH基团作为氢键受体比C=O基团表现更好，尽管第一种效应明显占主导。因此，预期的分子内氢键强度遵循[供体，受体]趋势：[OH，C=NH]>[OH，C=O]>[NH2，C=NH]>[NH2，C=O]。对于所有那些表现出分子内氢键的官能团为不饱和的化合物（I型），其分子内氢键比饱和对应物（II型）强得多。然而，当II型子集的饱和体系被限制为使氢键供体和氢键受体与相应的不饱和类似物处于同一平面且距离相同时，分子内氢键强度相似甚至更大。因此，我们得出结论，至少对于这一系列不饱和化合物，共振辅助氢键效应不是其分子内氢键强度的主要原因，这仅仅是体系σ骨架结构的结果，该结构使氢键供体和氢键受体共面且彼此更靠近。
