Hydrogen bonding in cyclic complexes of water with tautomeric pairs of molecules M⁰ and M¹ is calculated to be stronger by more than 25% for the less stable tautomer M¹ in all cases where the energy gap between the two tautomers is large (ΔE(M⁰ − M¹) > 10 kcal mol⁻¹). This is accompanied by a large red-shift (>200 cm⁻¹) of the N–H/O–H stretch frequency in the complexes involving M¹. Large barriers for double proton transfer in both directions should permit an experimental verification. Exceptions to this rule were found in heterocycles with an N–CO fragment incorporated into a conjugated cycle resulting in two nearly degenerate tautomers – keto and enol forms. The wavefunction of the keto form has a large contribution from a zwitterionic VB structure which is also aromatic. This increases the polarity of the keto group, making the oxygen atom a strong H-bond acceptor. It can also stabilize the keto form below the aromatic enol form. In this case the extra-HB stabilization is observed for the most stable tautomer (i.e. for the keto form). H-bonding enhances the aromatic character of less aromatic molecules, but the more aromatic tautomers partially loose aromaticity.

中文翻译：

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不稳定的互变异构体形成更强的氢键：水络合物的情况

与互变异构对的分子的水环状复合物的氢键中号⁰和M ¹被计算为更小的稳定是通过25％以上的强互变异构体中号¹在所有情况下，其中两个互变异构体之间的能隙大（Δ Ë（M ⁰ -M ¹）＞ 10kcal mol ^-1）。在涉及M ¹的配合物中，伴随着N–H / O–H伸缩频率的大红移（> 200 cm ^-1）。双向质子转移的较大障碍应允许进行实验验证。在带有NC的杂环中发现了该规则的例外将O片段掺入共轭循环中，产生两种简并的互变异构体-酮和烯醇形式。酮形式的波函数对也是芳族的两性离子VB结构有很大的贡献。这增加了酮基的极性，使氧原子成为强H键受体。它还可以将酮形式稳定在芳族烯醇形式以下。在这种情况下，对于最稳定的互变异构体（即酮形式）观察到了额外的HB稳定作用。H键增强了较少芳族分子的芳族特征，但是较高芳族互变异构体部分失去了芳族性。