Detailed NMR spectral analysis of DMSO-d6 solutions of the series of substituted 2-phenacylbenzimidazoles (ketimine form, K) reveals two from three tautomeric forms. Integrals of the 1H NMR signals are used in establishing the molar ratio of tautomers. The experimental analyses are supported by quantum-chemical calculations, which satisfactorily reproduced the experimental trends. Although the reported semiempirical quantum-chemical calculations show that enaminone E, i.e., 2-(1,3-dihydro-2H-benzo[d]imidazol-2-ylidene)-1-phenylethan-1-one, was thermodynamically most stable, the results of MP2 ab initio calculations reveal the following order of stability: ketimine > enolimine > enaminone (substituents do not affect this sequence). 13C CPMAS NMR spectral data reveal that in the crystalline state the enolimine tautomer O is predominant in the p-CH3 and p-NO2 substituted congeners.

中文翻译：

---

2-苯并咪唑溶液中的互变异构平衡

一系列取代的 2-苯甲酰苯并咪唑（酮亚胺形式，K）的 DMSO-d6 溶液的详细 NMR 光谱分析揭示了三种互变异构形式中的两种。1H NMR 信号的积分用于确定互变异构体的摩尔比。实验分析得到了量子化学计算的支持，该计算令人满意地再现了实验趋势。尽管报道的半经验量子化学计算表明烯胺酮 E，即 2-(1,3-dihydro-2H-benzo[d]imidazol-2-ylidene)-1-phenylethan-1-one，在热力学上是最稳定的， MP2 从头计算的结果揭示了以下稳定性顺序：酮亚胺 > 烯胺 > 烯胺酮（取代基不影响该序列）。