Institute of Chemistry, University of Sao Paulo, CP 26077, 05513-970 Sao Paulo, SP, Brazil.
Spectrochim Acta A Mol Biomol Spectrosc. 2013 Dec;116:196-203. doi: 10.1016/j.saa.2013.07.024. Epub 2013 Jul 25.
Literature data are controversial regarding the conformational equilibria of 2-acetylpyrrole (AP) and its N-methyl derivative (AMP). Now, a detailed study through infrared spectroscopy and theoretical calculations has shown that previous data were erroneously interpreted, since only a N,O-cis conformer is present in solution and that it is the stable conformer in the isolated state (ΔE(trans-cis) = 5.05 kcal mol(-1), for AP; ΔE(trans-cis) = 7.14 kcal mol(-1), for AMP). Carbonyl and NH absorption data in different solvents, supported by theoretical results taking into account the solvent effects [at IEFPCM-B3LYP/6-311++G(3df,3p) level of theory] clearly demonstrated that only the N,O-cis conformer is present in solution. However, a doublet was observed for AP, in CCl4, which can be attributed to this conformer and the lowest wavenumber component to the cis dimer form, stabilized through intermolecular hydrogen bonds (NH · · · OC). The overall preference for the N,O-cis conformer, in AP and AMP, as interpreted by the NBO analysis, indicated that the hyperconjugative effect is the main contribution to stabilize this rotamer, overcoming the possible steric repulsion. (13)C NMR experiments at low temperature in two different solvents (CS2/CDCl2 and acetone-d6) confirmed the occurrence of a single conformer since no separated signals were observed.
文献数据对于 2-乙酰基吡咯（AP）及其 N-甲基衍生物（AMP）的构象平衡存在争议。现在，通过红外光谱和理论计算的详细研究表明，以前的数据被错误地解释了，因为只有 N,O-顺式构象存在于溶液中，并且它是在孤立状态下稳定的构象（ΔE(trans-cis) = 5.05 kcal mol(-1)，对于 AP；ΔE(trans-cis) = 7.14 kcal mol(-1)，对于 AMP）。不同溶剂中羰基和 NH 吸收数据，通过考虑溶剂效应的理论结果（在 IEFPCM-B3LYP/6-311++G(3df,3p)理论水平上）得到支持，清楚地表明只有 N,O-顺式构象存在于溶液中。然而，在 CCl4 中观察到 AP 的双峰，可以归因于这种构象和最低波数组分的顺式二聚体形式，通过分子间氢键（NH····OC）稳定。通过 NBO 分析解释，AP 和 AMP 中 N,O-顺式构象的总体偏好表明超共轭效应是稳定这种旋转异构体的主要贡献，克服了可能的空间排斥。在两种不同溶剂（CS2/CDCl2 和丙酮-d6）中的低温下进行的 13C NMR 实验证实了单个构象的发生，因为没有观察到分离的信号。
