Department of Physics, Faculty of Science and Arts, Bitlis Eren University, 13000 Bitlis, Turkey.
CQC, Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal.
Molecules. 2021 May 10;26(9):2814. doi: 10.3390/molecules26092814.
A new Schiff base compound, 3-(5-bromo-2-hydroxybenzylideneamino)phenol (abbreviated as BHAP) was synthesized and characterized by H- and C- nuclear magnetic resonance and infrared spectroscopies. DFT/B3LYP/6-311++G(d,p) calculations were undertaken in order to explore the conformational space of both the - and - geometrical isomers of the enol-imine and keto-amine tautomers of the compound. Optimized geometries and relative energies were obtained, and it was shown that the most stable species is the -enol-imine form, which may exist in four low-energy intramolecularly hydrogen-bonded forms (I, II, V, and VI) that are almost isoenergetic. These conformers were concluded to exist in the gas phase equilibrium with nearly equal populations. On the other hand, the infrared spectra of the compound isolated in a cryogenic argon matrix (10 K) are compatible with the presence in the matrix of only two of these conformers (conformers II and V), while conformers I and VI convert to these ones by quantum mechanical tunneling through the barrier associated with the rotation of the OH phenolic group around the C-O bond. The matrix isolation infrared spectrum was then assigned and interpreted with help of the DFT(B3LYP)/6-311++G(d,p) calculated infrared spectra for conformers II and V. In addition, natural bond orbital (NBO) analysis was performed on the most stable conformer of the experimentally relevant isomeric form (-enol-imino conformer V) to shed light on details of its electronic structure. This investigation stresses the fundamental structural relevance of the O-H···N intramolecular H-bond in -hydroxyaryl Schiff base compounds.
一种新的席夫碱化合物，3-(5-溴-2-羟基亚苄基氨基)苯酚（简称 BHAP），通过氢核和碳核磁共振以及红外光谱进行了合成和表征。为了探索该化合物的烯醇-亚胺和酮-胺互变异构体的-和-几何异构体的构象空间，进行了 DFT/B3LYP/6-311++G(d,p)计算。得到了优化的几何形状和相对能量，表明最稳定的物种是-烯醇-亚胺形式，它可能以四种低能量的分子内氢键形式（I、II、V 和 VI）存在，这些形式几乎是等能量的。这些构象被认为在气相中处于平衡状态，其存在的比例几乎相等。另一方面，在低温氩气基质（10 K）中分离得到的化合物的红外光谱与基质中仅存在这两种构象（构象 II 和 V）是一致的，而构象 I 和 VI 通过围绕 C-O 键旋转的 OH 酚基团的量子力学隧穿转化为这两种构象。然后，通过对实验相关异构形式（-烯醇-亚胺构象 V）的最稳定构象进行 DFT(B3LYP)/6-311++G(d,p)计算红外光谱的帮助，对基质分离红外光谱进行了分配和解释。此外，对实验相关的异构形式（-烯醇-亚胺构象 V）的最稳定构象进行了自然键轨道（NBO）分析，以揭示其电子结构的细节。这项研究强调了 O-H···N 分子内氢键在-羟基芳基席夫碱化合物中的基本结构相关性。
