The conformational properties of 1,3-diindolylureas and thioureas were studied by a combination of heteronuclear NMR spectroscopy and quantum mechanics calculations. NOE experiments showed that the anti–anti conformer along the C7–N7α bonds was predominant in DMSO-d6 solution in the absence of anions. Anion-induced changes in the 1H and 15N chemical shifts confirm the weak binding of chloride anions with negligible conformational changes. Strong deshielding of ureido protons and moderate deshielding of indole NH was observed upon the addition of acetate, benzoate, bicarbonate and dihydrogen phosphate, which indicated that the predominant hydrogen bond interactions occurred at the urea donor groups. Binding of oxo-anions caused conformational changes along the C7–N7α bonds and the syn–syn conformer was preferred for anion–receptor complexes. The conformational changes upon anion binding are in good agreement with energetic preferences established by ab initio calculations.
1,3-二吲哚基脲和硫脲的构象性质通过异核NMR光谱和量子力学计算的结合研究。NOE实验表明,在缺乏阴离子的DMSO-d6溶液中,沿着C7-N7α键的anti-anti构象是主导的。氯离子引起的1H和15N化学位移的改变证实了氯离子的弱结合性,且构象变化可以忽略不计。在添加乙酸盐、苯甲酸盐、碳酸氢盐和二氢磷酸盐后,脲基质子的强脱屏和吲哚NH的适度脱屏表明主要氢键相互作用发生在脲供体基团处。氧化阴离子的结合导致了沿着C7-N7α键的构象变化,并且syn-syn构象更倾向于阴离子-受体复合物。阴离子结合引起的构象变化与从头算计算所确定的能量偏好基本一致。