The observed solvatochromism of the betaine 4-(2,4,6-triphenyl-1-pyridinio)phenolate (2a) in solvents of low and very high relative permittivity has been assessed both experimentally and theoretically using molecular orbital methods. The PM3/COSMO method suggests that there are a number of possible conformations for the betaine involving clockwise or anti-clockwise rotation of the four pendant phenyl groups relative to the central heterocyclic ring. The large thermochromic effect observed for the dye in acetone or tetrahydrofuran on moving from room temperature to −78 °C has been attributed to a combination of conformational changes coupled with an increase in the relative permittivity of the respective solvent. The calculated spectroscopic shifts for the betaine using a multi-electron configuration interaction treatment show similar trends to those found experimentally in aprotic solvents. In solvents with acidic hydrogens, the large hypsochromic shift observed for the betaine in the visible region arises from both a dielectric effect and a hydrogen bonding effect. Stable hydrogen bonded structures are predicted to be formed between either water, chloroform, or acetonitrile and the exocyclic oxygen atom of the betaine. The overall shifts observed in these solvents show a good correlation with those calculated for the postulated solvates using a version of the CNDO/S method.
利用分子轨道方法,对
甜菜碱 4-(2,4,6-三苯基-1-
吡啶)
苯酚(2a)在低和极高相对介电常数溶剂中的溶解变色现象进行了实验和理论评估。PM3/COSMO 方法表明,
甜菜碱有多种可能的构象,其中涉及四个悬垂苯基相对于中心杂环的顺时针或逆时针旋转。在
丙酮或
四氢呋喃中观察到的
染料从室温移至 -78 °C时的巨大热变色效应可归因于构象变化与相应溶剂相对介电常数增加的结合。利用多电子构型相互作用处理方法计算出的
甜菜碱光谱偏移显示出与在非沸腾溶剂中实验发现的相似趋势。在具有酸性氢的溶剂中,
甜菜碱在可见光区域观察到的大的低色度偏移来自介电效应和氢键效应。根据预测,
水、
氯仿或
乙腈与
甜菜碱的外环氧原子之间会形成稳定的氢键结构。在这些溶剂中观察到的总体偏移与使用 CNDO/S 方法计算出的假定溶质的偏移具有良好的相关性。