Kristinaitytė Kristina, Maršalka Aru Nas, Dagys Laurynas, Aidas Kęstutis, Doroshenko Iryna, Vaskivskyi Yevhenii, Chernolevska Yelyzaveta, Pogorelov Valeriy, Valevičienė Nomeda Rima, Balevicius Vytautas
Faculty of Physics , Vilnius University , Sauletekio al. 3 , LT-10257 Vilnius , Lithuania.
Faculty of Physics , Taras Shevchenko National University of Kyiv , Glushkova Avenue 4 , 03022 Kyiv , Ukraine.
J Phys Chem B. 2018 Mar 29;122(12):3047-3055. doi: 10.1021/acs.jpcb.8b00350. Epub 2018 Mar 15.
Temperature and composition effects in Sunset Yellow FCF (SSY) aqueous solutions were studied by the H, N NMR as well as Raman spectroscopy passing through all phase transitions between isotropic phase (I) and chromonic phases-nematic (N) and columnar (M). It was shown that the tautomeric equilibrium in SSY is strongly shifted toward the hydrazone form. The corresponding equilibrium constant p K = 2.5 was deduced using the density functional theory solvent model density model. The dominance of the hydrazone form was confirmed experimentally using the long-range H-N correlation, widely known as heteronuclear multiple bond correlation. The peak found in the H NMR spectra at ca. 14.5 ppm can be attributed to the proton in the intramolecular N-H···O bond. The existence of this signal shows that (i) the growth of the SSY aggregates is accompanied by the segregation of water in the intercolumnar areas with no access for exchange with the N-H protons in the internal layers of the columnar stacks and that (ii) the lifetime of those aggregates is ≥10 s or even longer. The temperature dependences of HO chemical shift and Raman O-H stretching band shape show that water confined in the intercolumnar areas behaves as in the neat substance. When the sample is heated and the transition from M phase to N phase occurs, the molecular motion of water is seen to change in a manner similar to that when water is melting. The equilibration time for N + M→ M is very long because of slow supramolecular restructuring, i.e., the growing of columnar stacks and building of hexagonal arrays. If the sample is cooled down to the temperature below N → M transition relatively fast, the structural changes are behind, and the system falls into supercooled state. In this case, the system evolves via long-lasting self-assembling from the supercooled state to the equilibrium. This process affects the shape of the H NMR signal and is easy to monitor.
通过氢核磁共振（¹H NMR）以及拉曼光谱研究了日落黄FCF（SSY）水溶液在各向同性相（I）与发色团相（向列相（N）和柱状相（M））之间所有相变过程中的温度和成分效应。结果表明，SSY中的互变异构平衡强烈地向腙形式偏移。使用密度泛函理论溶剂模型密度模型推导出相应的平衡常数pK = 2.5。通过长程¹H-¹⁵N相关（广为人知的异核多键相关）实验证实了腙形式的优势。在¹H NMR光谱中约14.5 ppm处发现的峰可归因于分子内N-H···O键中的质子。该信号的存在表明：（i）SSY聚集体的生长伴随着柱状区域间水的分离，使得柱状堆叠内层中的N-H质子无法进行交换；（ii）这些聚集体的寿命≥10 s甚至更长。¹H化学位移和拉曼O-H伸缩带形状的温度依赖性表明，限制在柱状区域间的水的行为与纯物质中的水相似。当样品加热并发生从M相到N相的转变时，可以看到水的分子运动变化方式类似于水熔化时的变化方式。由于超分子结构重组缓慢，即柱状堆叠的生长和六边形阵列的形成，N + M→M的平衡时间非常长。如果将样品相对快速地冷却到低于N→M转变的温度，结构变化滞后，系统进入过冷状态。在这种情况下，系统通过从过冷状态到平衡的持久自组装过程演化。这个过程会影响¹H NMR信号的形状，并且易于监测。
