Shubert V Alvin, James William H, Zwier Timothy S
Purdue University, Department of Chemistry, West Lafayette, Indiana 47907-2084, USA.
J Phys Chem A. 2009 Jul 16;113(28):8055-66. doi: 10.1021/jp904231d.
Laser-induced fluorescence (LIF), ultraviolet hole-burning (UVHB), and resonant ion-dip infrared (RIDIR) spectroscopies were carried out on isolated benzo-15-crown-5 ether (B15C) and 4'-amino-benzo-15-crown-5 ether (ABC) cooled in a supersonic expansion. Three conformational isomers of B15C and four of ABC were observed and spectroscopically characterized. Full optimizations and harmonic frequency calculations were undertaken for the full set of almost 1700 conformational minima identified in a molecular mechanics force field search. When compared with TDDFT predictions, the S(0)-S(1) origin positions serve as a useful diagnostic of the conformation of the crown ether near the phenyl ring responsible for the UV absorption and to the position of the NH(2) substituent. In-plane orientations for the beta carbons produce red-shifted S(0)-S(1) origins, while out-of-plane "buckling" produces substantial blue shifts of 600 cm(-1) or more. Comparison between the alkyl CH stretch spectra of B15C and ABC divide the spectra into common subgroups shared by the two molecules. The high-frequency CH stretch transitions (above 2930 cm(-1)) reflect the number of CH...O interactions, which in turn track in a general way the degree of buckling of the crown. On this basis, assignments of each of the observed conformational isomers to a class of structure can be made. All the observed structures have some degree of buckling to them, indicating that in the absence of a strong-binding partner, the crown folds in on itself to gain additional stabilization from weak dispersive and CH...O interactions.
对在超声速膨胀中冷却的孤立苯并 - 15 - 冠 - 5 醚（B15C）和 4'-氨基 - 苯并 - 15 - 冠 - 5 醚（ABC）进行了激光诱导荧光（LIF）、紫外空穴燃烧（UVHB）和共振离子偶极红外（RIDIR）光谱研究。观察到了 B15C 的三种构象异构体和 ABC 的四种构象异构体，并对其进行了光谱表征。针对在分子力学力场搜索中确定的近 1700 个构象极小值的完整集合进行了完全优化和谐波频率计算。与 TDDFT 预测相比，S(0)-S(1) 起源位置可作为一种有用的诊断方法，用于确定负责紫外吸收的苯环附近冠醚的构象以及 NH₂ 取代基的位置。β 碳的面内取向会产生红移的 S(0)-S(1) 起源，而面外“弯曲”会产生 600 cm⁻¹ 或更大的显著蓝移。B15C 和 ABC 的烷基 CH 伸缩光谱之间的比较将光谱分为两个分子共有的常见子组。高频 CH 伸缩跃迁（高于 2930 cm⁻¹）反映了 CH...O 相互作用的数量，这又大致跟踪了冠的弯曲程度。在此基础上，可以将每个观察到的构象异构体归为一类结构。所有观察到的结构都有一定程度的弯曲，这表明在没有强结合伙伴的情况下，冠会自身折叠以从弱色散和 CH...O 相互作用中获得额外的稳定性。
