Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan.
Phys Chem Chem Phys. 2018 Jul 11;20(27):18678-18687. doi: 10.1039/c8cp02707b.
We observe UV spectra of protonated dibenzylamine (dBAMH+) and its complexes with 15-crown-5 (dBAMH+-15C5), 18-crown-6 (dBAMH+-18C6), and 24-crown-8 (dBAMH+-24C8) under cold (∼10 K) gas-phase conditions by UV photodissociation (UVPD) and UV-UV hole-burning (HB) spectroscopy. The UVPD spectrum of the dBAMH+-15C5 complex shows an extensive low-frequency progression, which originates from a unique conformation of the dBAMH+ part with benzene rings facing closely to each other, while UVPD and calculation results suggest open conformations of the dBAMH+ part for dBAMH+-18C6 and dBAMH+-24C8. UV-UV HB spectra of the dBAMH+-24C8 complex indicate that there exist at least two conformers; multiple conformations can contribute to high stability of dBAMH+-24C8 pseudorotaxane due to "conformational" entropic effects. The UVPD experiment indicates that the dissociation probability of dBAMH+-24C8 into dBAMH+ and 24C8 is substantially smaller than that of dBAMH+-15C5 and dBAMH+-18C6, which can be related to the barrier height in the dissociation process. The energetics of the dBAMH+-24C8 complex is investigated experimentally with NMR spectroscopy and theoretically with the global reaction route mapping (GRRM) method. An energy barrier of ∼60 kJ mol-1 is present in the pseudorotaxane formation in solution, whereas there is no barrier in the gas phase. In the course of the photodissociation, excited dBAMH+-24C8 complexes can be trapped at many local minima corresponding to multiple conformations. This can result in effective dissipation of internal energy into degrees of freedom not correlated to the dissociation and decrease the dissociation probability for the dBAMH+-24C8 complex in the gas phase. The energy barrier for the pseudorotaxane formation in solution originates not simply from the slippage process but rather from solvent effects on the dBAMH+-24C8 complex.
我们通过紫外光解（UVPD）和紫外-紫外光致空穴烧蚀（HB）光谱法在冷（约 10 K）气相条件下观察了质子化二苄基胺（dBAMH+）及其与 15-冠-5（dBAMH+-15C5）、18-冠-6（dBAMH+-18C6）和 24-冠-8（dBAMH+-24C8）的复合物的紫外光谱。dBAMH+-15C5 复合物的 UVPD 光谱显示出广泛的低频进展，这源于 dBAMH+部分的独特构象，苯环彼此紧密相对，而 UVPD 和计算结果表明 dBAMH+部分的开构象用于 dBAMH+-18C6 和 dBAMH+-24C8。dBAMH+-24C8 复合物的紫外-紫外 HB 光谱表明存在至少两种构象；由于“构象”熵效应，多种构象可以为 dBAMH+-24C8 伪轮烷提供高稳定性。UVPD 实验表明，dBAMH+-24C8 解离为 dBAMH+和 24C8 的概率显著小于 dBAMH+-15C5 和 dBAMH+-18C6，这与解离过程中的势垒高度有关。通过 NMR 光谱实验和全局反应途径映射（GRRM）方法理论研究了 dBAMH+-24C8 复合物的能量学。在溶液中形成伪轮烷存在约 60 kJ mol-1 的能垒，而在气相中则没有能垒。在光解过程中，激发的 dBAMH+-24C8 复合物可以被捕获在许多对应于多种构象的局部极小值处。这可以导致内部能量有效地耗散到与解离不相关的自由度，并降低气相中 dBAMH+-24C8 复合物的解离概率。溶液中伪轮烷形成的能垒并非简单地源于滑移过程，而是源于溶剂对 dBAMH+-24C8 复合物的影响。
