Molecular Cage Reports on Its Contents: Spectroscopic Signatures of Cryo-Cooled K + - and Ba 2+ -Benzocryptand Complexes
Molecular Cage Reports on Its Contents: Spectroscopic Signatures of Cryo-Cooled K + - and Ba 2+ -Benzocryptand Complexes
复制标题
分子笼报告其内容：低温冷却 K - 和 Ba 2 -苯并隐配体配合物的光谱特征
DOI：
10.1021/acs.jpca.3c03457
复制
发表时间：
2023
期刊：
影响因子：
--
通讯作者：
Zwier, Timothy S.
中科院分区：
文献类型：
--
作者：
Foley, Casey D.;Allen, Cole D.;Au, Kendrew;Lee, Chin;Rempe, Susan B.;Ren, Pengyu;Sibert, Edwin L.;Zwier, Timothy S.
用紫外光碎片光谱和IR-UV双共振法测定了一个三维的[2.2.2]-苯并并笼(标记为K+-BzCrypt，Ba2+-BzCrypt)的结构和对K+或Ba2+掺入的光谱响应。我们研究了在低温条件下分离的离子-密码络合物，通过纳米电喷雾电离带入气相。在三个N-CH2-CH2-O-CH2-CH2-O-CH2-CH2-N链中的一个链中引入苯环取代中心乙基，提供了一个UV发色团，我们可以探测到它的S0-S1转变。K+-BzCrypt和Ba2+-BzCrypt分别在35,925和36,446 cm-1处发生S0-S1原点跃迁，分别比1，2-二甲氧基苯蓝移174和695 cm-1。这些起源被用来选择性地激发每个络合物的单一构象，并使用IR-UV DIP光谱记录它们的红外光谱。烷基CH伸展区域(2800-3000 cm-1)对被包裹离子的存在和性质非常敏感。我们对K+-BzCrypt和Ba2+-BzCrypt的笼形构象进行了详尽的构象搜索，确定了能量低于所有其他构象的两种构象(A和B)。我们将我们的CH拉伸区的局域模非简谐模型推广到这些强结合的离子笼络合物来预测构象特有的烷基CH拉伸谱，得到了与气相全局极小值构象A的定量一致的实验结果。电荷在O-和N-孤子对上的大的静电效应影响着与这些原子相邻的CH2基团的局域模式频率。局域的CH2剪刀模被相邻的O/N原子推高了频率，因此它们的泛音对烷基CH伸缩区的影响很小。然而，定域的CH2振动几乎是简并的，并且彼此强耦合，产生了一系列离域WG简正模，其最高频率成员可达1400 cm-1以上。因此，它们的泛音与CH伸缩模显著混合，最显著的是涉及全烷基链中中心乙基的CH2对称伸缩基元，以及与N原子相邻的CH2伸缩伸展和氮孤对的反周面伸展。
UV photofragment spectroscopy and IR–UV double resonance methods are used to determine the structure and spectroscopic responses of a three-dimensional [2.2.2]-benzocryptand cage to the incorporation of a single K+or Ba2+imbedded inside it (labeled as K+-BzCrypt, Ba2+-BzCrypt). We studied the isolated ion-cryptand complex under cryo-cooled conditions, brought into the gas phase by nano-electrospray ionization. Incorporation of a phenyl ring in place of the central ethyl group in one of the three N-CH2-CH2-O-CH2-CH2-O-CH2-CH2-N chains provides a UV chromophore whose S0–S1transition we probe. K+-BzCrypt and Ba2+-BzCrypt have their S0–S1origin transitions at 35,925 and 36,446 cm–1, respectively, blue-shifted by 174 and 695 cm–1from that of 1,2-dimethoxybenzene. These origins are used to excite a single conformation of each complex selectively and record their IR spectra using IR–UV dip spectroscopy. The alkyl CH stretch region (2800–3000 cm–1) is surprisingly sensitive to the presence and nature of the encapsulated ion. We carried out an exhaustive conformational search of cage conformations for K+-BzCrypt and Ba2+-BzCrypt, identifying two conformations (A and B) that lie below all others in energy. We extend our local mode anharmonic model of the CH stretch region to these strongly bound ion-cage complexes to predict conformation-specific alkyl CH stretch spectra, obtaining quantitative agreement with experiment for conformer A, the gas-phase global minimum. The large electrostatic effect of the charge on the O- and N-lone pairs affects the local mode frequencies of the CH2groups adjacent to these atoms. The localized CH2scissors modes are pushed up in frequency by the adjacent O/N-atoms so that their overtones have little effect on the alkyl CH stretch region. However, the localized CH2wags are nearly degenerate and strongly coupled to one another, producing an array of delocalized wag normal modes, whose highest frequency members reach up above 1400 cm–1. As such, their overtones mix significantly with the CH stretch modes, most notably involving the CH2symmetric stretch fundamentals of the central ethyl groups in the all-alkyl chains and the CH stretches adjacent to the N-atoms and antiperiplanar to the nitrogen lone pair.