Department of Chemistry and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon 24341, Korea.
Phys Chem Chem Phys. 2021 Jan 21;23(2):1414-1423. doi: 10.1039/d0cp05969b.
Isolating and identifying the conformational forms of molecules are imperative processes to investigate the chemical reaction pathways of individual conformers. Herein, we explored the conformational structures of tetrahydropyran in the neutral (S0) and cationic (D0) states by varying the supersonic expansion conditions using one-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The constructed 2D potential energy surfaces associated with conformational interconversion between the chair and boat forms in the S0 and D0 states revealed that the ionic transitions observed in the MATI spectra correspond to the most stable chair conformer. Accordingly, based on the 0-0 band in the VUV-MATI spectrum supported by the VUV photoionization efficiency curve, the adiabatic ionization energy for the conversion of the chair conformer to a cationic state was determined to be 74 687 ± 4 cm-1 (9.2600 ± 0.0005 eV). Definitive vibrational assignment of the measured MATI spectra using Franck-Condon fitting revealed the cationic structure of the twisted chair conformer. The geometrical change upon ionization promoted the vibrational modes associated with ring inversion and deformation motions in the cationic state. This behavior, which was attributed to the effect of electron removal from the highest occupied molecular orbital (HOMO) consisting of the nonbonding orbital of the oxygen atom, reveals the role of electrons in the HOMO.
分离和鉴定分子的构象形式对于研究单个构象体的化学反应途径至关重要。在此，我们通过单光子真空紫外光质量分析阈值电离（VUV-MATI）光谱，改变超声速膨胀条件，探索了四氢吡喃在中性（S0）和阳离子（D0）态下的构象结构。构建的与 S0 和 D0 态中椅式和船式构象之间转化相关的二维势能面表明，MATI 光谱中观察到的离子跃迁对应于最稳定的椅式构象。因此，基于 VUV-MATI 光谱的 0-0 带和 VUV 光致电离效率曲线，确定了椅式构象转化为阳离子态的绝热电离能为 74687±4cm-1（9.2600±0.0005 eV）。使用 Franck-Condon 拟合对测量的 MATI 光谱进行明确的振动归属，揭示了扭曲椅式构象的阳离子结构。电离引起的几何变化促进了阳离子态中环反转和变形运动的振动模式。这种行为归因于从由氧原子非键轨道组成的最高占据分子轨道（HOMO）中去除电子的影响，揭示了电子在 HOMO 中的作用。
