Lee Yu Ran, Kim Kyeung Eun, Kim Myung Hwa, Kim Hong Lae, Kwon Chan Ho
New and Renewable Energy Research Center, Ewha Womans University, Seoul 03760, South Korea.
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, South Korea.
J Chem Phys. 2019 Jul 7;151(1):014304. doi: 10.1063/1.5110436.
We report conformational and vibrational assignments of vacuum ultraviolet mass-analyzed threshold ionization spectrum of the isolated gauche conformer, based on previously determined conformer-specific photoionization and conformational stabilities of isobutanal. The vibrational spectrum of the pure cationic gauche conformer was acquired by removing the trans conformer via conformationally effective cooling with Ar carrier gas. The peaks in the spectrum were assigned by Franck-Condon (FC) fitting by adjusting the cationic geometrical parameters of the gauche conformer at the CAM-B3LYP/cc-pVTZ level. Based on the good agreement between the experimental and calculated results, we were able to determine the precise structure of the cationic gauche conformer of isobutanal with C symmetry. Notably, the unveiled vibrational structure was mainly attributed to a geometrical change along the vibrational motions associated with the formyl torsion and CC stretching upon ionization, resulting in their prominent spectral overtones and combination bands with other fundamental vibrations. On the potential energy curve for the formyl torsion of the cationic gauche conformer determined by FC fitting, the transition barrier at 313 cm preserved the hindered formyl torsion in the case of a harmonic potential well, which was confirmed by the progression of formyl torsion, namely, 33, 33, and 33 observed at 60, 120, and 180 cm, respectively.
基于先前确定的异丁醛构象特异性光电离和构象稳定性，我们报告了分离的gauche构象体的真空紫外光质谱分析阈值电离光谱的构象和振动归属。通过用Ar载气进行构象有效冷却去除反式构象体，获得了纯阳离子gauche构象体的振动光谱。通过在CAM-B3LYP/cc-pVTZ水平上调整gauche构象体的阳离子几何参数，用弗兰克-康登（FC）拟合对光谱中的峰进行了归属。基于实验和计算结果之间的良好一致性，我们能够确定具有C对称性的异丁醛阳离子gauche构象体的精确结构。值得注意的是，揭示的振动结构主要归因于电离时与甲酰基扭转和CC拉伸相关的振动运动导致的几何变化，从而产生了它们突出的光谱泛音以及与其他基本振动的组合带。在通过FC拟合确定的阳离子gauche构象体甲酰基扭转的势能曲线上，在313 cm处的过渡势垒在简谐势阱的情况下保留了受阻的甲酰基扭转，这通过甲酰基扭转的进展得到证实，即在60、120和180 cm处分别观察到33、33和33。
