Beijing National Laboratory of Molecular Science, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China; University of Chinese Academy of Sciences, Beijing 100049, PR China.
Beijing National Laboratory of Molecular Science, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China.
Spectrochim Acta A Mol Biomol Spectrosc. 2020 Jan 5;224:117398. doi: 10.1016/j.saa.2019.117398. Epub 2019 Jul 17.
The resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra of 3-fluoro-5-methylanisole (3F5MA) were recorded to explore the conformers arising from the rotation of meta-methyl group (m-CH) and methoxy group (OCH), namely the staggered (s)/eclipsed (e)-cis/trans 3F5MA. The theoretical calculations predicted that the stable conformer of cis 3F5MA is staggered in the S and S states, but eclipsed in the D state. While for trans 3F5MA, the staggered conformer is stable only in the S state and the eclipsed one is stable in the S and D states. The first electronic excitation energies (Es) of cis and trans 3F5MA were determined to be 36,709 ± 3 and 36,615 ± 3 cm by the R2PI spectroscopy. Correspondingly, by the MATI spectroscopy, the adiabatic ionization energies (IEs) were measured to be 66,908 ± 5 and 66,692 ± 5 cm. Compared with the cis 3F5MA, more low-frequency vibronic bands assigned to m-CH torsions are observed for the trans conformer in the R2PI spectrum, supporting the theoretically predicated e-trans → s-trans isomerization upon excitation. The MATI spectra of the cis and trans conformers are similar. Most of the observed cationic bands are related to the m-CH torsion modes, revealing the s-cis → e-cis and s-trans → e-trans isomerizations upon the D ← S ionization. By comparing with several analogues, it is found that the s/e conformational preference in each electronic state is mainly influenced by OCH group instead of the F atom. The combined effects of meta-substituents on molecular conformation and transition energies are discussed in detail. A kind of additivity of meta-substituent effects on s/e preference is qualitatively true for the S and S states but false for the D state. This is different from that of ortho-substituent effect, which is previously reported that the additivity of ortho-substituent effects on methyl rotation barriers in 2-fluoro-6-chlorotoluene is applicable to all the three electronic states while the additivity of meta-substituent effect on cis/trans preference is found to be true in all three electronic states of 3F5MA.
记录了 3-氟-5-甲基苯甲醚（3F5MA）的共振增强双光子电离（R2PI）和质量分析阈电离（MATI）光谱，以研究来自间甲基（m-CH）和甲氧基（OCH）旋转的构象，即交错（s）/重叠（e）-顺/反式 3F5MA。理论计算预测，顺式 3F5MA 的稳定构象在 S 和 S 态下是交错的，而在 D 态下是重叠的。而对于反式 3F5MA，交错构象仅在 S 态下稳定，重叠构象在 S 和 D 态下稳定。通过 R2PI 光谱，确定顺式和反式 3F5MA 的第一电子激发能（Es）分别为 36,709 ± 3 和 36,615 ± 3 cm。相应地，通过 MATI 光谱，测量得到的绝热电离能（IEs）分别为 66,908 ± 5 和 66,692 ± 5 cm。与顺式 3F5MA 相比，在 R2PI 光谱中观察到更多分配给 m-CH 扭转的低频振动态带，支持激发后理论预测的 e-顺式 → s-顺式异构化。顺式和反式构象的 MATI 光谱相似。观察到的大多数阳离子带与 m-CH 扭转模式有关，表明 D → S 电离后发生 s-顺式 → e-顺式和 s-反式 → e-反式异构化。通过与几个类似物进行比较，发现每个电子态中 s/e 构象偏好主要受 OCH 基团而不是 F 原子的影响。详细讨论了间位取代基对分子构象和跃迁能的综合影响。间位取代基对 s/e 偏好的加和性在 S 和 S 态下定性上是正确的，但在 D 态下是错误的。这与先前报道的邻位取代基效应不同，邻位取代基对 2-氟-6-氯甲苯中甲基旋转势垒的加和性适用于所有三个电子态，而在 3F5MA 的所有三个电子态中都发现间位取代基对顺/反式偏好的加和性是正确的。
