Department of Chemistry and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon 24341, Korea.
Phys Chem Chem Phys. 2020 Oct 15;22(39):22823-22832. doi: 10.1039/d0cp04407e.
Piperidine has received attention in pharmaceutical synthesis and biochemical degradation because of its conformational activity. We explored the conformational structures of piperidine in the neutral (S0) and cationic (D0) ground states by conformer-specific vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy, which provides high-resolution vibrational spectra for the corresponding cationic conformer. To identify conformers corresponding to the obtained VUV-MATI spectra, equilibrium structures of piperidine conformers in the S0 and D0 states were determined at various density functional theory levels, and potential energy surfaces associated with the conformational changes were constructed. Notably, the chair form interconverting between the equatorial NH and the axial NH conformers (Chair-Eq and Chair-Ax) in piperidine lies on the global minimum of the S0 state, but only the axial-like NH conformer (Chair-Ax-like) in chair form exists in the D0 state. The vibrational assignment of the observed spectra was accomplished through Franck-Condon (FC) analysis for adiabatic transitions between two Chair-Eq and Chair-Ax conformers and a cationic Chair-Ax-like conformer. Rigorous FC analysis revealed the precise structure of a cationic Chair-Ax-like conformer induced by removal of an electron from the lone-pair sp3 orbital of the nitrogen atom in piperidine. The adiabatic ionization energies of Chair-Eq and Chair-Ax conformers converting to a cationic state were determined to be 64 704 ± 4 cm-1 (8.0223 ± 0.0005 eV) and 64 473 ± 4 cm-1 (7.9936 ± 0.0005 eV), respectively. Consequently, the difference between their adiabatic ionization energies allowed the accurate determination of the conformational stability of Chair-Eq and Chair-Ax conformers in piperidine (231 ± 4 cm-1).
哌啶因其构象活性而在药物合成和生化降解方面受到关注。我们通过构象特异性真空紫外光质量分析阈电离（VUV-MATI）光谱研究了中性（S0）和阳离子（D0）基态下哌啶的构象结构，该光谱为相应的阳离子构象提供了高分辨率的振动光谱。为了识别与所获得的 VUV-MATI 光谱相对应的构象，我们在不同密度泛函理论水平下确定了 S0 和 D0 态下哌啶构象的平衡结构，并构建了与构象变化相关的势能面。值得注意的是，哌啶中在赤道 NH 和轴向 NH 构象（Chair-Eq 和 Chair-Ax）之间互变的椅式构象位于 S0 态的全局最低点，但仅存在于 D0 态的轴向 NH 构象（Chair-Ax-like）。通过对两个 Chair-Eq 和 Chair-Ax 构象以及阳离子 Chair-Ax-like 构象之间的绝热跃迁的 Franck-Condon（FC）分析，完成了观察到的光谱的振动归属。严格的 FC 分析揭示了从哌啶氮原子的孤对 sp3 轨道中除去一个电子后诱导的阳离子 Chair-Ax-like 构象的精确结构。将 Chair-Eq 和 Chair-Ax 构象转化为阳离子状态的绝热电离能分别确定为 64704 ± 4 cm-1（8.0223 ± 0.0005 eV）和 64473 ± 4 cm-1（7.9936 ± 0.0005 eV）。因此，它们的绝热电离能之差允许准确确定哌啶中 Chair-Eq 和 Chair-Ax 构象的构象稳定性（231 ± 4 cm-1）。
