Issue 39, 2020

Accurate conformational stability and cationic structure of piperidine determined by conformer-specific VUV-MATI spectroscopy

Abstract

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).

Graphical abstract: Accurate conformational stability and cationic structure of piperidine determined by conformer-specific VUV-MATI spectroscopy

Supplementary files

Article information

Article type
Paper
Submitted
19 Aug 2020
Accepted
28 Sep 2020
First published
29 Sep 2020

Phys. Chem. Chem. Phys., 2020,22, 22823-22832

Accurate conformational stability and cationic structure of piperidine determined by conformer-specific VUV-MATI spectroscopy

S. Y. Eom, Y. R. Lee and C. H. Kwon, Phys. Chem. Chem. Phys., 2020, 22, 22823 DOI: 10.1039/D0CP04407E

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