Conformational preferences and isomerization upon excitation/ionization of 2-methoxypyridine and 2-N-methylaminopyridine

Abstract

Conformers from the rotations of the methyl group and the methoxy or methylamino group, namely staggered (s)/eclipsed (e)-cis/trans 2-methoxypyridine (2MOP) and 2-N-methylaminopyridine (2NMP), are studied using theoretical calculations in combination with one-color resonance-enhanced two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopies. The calculations predict that, for cis 2MOP, trans 2MOP and trans 2NMP, only the s conformers are stable in the S, S and D states. However, for cis 2NMP, the stable conformer is staggered in the S state but eclipsed in the S and D states, indicating an isomerization upon the excitation or ionization from the S state. This is experimentally supported by the 1C-R2PI and MATI spectra of 2NMP. Due to the relative instability, the number density of trans 2MOP is too low in the sample to be detected. All the bands in the 1C-R2PI and MATI spectra of 2MOP are assigned to s-cis 2MOP. The energy differences between cis and trans conformers are derived from excitation and ionization energies, indicating another conformational isomerization: stable trans 2NMP in the S and S states but stable cis 2NMP in the D state. For 2MOP, the so-called syn preference previously found for the S state is also observed in the S and D states. The conformational preference and isomerization are discussed with natural bond orbital calculations and reduced density gradient analysis. For 2MOP, the syn preferences are mainly caused by the exchange repulsion among several σ-orbitals of the OCH group and the pyridine ring. While the relative stabilities of the s and e conformers of cis 2MOP and cis 2NMP are simultaneously influenced by steric repulsion and orbital interactions.

Modulation of Conformational Preferences of Heteroaromatic Ethers and Amides through Protonation and Ionization: Charge Effect

“…It can be seen that, for all the ethers having an ortho ‐N heteroatom 1 – 28 , the syn conformers have lower potential energy and are more stable than the corresponding anti ones, suggesting the syn preferences . This means that the ortho ‐N heteroatom has a strong syn ‐preferring effect, which is also verified by performing resonance enhanced two‐photon ionization experiments on 2‐[6]‐1N 1 (Figure ) and the cationic spectroscopy . On the other hand, for ethers having an ortho ‐O or ortho ‐S but no ortho ‐N (ethers 29 – 36 ), most have anti preferences (ethers 29 – 34 , 36 ), except for the 2‐[5]1O4N5N 35 (with very weak syn conformational preference).…”

“…The E 1 of anti conformer is predicted by CIS/cc‐pVTZ calculations (the electronic energy corrected by EOM‐CCSD/cc‐pVTZ) at about 107 cm −1 lower than that of the syn conformer, which is not observed in the 1C‐R2PI spectrum, indicating too low population to be detected. The observed bands are assigned based on the cationic spectroscopy …”

“…For example, the pH‐triggered conformational switch of heteroaromatic lipids can control the channels on membrane . The ionization/oxidation‐induced conformational switching of some aromatic compounds is also found . If the protonation or ionization can modulate the conformation of heteroaromatics, the heterocyclic hinges can operate by pH or redox.…”

“…[17] The ionization/oxidation-induced conformational switching of some aromatic compounds is also found. [18,19] If the protonation or ionization can modulate the conformation of heteroaromatics, the heterocyclic hinges can operate by pH or redox. Since both the protonation and ionization introduce one positive charge into the molecule, the effect of the positive charge on stability of molecular conformers will provide information helpful for understanding the origin of conformational preferences.…”

“…[11,23] This means that the ortho-N heteroatom has a strong synpreferring effect, which is also verified by performing resonance enhanced two-photon ionization experiments on 2-[6]-1N 1 (Figure 2) and the cationic spectroscopy. [18] On the other hand, for ethers having an ortho-O or ortho-S but no ortho-N (ethers 29-36), most have anti preferences (ethers 29-34, 36), except Figure 1. The syn conformers of o-methoxyheteroarenes.…”