Conformational Analysis of Quinine and Its Pseudo Enantiomer Quinidine: A Combined Jet-Cooled Spectroscopy and Vibrational Circular Dichroism Study

Abstract

Laser-desorbed quinine and quinidine have been studied in the gas phase by combining supersonic expansion with laser spectroscopy, namely, laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and IR-UV double resonance experiments. Density funtional theory (DFT) calculations have been done in conjunction with the experimental work. The first electronic transition of quinine and quinidine is of π-π* nature, and the studied molecules weakly fluoresce in the gas phase, in contrast to what was observed in solution (Qin, W. W.; et al. J. Phys. Chem. C2009, 113, 11790). The two pseudo enantiomers quinine and quinidine show limited differences in the gas phase; their main conformation is of open type as it is in solution. However, vibrational circular dichroism (VCD) experiments in solution show that additional conformers exist in condensed phase for quinidine, which are not observed for quinine. This difference in behavior between the two pseudo enantiomers is discussed.

Photophysical behavior and fluorescence quenching by halides of quinidine dication: Steady state and time resolved study

“…This can be explained by observed a small shift in excitation spectra when monitored across the emission profile and a double exponential decay for shorter wavelengths. The difference in the photophysical behavior when compared to quinine sulfate cation (QS þ þ ) may be due to structural difference in these two compounds as discussed earlier [8,[28][29][30]. In earlier works, in case of quinine dication, an intramolecular bond has been suggested which, however, is absent in quinidine [8].…”

Chirality effects in gas-phase spectroscopy and photophysics of molecular and ionic complexes: contribution of low and room temperature studies

“…They are of more limited use because of the difficulty of putting non-labile neutral molecules in the gas phase, despite the possibility offered by laser-ablation sources [18][19][20]. However, coupled to well-resolved spectroscopic technique, they provide a unique tool for getting precise structural information and for understanding the intimate nature of the forces responsible for chiral recognition [21,22].…”

“…The aim of our study is to compare the properties of these four cinchona alkaloids isolated in the gas phase, to see whether the solid-state differences manifest themselves in isolated conditions too, and to evidence the consequences of pseudo-enantiomerism on the spectroscopy and photophysics of neutral and protonated species [20,22,220]. Lowtemperature experiments allow separating the effects of energy from those of entropy which are visible at higher temperature.…”

“…It is therefore a sensitive probe of the presence of higher energy conformers which cannot be accessed in jet-cooled experiments. VCD spectroscopy has been applied to the pseudo-enantiomers Qn and Qd [20]. Going from gas phase to CDCl 3 solution leads to the stabilisation of higher energy conformers.…”

Diastereo-specific conformational properties of neutral, protonated and radical cation forms of (1R,2S)-cis- and (1R,2R)-trans-amino-indanol by gas phase spectroscopy

“…[3][4][5][6][7][8][9][10] The stereochemical effects observed in diastereomers isolated in the gas phase are often weak. Indeed most of the neutral systems studied so far like peptides, 11,12 polyols, 13 or amino-alcohols like neurotransmitters 14 and cinchona alkaloids 15,16 only display minor structural and spectroscopic differences. However, larger differences are observed between diastereomers interacting with a cation.…”