The role of weak hydrogen bonds in chiral recognition

Abstract

Chiral recognition has been studied in neutral or ionic weakly bound complexes isolated in the gas phase by combining laser spectroscopy and quantum chemical calculations. Neutral complexes of the two enantiomers of lactic ester derivatives with chiral chromophores have been formed in a supersonic expansion. Their structure has been elucidated by means of IR-UV double resonance spectroscopy in the 3 μm region. In both systems described here, the main interaction ensuring the cohesion of the complex is a strong hydrogen bond between the chromophore and methyl-lactate. However, an additional hydrogen bond of much weaker strength plays a discriminative role between the two enantiomers. For example, the 1:1 heterochiral complex between R-(+)-2-naphthyl-ethanol and S-(+) methyl-lactate is observed, in contrast with the 1:1 homochiral complex which lacks this additional hydrogen bond. On the other hand, the same kind of insertion structures is formed for the complex between S-(±)-cis-1-amino-indan-2-ol and the two enantiomers of methyl-lactate, but an additional addition complex is formed for R-methyl-lactate only. This selectivity rests on the formation of a weak CHπ interaction which is not possible for the other enantiomer. The protonated dimers of Cinchona alkaloids, namely quinine, quinidine, cinchonine and cinchonidine, have been isolated in an ion trap and studied by IRMPD spectroscopy in the region of the ν(OH) and ν(NH) stretch modes. The protonation site is located on the alkaloid nitrogen which acts as a strong hydrogen bond donor in all the dimers studied. While the nature of the intermolecular hydrogen bond is similar in the homochiral and heterochiral complexes, the heterochiral complex displays an additional weak CHO hydrogen bond located on its neutral part, which results in slightly different spectroscopic fingerprints in the ν(OH) stretch region. This first spectroscopic evidence of chiral recognition in protonated dimers opens the way to the study of the complexes of Cinchona alkaloids involved in enantioselective catalysis. These examples show how secondary hydrogen bonds controlled by stereochemical factors govern molecular recognition processes.

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

“…However, the agreement is partly due to error compensation as this hybrid density functional lacks dispersion, which does not make it amenable for large system and for the description of the ancillary forces responsible for chiral recognition. In particular non-conventional OH…π, CH…π or bifurcated hydrogen bonds which often involve dispersion are delicate to describe properly [22,24,153,164,165]. First principle correlated methods should correct for these deficiencies, they involve however prohibitive computation times.…”

“…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].…”

“…MS approaches of chiral recognition have been the subject of several review articles [35,[47][48][49]. However, studies of chiral recognition coupling spectroscopy and MS are still scarce [22,50,51] and this review will focus on them as well as on the photophysical properties of the studied systems. Low temperature as achieved in supersonic expansions and room-temperature experiments, in ion trap or in the bulk, give complementary information.…”

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

“…24 Its hydrates 25 as well as its chiral recognition properties have also been investigated with the same experimental approach in the neutral ground state. 5,26 However, no spectroscopic and theoretical data are available for trans-AI and any of the radical cation or protonated species.…”