Ultraviolet and infrared spectroscopy of neutral and ionic non-covalent diastereomeric complexes in the gas phase

Abstract

Non-covalent intermolecular interactions responsible for chiral discrimination have been investigated in the gas phase both in neutral and ionic complexes. Mass-selected resonant two-photon ionization (R2PI) as well as infrared depleted R2PI (IR–R2PI) techniques have been applied to investigate the role of fluorine substitution in the chiral recognition process between (R)-1-phenyl 1-ethanol (ER), (S)-1-(4-fluorophenyl)-ethanol (pFES), (R)-1-phenyl-2,2,2-trifluoroethanol (FER) and the two enantiomers of butan-2-ol (BR/S), generated in a supersonic molecular beam. The results have been interpreted with the aid of theoretical predictions at the D-B3LYP/6-31G** level of theory. The diastereomeric complexes of ER and pFES with R- and S-butan-2-ol are structurally similar, and dispersive interactions between the aliphatic chain of the alcohol and the π system of the chromophore as well repulsive interactions are mainly responsible for chiral recognition. FER forms, predominantly with S-butan-2-ol, also stable complexes in which the alcohol is oriented away from the aromatic ring. The ionic complexes between pure enantiomers of the bis (diamido)-bridged basket resorcin[4]arene and cytarabine are generated in the gas phase by electrospray ionization and investigated by IRMPD. The proton-bound diastereomers show clearly different IRMPD spectra which, in light of ONIOM (B3LYP/6-31(d):UFF) calculations, are consistent with the occurrence of several isomeric structures, in which the N(3)-protonated guest is either accommodated inside the host cavity or outside it. The spectral differences are attributed to the effects of the intramolecular hydrogen bonding between the C(2′)_OH group and the aglycone oxygen atom of the nucleosidic guest upon repulsive interactions between the same oxygen atom and the aromatic rings of the host.