Formation of Hydrogen-Bonded Structures in Jet-Cooled Complexes of a Chiral Chromophore Studied by IR/UV Double Resonance Spectroscopy:  Diastereoisomeric Complexes of (±)-2-Naphthyl-1-ethanol with (±)-2-Amino-1-propanol

Abstract

The structure of weakly bound diastereoisomeric complexes of (±)-2-naphthyl-1-ethanol with (±)-2-amino-1-propanol (alaninol) has been interrogated by means of laser-induced fluorescence and IR fluorescence dip spectroscopy. The diastereoisomers have been discriminated on the basis of the complexation-induced shift of the S0−S1 transition of the chromophore. The heterochiral complex exists under one dominant fluorescent form, while two isomers have been detected for the homochiral complex, with a dramatically different spectroscopic signature in the ν(OH) region. The solely observed heterochiral complex involves an OH···O bond from the chromophore to alaninol, with a quasi retention of the most stable structure of the amino alcohol molecule (intramolecular hydrogen bond). One of the homochiral isomers adopts the same OH···O structure while the second homochiral one involves a OH···N bond to alaninol, whose conformation differs from that of the most stable isolated fragment (absence of the intramolecular hydrogen bond). The observation of both structures of the homochiral complex leaves no doubt as to the fact that isomers containing energetically disfavored fragments can be formed in the supersonic expansion. Moreover, the OH···N structure is only observed in the homochiral complex, which seems to indicate that the two enantiomers of the chromophore do not select the same conformation of alaninol during the complexation process.