VAN-DER-WAALS COMPLEXES BETWEEN CHIRAL MOLECULES IN A SUPERSONIC JET - A NEW SPECTROSCOPIC METHOD FOR ENANTIOMERIC DISCRIMINATION

van der Waals complexation in a supersonic expansion combined with las er-induced fluorescence techniques have been used to discriminate betw een isolated diastereoisomer pairs of weakly bound species. By using 2 -naphthyl-1-ethanol (2-NetOH) as the chiral chromophore, we have inves tigated the effect of complexation with different aliphatic alcohols o n the microscopic shifts of the S-0-S-1 transition and on the fluoresc ence decay times of the chromophore. The fluorescence excitation spect ra of the complexes of 2-NetOH with nonchiral primary alcohols have be en first examined, and the binding energy of the complex of Z-NetOH wi th methanol has been determined to be on the order of 1000 cm(-1). In the case of complexation with chiral solvents such as 2-methyl-1-butan ol or secondary alcohols, the homochiral and heterochiral pairs give r ise to specific spectral shifts and patterns which allow them to be cl early distinguished. The fluorescence lifetimes following; excitation of alcoholic complexes in every case are longer than those of the unco mplexed chromophore and also depend on the particular diastereoisomer excited. The chiral recognition evidenced on the spectral properties a nd on the dynamical relaxation processes of isolated enantiomeric pair s shows the nonequivalence of their interaction energy in both the gro und and excited states. The nature of the stereochemically dependent i nteractions can be tentatively described on the grounds of a hydrogen bonded intermolecular structure involving a folded geometry of the alk yl chain with respect to the naphthalene nucleus.