One- and two-color, mass-selected R2PI spectra of the S1<--S0 transitions in the bare chiral chromophore R-(+)-1-phenyl-1-propanol (R) and its complexes with a variety of alcoholic solvent molecules (solv), namely methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, S-(+)-2-butanol, R-(-)-2-butanol, 1-pentanol, S-(+)-2-pentanol, R-(-)-2-pentanol, and 3-pentanol, were recorded after a supersonic molecular beam expansion. Spectral analysis, coupled with theoretical calculations, indicate that several hydrogen-bonded [R.solv] conformers are present in the beam. The R2PI excitation spectra of [R.solv] are characterized by significant shifts of their band origin relative to that of bare R. The extent and direction of these spectral shifts depend on the structure and configuration of solv and are attributed to different short-range interactions in the ground and excited [R.solv] complexes. Measurement of the binding energies of [R.solv] in their neutral and ionic states points to a subtle balance between attractive (electrostatic and dispersive) and repulsive (steric) forces, which control the spectral features of the complexes and allow enantiomeric discrimination of chiral solv molecules.

Enantiomeric discrimination takes place when a chiral selector (C) forms with a pair of enantiomers (M) two diastereomeric molecular complexes (MCs) of different stability (thermodynamic enantioselectivity). This is the basis of chiral chromatography, enzymic resolution, asymmetric synthesis, and NMR spectroscopic discrimination with chiral auxiliaries. [1,2] Although the principles of thermodynamic enantioselectivity find eminent applications in many fields, very few experiments have been designed so far to establish at the microscopic level the nature and energetics of the various interactions in diastereomeric MCs.Thermometric measurements of the energetics of aggregation of chiral ions in solution to make diastereomeric ion pairs were first carried out by Arnett and Zingg. [3] These authors pointed out that the average difference in thermodynamic stability between diastereomeric combinations of several chiral amines with mandelic acid enantiomers may span from zero to 200 ± 350 cal mol À1 , depending upon the structure of the amine and the nature of the solvent.A way to evaluate the intrinsic nature of the various interactions in diastereomeric MCs and to eliminate the moderating effects of solvent on their energetics is to study their features in the isolated state. Here we report on the first spectroscopic determination of the binding energy in isolated diastereomeric MCs, with special regard to the dependence of the binding energy on the configuration of the chiral solvent molecule.Weakly bound MCs, which would be unobservable at room temperature, can be readily generated in the isolated state by supersonic expansion of their components. Under these conditions, they are formed at an average temperature of a few Kelvins [4] and can be spectroscopically discriminated. The spectral analysis is facilitated by the fact that, at low temperatures, only the lowest rotational and vibrational levels are populated. Lahmani and co-workers recently characterized jet-cooled diastereomeric MCs by different laser-induced fluorescence (LIF) spectra. [5±7] Discrimination between diastereomeric MCs was obtained by us [8] with resonanceenhanced multiphoton ionization (REMPI) spectroscopy combined with time-of-flight (TOF) mass spectrometry. [4, 9] Accordingly, the REMPI-TOF spectra of the diastereomeric molecular complexes Rr and RsÐformed from the combination of the chiral chromophore (R)-()-1-phenyl-1-propanol (R) with (R)-(À) (r) and (S)-()-2-butanol (s), respectivelyÐ display different features. In particular, their most intense band, assigned to the electronic band origin, is red-shifted relative to the S 1 2 S 0 band origin of the isolated R molecule (Dn À 79 cm À1 (Rr), À 92 cm À1 (Rs)), indicating an enhancement of binding energy of both Rr and Rs adducts in the S 1 state relative to the S 0 state. The difference between the red shifts (Dn[Rr] À Dn[Rs] 13 cm À1 ) reflects a S 1 2 S 0 energy gap for Rs which is smaller than for Rr.The binding energy of the diastereomeric Rr and Rs adducts was measured by two color ...

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