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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Mit einer Kombination aus hochauflösender Spektroskopie und Flugzeit‐Massenspektrometrie konnten erstmals die grundlegenden Eigenschaften der in einem Überschallstrahl expandierten van‐der‐Waals‐Komplexe zwischen (R)‐(+)‐1‐Phenyl‐1‐propanol (PR) und den Enantiomeren von 2‐Butanol (BR und BS) auf mikroskopischem Niveau untersucht werden. Die chirale Erkennung kommt sowohl in den unterschiedlichen Fragmentierungsmustern von PRBR und PRBS als auch in der unterschiedlichen bathochromen Verschiebung ihrer elektronischen Bandenzentren bezüglich der reinen Substanz PR zum Ausdruck.

A combination of high‐resolution spectroscopy and time‐of‐flight mass spectrometry was used for the first time to study the fundamental properties of supersonically expanded van der Waals complexes between (R)‐(+)‐1‐phneyl‐1‐propanol (PR) and the enantiomers of 2‐butanol (BR or BS) at the microscopic level. Chiral discrimination is based on the different fragmentation patterns of PRBR and PRBS or on the different bathochromic shift of their electronic band origin relative to that of pure PR.

One colour resonance enhanced two photon ionization of van der Waals complexes of(R +)-1-phenyl-1-propanol clustered with chiral and non chiral compounds have been studied. Enantiodifferentiation, different fragmentation patterns and reactivity are discussed on the basis of experiments.