Binding energy of hydrogen-bonded complexes of the chiral molecule 1-phenylethanol, as studied by 2C-R2PI: comparison between diastereoisomeric complexes with butan-2-ol and the singly hydrated complex

The relative gas phase binding energy of the two diastereoisomeric complexes of R-1-phenylethanol with R- and S-butan-2-ol formed in a supersonic expansion has been obtained from fragmentation measurements following two-colour resonance two-photon ionisation of the complex. The homochiral species (Rr) is found to be more stable than the heterochiral complex (Rs) by about 0.7 kcal mol(-1). The present study also points out two possible reasons for an underestimation of the absolute binding energy when using a photofragmentation technique: (i) when the adiabatic ionisation threshold of the chromophore (here, 1-phenylethanol) is not accessible by photoionisation because of a too large geometry change between the neutral species and the ion, only lower limits of the binding energies can be drawn; and (ii) when the solvent (here, butan-2-ol) exists under different conformers, the most stable form of the complex can involve a conformation different from the most stable one in the isolated fragment. The lowest-energy fragmentation channel in the ion nevertheless corresponds to the formation of the most stable form of the fragments. In the present case, this latter effect may contribute to an anomalous low binding energy of the complexes of 1-phenylethanol with butan-2-ol compared to the 1:1 hydrate.