The present study combines both experiment and molecular dynamics simulations in order to document the ionization behavior of the C 6 H 6 -H 2 O and C 6 H 6 -D 2 O complexes close to the ionization threshold, in particular its nonadiabatic character. Using the two-color two-photon resonant ionization laser technique, the ionization thresholds of these species have been measured together with the threshold for dissociative ionization. A binding energy has been deduced for the neutral species: D 0 (C 6 H 6 -H 2 O) ) 106 ( 4 meV and D 0 (C 6 H 6 -D 2 O) ) 116 ( 5 meV, which significantly increases the precision compared to literature. Using a semiempirical potential model, the minimum energy structures of the neutral and ionic species have been determined, and the potential energy surfaces have been analyzed using a two-dimensional approach. As a result, the formation of a stable C 6 H 6 + -H 2 O complex close to the threshold is found to be controlled by a pure quantum effect and is ascribed to the classically forbidden region of the neutral ground state wave function for the intermolecular vibrational motion. Using classical molecular dynamics simulations in order to sample this region, it has been shown that the neutral conformations involved in the production of stable ions at the ionization threshold exhibit a strong geometry change compared to the neutral equilibrium conformation; i.e., the water molecule is strongly shifted off the benzene C 6 axis and is also flipped over backward the benzene ring. The difference in the ionization energy of the C 6 H 6 -H 2 O and C 6 H 6 -D 2 O complexes, which cannot be explained by the difference in the neutral binding energies alone, supports this result.
Isomer formation in dimeric complexes of a chiral naphthalene derivative (2-naphthyl-1-ethanol) with nonchiral or chiral primary and secondary alcohols (n-propanol, 2-methyl-1-butanol, 2-butanol, 2-pentanol) has been studied by hole-burning spectroscopy. Besides the spectroscopic discrimination between the homochiral and heterochiral complexes, previously observed in the fluorescence excitation spectra, ground-state depletion experiments have shown that each diastereoisomer is cooled in the jet in several isomeric forms. To get information on the structures of the complexes and on the influence of the solvent conformations of these structures, semiempirical calculations that rely on the exchange perturbation method have been performed. It has been shown that the most stable complexes involve a H-bond between the chromophore acting as the donor and the solvent and that they involve anti and gauche conformations of the solvent. The binding energy of the complexes results from a subtle balance between electrostatic and dispersive forces: the complexes involving the gauche and anti conformers of the solvent differ from each other by the amount of dispersion energy relative to the total interaction energy. The increase in the dispersive forces calculated for the complexes with the anti conformers has been related to a larger red shift of the absorption spectrum and is suggested to play a role in the observed chiral discrimination.
gamma-Turn, the shortest secondary structure of peptides, exists as two helical forms gamma(l) and gamma(d) of opposite handedness. The present gas phase study of capped l-Phe-Xxx peptides (Xxx = l-Ala, d-Ala or Aib: aminoisobutyric acid) provides a unique example of intramolecular chiral recognition of the gamma-turn helicity on Ala or Aib by the neighbouring residue Phe within the chain. With the chiral l- or d-Ala residues, the presence of a side-chain operates a discrimination between the two helical forms: one of them is widely favoured over the other (gamma(l) or gamma(d), respectively). This enables us to validate and calibrate the recognition capabilities of the nearby l-Phe residue. The discriminating interactions have been precisely characterized from their spectroscopic UV and IR signatures and identified by comparison with quantum chemistry calculations. Then, in the case of the non-chiral residue Aib, the two helical forms of the gamma-turn, which are simultaneously observed in the jet, have been discriminated and assigned by comparison with the chiral residues. The relative abundances of the diastereomeric forms l-Phe-Aib(gamma(l)) and l-Phe-Aib(gamma(d)) enable us to determine the most efficient recognition configuration.
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