An Experimental and Theoretical Study of Jet-Cooled Complexes of Chiral Molecules:  The Role of Dispersive Forces in Chiral Discrimination

Abstract

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.

Photoinduced Electron Transfer in Jet Cooled Molecular Complexes

“…6b and c). This result contrasts with the properties of a previously studied chiral naphthalene derivative, that bears similar substituents, and that has been shown to exist under a single isomeric form under supersonic-jet conditions [12]. These differences in behaviour may be related to a larger steric hindrance in the case of an anthracene ring.…”

Chirality Recognition between Neutral Molecules in the Gas Phase

“…[62,66] The LIF spectra of all complexes (see Figure 4) share common trends. The transition origin is always shifted down in energy relative to the isolated chromophore, indicating that the chromophore acts as the hydrogen-bond donor.…”

“…The enantioselective shift of the electronic transition relative to the bare chromophore is a signature of chirality discrimination. It can be used to obtain information, though indirect, on the structure of the complexes, in particular on the role of the chromophore as a hydrogen-bond donor or acceptor, [62][63][64][65] the role of dispersion and conformation in the binding energy, [66] and the existence of OH···p interactions. [67][68][69][70] Besides providing mass-selective electronic spectra, REMPI experiments allow measuring the binding energy of the diastereomeric complexes.…”

Requirements for the Formation of a Chiral Template

“…Gas-phase 2-butanol and 2-methylbutanoic acid can adopt several conformations as shown in Figure 4. 34 These are designated ga, ag, and gg (g ) gauche, a ) anti) and are illustrated for the (R)-enantiomer in Figure 4. Previous MP2 calculations have shown that, for 2-butanol, the ga conformation is ∼0.8 kcal/mol more stable than the ag conformer, which in turn is ∼1.2 kcal/mol more stable than the gg structure.…”