Short-Range Interactions within Molecular Complexes Formed in Supersonic Beams: Structural Effects and Chiral Discrimination

Abstract

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.

Photochemical R2PI study of chirality and intermolecular forces in supersonic beam

“…As already found in the clusters P R T R and P R T S [3] the excitation spectrum of E R T R is red shifted (∆ν = −101 cm −1 ) relative to that of the bare chromophore, while the excitation spectrum of E R T S is characterized by a multiplet that is blue shifted relative to the band origin of the bare E R (∆ν = +5 cm −1 ). The different extent of the spectral shifts of these diastereomeric clusters reflects the combined effects of attractive (electrostatic and dispersive) and repulsive (steric) interactions on the HOMO and LUMO energies of E R solv, and indicates that steric factors play a different role, which reduce the effect of the attractive electrostatic and dispersive interaction.…”

“…The most intense band of the spectrum, assigned to the electronic band origin of the cluster, is red-shifted relative to that of the bare chromophore, at 37618 cm −1 . This indicates that the chromophore acts as a proton donor [3] and that the binding energy of the adducts is greater in the S 1 state than in the S 0 state [3,11,12]. The red shift of the heterochiral complex (∆ν = −131 cm −1 ) exceeds that of the homochiral one (∆ν = −119 cm −1 ) by ∆(∆ν) = 12 cm −1 .…”

“…These studies gave insights into the attractive and repulsive forces operating in these systems, and it was found that the homochiral complexes are more stable than the heterochiral ones [1][2][3][4][5].…”

Molecular and supramolecular chirality: R2PI spectroscopy as a tool for the gas‐phase recognition of chiral systems of biological interest

“…R2PI spectroscopy of diastereomeric clusters has been employed in the past, mainly by our group, to study the spectroscopic properties and the interaction energies in hydrogen bonded clusters of sev- eral chiral aromatic alcohols (cromophores) bonded to a variety of solvent molecules, including chiral mono and bifunctional alcohols, amines, and water. [115][116][117] The shifts of the electronic transitions, excited state vibrational frequencies, binding energies, and activation barriers of these complexes were determined, showing their sensitivity to the chirality of their components. More recently, we focused our attention on the study of chiral molecules of biological or pharmaceutical interest, or tailor-made complexes containing components which mimic the building blocks of large biomolecules.…”

“…The most stable anti conformer and at least one gauche rotamer have been identified in the electronic spectrum of the bare molecule. 115,125 The intramolecular hydrogen bond, usually operative in bifunctional alcohols (solv) in the gas phase, can be either retained or disrupted and replaced by a more stabilizing intermolecular hydrogen bond network when solv is associated with a protic chromophore, like P R . The hydrogen bond networks can be classified in three groups (Fig.…”