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. Sono stati investigati gli spettri di ionizzazione bifotonica risonante (R2PI) a uno e due colori delle transizioni S1←S0 del cromoforo R-(+)-1-fenil-1-propanolo (R) allo stato isolato e dei suoi complessi con alcune molecole alcooliche (solv), quali metanolo, etanolo, 1-propanolo, 2-propanolo, 1-butanolo, S-(+)-2-butanolo, R-(−)-2-butanolo, 1-pentanolo, S-(+)-2-pentanolo, R-(−)-2-pentanolo, and 3-pentanolo, in un fascio molecolare espanso supersonicamente. L'analisi spettroscopica, supportata da calcoli teorici, indica la presenza nel fascio supersonico di alcuni conformeri degli addotti molecolari [R⋅solv]. Gli spettri di eccitazione di [R⋅solv] sono caratterizzati da significative deviazioni delle loro bande fondamentali rispetto a quelle dei rotameri di R. L'entità e la direzione di tali deviazioni risultano dipendenti dalla struttura e dalla configurazione di solv e sono attribuite alle diverse interazioni intime presenti nei complessi [R⋅solv] allo stato fondamentale ed eccitato. La misura delle energie di interazione in [R⋅solv] allo stato neutro e ionizzato rivela una sottile interdipendenza tra le forze attrattive (elettrostatiche e di polarizzazione) e repulsive (steriche) che sono responsabili delle caratteristiche spettroscopiche dei complessi e che permettono la discriminazione fra le forme enantiomeriche di solv chirali. An excellent tool for enantiodifferentiating chiral molecules (solv = aliphatic alcohols) and for investigating their interactions with a chiral selector (R = R-(+)-1-phenyl-1-propanol) in the isolated state was found to be resonance-enhanced two-photon ionization spectroscopy, coupled with time-of-flight detection (R2PI/PI). Supersonically expanded [R⋅solv] complexes have R2PI spectral bands that are shifted relative to the band origin region of the bare chromophore R. The magnitude and the direction of these spectral shifts, as well as the [R⋅solv] binding energies, depend on the structure and the configuration of both R and solv and reflect a subtle interplay between short-range attractive (electrostatic and dispersive) and repulsive (steric) forces in the ground- and excited-state [R⋅solv] complex.