S. Piccirillo

Gas-phase enantioselectivity

Abstract Determination of the intrinsic noncovalent interactions governing chiral recognition in diastereomeric complexes constitutes the basis for understanding information transfer between molecules in living systems as well as in synthetic supramolecular structures. The most important experimental methodologies so far employed for this task are illustrated in the present review. Emphasis is put on the principles and the applications of techniques, such as radiolysis, Fourier transform ion cyclotron resonance (FTICR) and collision-induced dissociation (CID) mass spectrometry, and resonance-enhanced multiphoton ionization time-of-flight (REMPI-TOF) spectroscopy, that allow measurement of the relative stability of diastereomeric ion/molecule and molecule/molecule complexes and quantification of the short-range forces controlling their enantioselective evolution to products.

ENERGETICS OF MOLECULAR COMPLEXES IN A SUPERSONIC BEAM : A NOVEL SPECTROSCOPIC TOOL FOR ENANTIOMERIC DISCRIMINATION

A subtle balance between attractive (electrostatic and dispersive) and repulsive (steric) forces is responsible for the different stabilities and spectral features of supersonically expanded van der Waals complexes between (R)-(+)-1-phenyl-1-propanol (R) and (R)-(−)- (r) or (S)-(+)-2-butanol (s). The homochiral Rr complex is more stable than the heterochiral Rs diastereomer in both the ground and excited states (see drawing). This was determined by the first direct measurement of the binding energies of the diastereomeric complexes by two color resonance two photon ionization/time-of-flight mass spectrometry.

Chiral aggregates of indan-1-ol with secondary alcohols and water: Laser spectroscopy in supersonic beams

One color, mass selected resonant two-photon ionization (1cR2PI) spectra of supersonically expanded bare (R)-(−)indan-1-ol (IR) and its complexes with chiral and achiral molecules (solv) have been investigated. The excitation spectrum of bare IR has been analyzed and discussed on the basis of theoretical predictions at the B3LYP/6-31G** level of theory. The excitation spectra of its diastereomeric complexes with (R)-(−)-and (S)-(+)hexan-2-ol (XR or XS, respectively) and water (W) are characterized by significant shifts of their S0 ← S1 band origin relative to that of bare IR. The extent and the direction of these shifts are found to depend upon the structure and the configuration of solv and are attributed to different short-range interactions in the ground and excited [IR·solv] complexes. In particular the [IR·W]n complexes display band origins blue-shifted relative to that of bare IR, attributed to the presence of an O–H⋯π electrostatic interaction between IR and W in [IR·W]n. The [IR·XR] and [IR·XS] equilibrium structures have been calculated by a molecular dynamical (MM3) sampling and PM3 semiempirical local optimization.

Chirality and intermolecular forces: studies using R2PI experiments in supersonic beams

One- and two-color, mass-selected resonant two-photon ionization (R2PI) spectra of the S1←S0 transitions in the bare (R)-(+)-1-phenylethan-1-ol (ER) and its complexes with a solvent molecule (solv: (S)-(+)butan-2-ol (BS), (R)-(−)butan-2-ol (BR), or water (W)) have been recorded after a supersonic molecular beam expansion. The excitation spectrum of bare ER conforms to theoretical predictions at the B3LYP/6-31G** level of theory by pointing to the formation of a single conformer. The one-color R2PI excitation spectra of the diastereomeric complexes [ER–solv] (solv: BS or BR) are characterized by significant shifts of their band origin relative to that of bare ER. The extent and the direction of these spectral shifts are found to depend upon the structure and the configuration of solv and are attributed to different short-range interactions in the ground and excited [ER–solv] complexes. In analogy with strictly related diastereomeric complexes, the phenomenological binding energy of the homochiral [ER–BR] is found to be greater that of the heterochiral one [ER–BS]. The one-color R2PI excitation spectra of the [ER–W] complex displays two signals blue shifted by 54 and 73 cm−1, relative to the S1←S0 band origin of bare ER, which indicate the presence of a O–H···π electrostatic interaction between ER and W.

Short-range interactions within molecular complexes formed in supersonic beams: structural effects and chiral discrimination

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.

Energetics of monohydrated chiral R(+)-1-phenyl-1-propanol: supersonic beam experiments and density functional calculations

Abstract Isolated hydrated clusters of chiral R(+)-1-phenyl-1-propanol have been studied in a supersonic molecular beam by means of one- and two-color resonant two-photon ionization mass-resolved spectroscopy. Excitation and threshold ionization spectra have been determined for monohydrated clusters together with the electronic ground state binding energy. Cluster spectra have been analyzed with the aid of ab initio molecular orbital calculations conducted at the B3LYP/6-31G ∗∗ level of the theory. The analysis gives information on the cluster structures and the binding energies.

Spectroscopic enantiodifferentiation of chiral molecules in the gas phase

Enantiodiscrimination of several chiral molecules, i.e. S-(+)- (s) vs R-(−)-2-butanol (r) and S-(+)- (s′) vs R-(−)-2-butylamine (r′), is obtained by two-color resonance-enhanced two-photon ionization time-of-flight (2cR2PI-TOF) spectroscopy of their supersonically expanded complexes with a suitable chromophore, i.e. R-(+)-1-phenyl-1-propanol (R). Analysis of the 2cR2PI-TOF excitation spectra indicates that these complexes are formed in the supersonic beam in several isomeric hydrogen-bonded structures. The different batochromic shifts of the band origin regions of the molecular diastereomeric complexes relative to that of bare R provide a viable means for spectroscopically discriminating the enantiomers. A further tool for their differentiation is provided by the very reproducible differences in the mass spectral fragmentation patterns of the jet-cooled diastereomeric aggregates. Chirality 11:376–380, 1999.

R2PI detection and spectroscopy of van der Waals complexes of 4-fluorostyrene with rare gases

Abstract One-color resonance-enhanced two-photon ionization spectra of jet-cooled 4-fluorostyrene van der Waals complexes with Ne, Ar, Kr, Xe are reported. The measured spectra display discrete structure and allow the identification of electronic spectral shirts as well as stretching and bending frequencies of the various complexes. These spectra are interpreted with the help of simple model calculations.

Spectroscopy of 4-fluorostyrene clusters

Abstract The R2PI mass spectra of jet cooled 4-fluorostyrene (4-FSTY) pure or mixed with various atoms and compounds such as Ar, Kr, N 2 , C 2 H 5 , NH 2 , CCl 4 show the presence of eterogeneous cluster ions. The wavelength spectra of these clusters show specific shift of the O 0 0 band due to vdW interactions.

Chiral discrimination of monofunctional alcohols and amines in the gas phase

Abstract A methodology has been developed for enantiodiscriminating chiral monoalcohols and monoamines by mass spectrometry. The approach is based on the generation of supersonically expanded complexes of these molecules with suitable chromophores, i.e. R-(+)-1-phenyl-ethanol (ER) or R-(+)-1-phenyl-1-propanol (PR). The jet-cooled diastereomeric complexes, otherwise elusive at room temperature, have been ionized by one-color resonant two-photon absorption (R2PI) and their fragmentation pattern analyzed by time-of-flight (TOF) spectrometry. Enantiodifferentiation of the chiral monoalcohols and monoamines is based on: (1) the different spectral shifts of the band origin of their molecular complexes relative to that of the bare chromophore (Δ) and (2) the different mass spectral fragmentation patterns of the jet-cooled diastereomeric adducts. Detection of stable aggregates of methane, n-butane, and other simple molecules with the selected chromophores suggests that the R2PI/TOF method can be a potential tool for enantiodifferentiating chiral hydrocarbons in the gas phase.