D. Scuderi

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.

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.

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.

Gas-phase complexes: noncovalent interactions and stereospecificity

Abstract Chiral recognition is a fundamental phenomenon in life sciences based on the enantioselective complexation of a chiral molecule with a chiral selector. The diastereomeric aggregates, formed by complexation, are held together by a different combination of intermolecular forces and are, therefore, endowed with different stability and reactivity. Determination of these forces, which are normally affected in the condensed phase by solvent and supramolecular interactions, requires the generation of the diastereomeric complexes in an isolated state and their kinetic and spectroscopic investigation. This paper concerns enantiodiscrimination of chiral molecules in the gas phase through the application of various ESI-MS n -CID and REMPI-TOF methodologies. The measurement of the fragmentation thresholds of diastereomeric clusters by these techniques allowed to shed light upon the nature and the magnitude of the intrinsic interactions which control their formation and which affect their stability and reactivity.

Chiral recognition of diols by complexation with (R)-(+)-1-phenyl-1-propanol: a R2PI approach in supersonic beam

Wavelength and mass resolved resonance-enhanced multiphoton ionization (REMPI) excitation spectra of (R)-(+)-1-phenyl-1-propanol (PR) and its complexes with some chiral diols, i.e. 1,2-propanediols, 2,3-butanediols, and 2,4-pentanediols, have been recorded after a supersonic molecular beam expansion and interpreted in the light of molecular dynamic (MD) conformational minima searches. The spectral features of the selected complexes were found to depend on cooperative hydrogen-bond interactions between the two components, whose intensity depends upon the specific configuration of the diol moiety and the relative position of its hydroxy groups. The study further confirms resonant two-photon ionization spectroscopy, coupled with time-of-flight mass resolution (R2PI-TOF), as an excellent tool for gathering valuable information on the interactive forces in molecular clusters and for enantiodiscrimination of chiral molecules in the gas phase.

Chiral discrimination of 2,3-butanediols by laser spectroscopy

The resonance enhanced two-photon ionization time-of-flight (R2PI-TOF) excitation spectra of supersonically expanded complexes of isomeric 2,3-butanediols with a suitable chromophore, i.e. R-(+)-1-phenyl-1-propanol, represent powerful means for structurally discriminating the diol moiety and for investigating the nature of the intra- and intermolecular interactions involved in the complexes.

Mass resolved laser spectroscopy of micro-solvated R-(+)-1-phenyl-1- propanol: A chiral molecule of biological interest

The effects of micro-solvation on the radical cation of R-(+)-1-phenyl-1-propanol, have been investigated. The energy thresholds of the homolytic Cα–Cβ bond breaking of R-(+)-1-phenyl-1-propanol radical cation, its mono-hydrated cluster, and its clusters with (2R,3R)-(−)-2,3-butanediol and (2S,3S)-(+)-2,3-butanediol have been studied by two color resonant two photon ionization and photodissociation. The barrier of the Cα–Cβ fragmentation is appreciably higher for the unsolvated molecular ion than for its adducts with solvent molecules. Marked differences in the ethyl loss fragmentation energy are observed for clusters with water and with the two diols. In particular, the homochiral cluster with (2R, 3R)-(−)-2,3-butanediol exhibits a fragmentation barrier higher than that of the corresponding heterochiral adduct with (2S, 3S)-(+)-2,3-butanediol. At variance with the water adduct, the fragmentation of the covalent Cα–Cβ bond in the diol-clusters is energetically preferred to the loss of solvent.

Laser spectroscopy of 1,2,3,4-tetrahydro-1-naphthol·ROH (R = H, CH3) clusters

Wavelength and mass selected resonant enhanced multi-photon ionization (REMPI) spectroscopy is an excellent tool for characterising molecular clusters. Mass selected resonant two photon ionisation (R2PI) spectra of the S1 ← S0 transition of 1,2,3,4-tetrahydro-1-naphthol (TR) and their complexes with solvent molecules such as water (W) and methanol (MeOH) have been recorded after a supersonic molecular beam expansion. Spectra have been analysed with the aid of ab initio molecular orbital calculations conducted at the B3LYP/6-31G** level of the theory.

Photochemical R2PI study of chirality and intermolecular forces in supersonic beam

One and two-color, mass selected R2PI spectra of the S1 ← S0 transitions in the bare (+)-(R)- 1-phenyl-1-ethanol (ER) and its complexes with different solvent molecules (solv) (−)-(R)-2-butanol (BR) or (+)-(S)-2-butanol (BS), (−)-(R)-2-pentanol (TR )o r(+)-(S)-2-pentanol (TS) and (−)-(R)-2-butylamine (AR )o r (+)-(S)-2-butylamine (AS), have been recorded after a supersonic molecular beam expansion. The one-color R2PI excitation spectra of the diastereomeric complexes 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 states of the complexes. In analogy with other diastereomeric complexes, the phe- nomenological binding energy of the homochiral cluster is found to be greater than that of the heterochiral one. Preliminary measurements of excitation spectrum of (+)-(R)-1-Indanol (IR) is also reported.