A. Paladini

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.

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.

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

In life sciences, diastereomeric chiral molecule/chiral receptor complexes 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, can be accomplished through the generation of diastereomeric complexes in the isolated state and their spectroscopic investigation. This review presents a detailed discussion of the mass resolved Resonant Two Photon Ionization (R2PI-TOF) technique in supersonic beams and introduces an overview of various other technologies currently available for the spectroscopic study of gas phase chiral molecules and supramolecular systems. It reports case studies primarily from our recent work using R2PI-TOF methodology for chiral recognition in clusters containing molecules of biological interest. The measurement of absorption spectra, ionization and fragmentation thresholds of diastereomeric clusters by this technique allow the determination of the nature 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.

Laser excited charge transfer processes in oxygen organic molecules mixtures : O(3Pj) formation

Abstract Laser induced charge transfer (CT) processes have attracted attention for many years. These reactions play an important role in chemistry as well as biophysics and biology. When the reaction is induced by laser on a complex formed in a supersonic beam, the impact geometry is well defined and reaction details are more evident. This paper deals with features relevant to O 2 photodissociation induced in clusters of oxygen molecules and rare gases, water or organic molecules. Our studies, performed in a supersonic beam by using lasers of different wavelength to induce the reaction and to detect the reaction products by multiphoton ionization, show that production of atomic oxygen is strongly dependent on the ionization potential of the electron donor.

The effect of fluorine substitution on chiral recognition: interplay of CH⋯π, OH⋯π and CH⋯F interactions in gas-phase complexes of 1-aryl-1-ethanol with butan-2-ol

The molecular diastereomeric complexes between R-1-phenyl-1-ethanol, S-1-(4-fluorophenyl)ethanol and S-1-(2-fluorophenyl)ethanol and R and S-butan-2-ol, isolated under molecular beam conditions in the gas phase, have been investigated by mass-selective resonant two-photon ionization (R2PI) and infrared depleted R2PI (IR-R2PI). The comparison of the three systems allowed us to highlight the significance of specific intermolecular interactions in the chiral discrimination process. The interpretation of the results is based on theoretical predictions mainly at the D-B3LYP/6-31++G** level of theory. The homo and heterochiral complexes are endowed with fine differences in intermolecular interactions, namely strong OH···O, and weaker CH···π, OH···π, CH···F as well as repulsive interactions. The presence of a fluorine atom in the para position of the aromatic ring does not influence the overall geometry of the complex whilst it affects the electron density in the π system and the strength of CH···π and OH···π interactions. The role and the importance of CH···F intermolecular interactions are evident in the complexes with fluorine substitution in the ortho position. While the ortho hetero complex is structurally analogous to the hetero para and non-fluorinated structures, butan-2-ol in the ortho homo adduct adopts a different conformation in order to establish a CH···F intermolecular interaction.

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.

Analysis of the plume expansion from laser ablated SmBaCuO target

Abstract The plasma produced by laser ablation of a Sm 1 Ba 2 Cu 3 O 7− x target was investigated by means of optical emission spectroscopy. Optical spectroscopy provides information on the composition of the species presented in the plume and on their time evolution. Intensified charge coupled device photography gives information on the velocities of the charged and neutral species in the expanding plume. The deposited films have been analyzed by means of time of flight mass spectroscopy.