D. Catone

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.

Laboratory Studies of Molecular Growth in the Titan Ionosphere

Experimental simulations of the initial steps of the ion-molecule reactions occurring in the ionosphere of Titan were performed at the synchrotron source Elettra in Italy. The measurements consisted of irradiating gas mixtures with a monochromatic photon beam, from the methane ionization threshold at 12.6 eV, up to and beyond the molecular nitrogen dissociative ionization threshold at 24.3 eV. Three gas mixtures of increasing complexity were used: N(2)/CH(4) (0.96/0.04), N(2)/CH(4)/C(2)H(2) (0.96/0.04/0.001), and N(2)/CH(4)/C(2)H(2)/C(2)H(4) (0.96/0.04/0.001/0.001). The resulting ions were detected with a high-resolution (1 T) FT-ICR mass spectrometer as a function of time and VUV photon energy. In order to interpret the experimental results, a Titan ionospheric model was adapted to the laboratory conditions. This model had previously allowed the identification of the ions detected in the Titan upper atmosphere by the ion neutral mass spectrometer (INMS) onboard the Cassini spacecraft. Comparison between observed and modeled ion densities validates the kinetic model (reactions, rate constants, product branching ratios) for the primary steps of molecular growth. It also reveals differences that we attribute to an intense surface chemistry. This result implies that heterogeneous chemistry on aerosols might efficiently produce HCN and NH(3) in the Titan upper atmosphere.

Chirality Transfer from a Single Chiral Molecule to 2D Superstructures in Alaninol on the Cu(100) Surface

The formation of 2D chiral monolayers obtained by self-assembly of chiral molecules on surfaces has been widely reported in the literature. Control of chirality transfer from a single molecule to surface superstructures is a challenging and important aspect for tailoring the properties of 2D nanostructures. However, despite the wealth of investigations performed in recent years, how chiral transfer takes place on a large scale still remains an open question. In this paper we report a coupling of scanning tunneling microscopy and low energy electron diffraction measurements with an original theoretical approach, combining molecular dynamics and essential dynamics with density functional theory, to investigate self-assembled chiral structures formed when alaninol adsorbs on Cu(100). The peculiarity of this system is related to the formation of tetrameric molecular structures which constitute the building blocks of the self-assembled chiral monolayer. Such characteristics make alaninol/Cu(100) a good candidate to reveal chiral expression changes. We find that the deposition of alaninol enantiomers results in the formation of isolated tetramers that are aligned along the directions of the substrate at low coverage or when geometrical confinement prevents long-range order. Conversely, a rotation of 14° with respect to the Cu(100) unit vectors is observed when small clusters of tetramers are formed. An insight to the process leading to a 2D globally chiral surface has been obtained by monitoring molecular assemblies as they grow from the early stages of adsorption, suggesting that the distinctive orientation of the self-assembled monolayer originates from a balance of cooperating forces which start acting only when tetramers pack together to form small clusters.

Conformational Effects in Photoelectron Circular Dichroism of Alaninol

A photoelectron circular dichroism (CD) study of the valence states of 2-amino-1-propanol (alaninol) in the gas phase is presented. The aim of the investigation is to reveal conformer population effects in the valence-state photoelectron spectrum. The experimental dispersion of the dichroic D parameter of valence states as a function of the photon excitation energy is compared with its theoretical value calculated by employing a multicentric basis set of B-spline functions and a Kohn-Sham Hamiltonian. The theoretical values are in very good agreement with the experimental data when the conformer population distribution is taken into account. Moreover, thanks to a comparison between experiment and theory, a clear assignment of the molecular orbital character and conformer geometry is given to the features of the photoelectron spectrum. This work indicates in a detailed experimental analysis that CD in photoelectron spectroscopy is an effective technique to disentangle the conformer assignment in photoelectron spectra.

Vibrational state dependence of β and D asymmetry parameters: The case of the highest occupied molecular orbital photoelectron spectrum of methyl-oxirane

The beta angular asymmetry and D dichroic asymmetry parameters of the methyl-oxirane highest occupied molecular orbital (HOMO) band have been experimentally investigated with vibrational resolution using synchrotron radiation. A theoretical calculation of the Franck-Condon factors between vibrational ground state and different ionic vibrational states, in the Born-Oppenheimer harmonic approximation, has been performed in order to gain information on the vibrational states mainly involved in the HOMO photoelectron band. The general good agreement between theoretical and experimental results allows a reliable assignment of the major features. The experimental determination of beta and D shows their dependence on the different final vibrational states. This paper reports, for the first time, experimental evidence of the dependence of the dichroic D parameter on the vibrational excitation of the ion.

Conformational Sensitivity in Photoelectron Circular Dichroism of 3-Methylcyclopentanone

A study of (R)-3-methylcyclopentanone [(R)-3-MCP] by photoelectron spectroscopy and photoelectron circular dichroism (PECD) is presented. The synchrotron radiation gas-phase photoelectron spectra of (R)-3-MCP were measured and are discussed on the basis of different theoretical methodologies. The experimental dichroism of (R)-3-MCP for selected deconvoluted valence states and for the carbonyl carbon 1s core state are reported and reproduced well by calculated dispersions generated by considering the contributions of two different conformers. The theoretical dichroic parameters are calculated by employing a multicentre basis set of B-spline functions and a Kohn-Sham Hamiltonian. Temperature-dependent PECD studies of the HOMO state and the carbonyl carbon 1s core level allowed the separation of the contributions of each conformer by photoelectron dichroism. This new approach clearly shows how the PECD methodology is sensitive to conformational and structural changes of unoriented (R)-3-MCP in the gas phase, opening up new perspectives in the characterisation of chiral molecular systems.

An experimental and computational study of the valence photoelectron spectra of halogenated pyrimidines

The electronic structures of pyrimidine and a selection of its halogen-substituted derivatives have been investigated using ultraviolet photoelectron spectroscopy and ab initio quantum chemical methods. Assignments are proposed for all of the features in the PES spectra by comparison with the vertical ionization energies of the molecular orbitals calculated using the partial third-order quasiparticle approximation as applied to electron propagator theory and a corrected density functional method based on the B3LYP functional. The shifts of the outermost five molecular orbitals of the pyrimidine ring structure in the halogen-substituted derivatives with respect to the binding energies of the equivalent orbitals in the parent pyrimidine molecule are discussed as a function of the identity and ring position of the halogen atom.

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.