97 research outputs found
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
No full textThe 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. © 2013 the Owner Societies
Ultraviolet and infrared spectroscopy of neutral and ionic non-covalent diastereomeric complexes in the gas phase
No full textNon-covalent intermolecular interactions responsible for chiral discrimination have been investigated in the gas phase both in neutral and ionic complexes. Mass-selected resonant two-photon ionization (R2PI) as well as infrared depleted R2PI (IR-R2PI) techniques have been applied to investigate the role of fluorine substitution in the chiral recognition process between (R)-1-phenyl 1-ethanol (ER), (S)-1-(4-fluorophenyl)-ethanol (pFE S), (R)-1-phenyl-2,2,2-trifluoroethanol (FER) and the two enantiomers of butan-2-ol (BR/S), generated in a supersonic molecular beam. The results have been interpreted with the aid of theoretical predictions at the D-B3LYP/6-31G**level of theory. The diastereomeric complexes of ER and pFES with R- and S-butan-2-ol are structurally similar, and dispersive interactions between the aliphatic chain of the alcohol and the π system of the chromophore as well repulsive interactions are mainly responsible for chiral recognition. FER forms, predominantly with S-butan-2-ol, also stable complexes in which the alcohol is oriented away from the aromatic ring. The ionic complexes between pure enantiomers of the bis (diamido)-bridged basket resorcin[4]arene and cytarabine are generated in the gas phase by electrospray ionization and investigated by IRMPD. The proton-bound diastereomers show clearly different IRMPD spectra which, in light of ONIOM (B3LYP/6-31(d):UFF) calculations, are consistent with the occurrence of several isomeric structures, in which the N(3)-protonated guest is either accommodated inside the host cavity or outside it. The spectral differences are attributed to the effects of the intramolecular hydrogen bonding between the C(2′) -OH group and the aglycone oxygen atom of the nucleosidic guest upon repulsive interactions between the same oxygen atom and the aromatic rings of the host. © 2013 Accademia Nazionale dei Lincei
Competition between electron-donor and electron-acceptor substituents in nitrotoluene isomers: A photoelectron spectroscopy and ab initio investigation
No full textWe present an investigation of the close relationship between chemical structure, physical properties and reactivity of the three nitrotoluene isomers: a joint experimental and theoretical study, based on X-ray photoelectron spectroscopy (XPS) measurements and ab initio calculations, addressing the complex interplay between the competing electron-donor and electron-acceptor effects of the nitro- and methyl-substituents on the chemical properties of the nitrotoluene isomers. As the main results of the investigation we: (i) point out that accurate ab initio calculations play a key role in the complete assignment of photoemission measurements, as well as in the estimate of proton affinities in the case of all the eligible sites; (ii) revisit, at a more quantitative level, textbook models based on inductive and resonant effects of different substituents of the aromatic ring, as well as on the hyper-conjugative connection of the methyl group to the π-conjugated system; (iii) provide an accurate analysis of correlation patterns between calculated proton affinities and core-ionization energies, which represent a powerful tool, capable of predicting site-specific reactivities of polysubstituted molecules in the case of electrophilic aromatic substitution reactions. © 2014 The Royal Society of Chemistry
Conformational sensitivity in photoelectron circular dichroism of 3-methylcyclopentanone
No full textA 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. Dichroism and conformers: Temperature-dependent photoelectron circular dichroism (PECD) studies of the HOMO state and the carbonyl carbon 1s core level of R-3-methylcyclopentanone (R-3-MCP) allowed to separate the contributions of each conformer, showing how the PECD methodology can be sensitive to the conformational and structural changes of unoriented chiral molecules. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Probing the competition among different coordination motifs in metal-ciprofloxacin complexes through IRMPD spectroscopy and DFT calculations
No full textThe vibrational spectra of ciprofloxacin complexes with monovalent (Li +, Na+, K+, Ag+) and polyvalent (Mg2+, Al3+) metal ions are recorded in the range 1000-1900 cm-1 by means of infrared multiple-photon dissociation (IRMPD) spectroscopy. The IRMPD spectra are analyzed and interpreted in the light of density functional theory (DFT)-based quantum chemical calculations in order to identify the possible structures present under our experimental conditions. For each metal-ciprofloxacin complex, four isomers are predicted, considering different chelation patterns. A good agreement is found between the measured IRMPD spectrum and the calculated absorption spectrum of the most stable isomer for each complex. Metal ion size and charge are found to drive the competition among the different coordination motifs: small size and high charge density metal ions prefer to coordinate the quinolone between the two carbonyl oxygen atoms, whereas large-size metal ions prefer the carboxylate group as a coordination site. In the latter case, an intramolecular hydrogen bond compensates the weaker interaction established by these cations. The role of the metal cation on the stabilization of ionic and nonionic structures of ciprofloxacin is also investigated. It is found that large-size metal ions preferentially stabilize charge separated motifs and that the increase of metal ion charge has a stabilizing effect on the zwitterionic form of ciprofloxacin. © 2012 American Chemical Society
Photoionization mass spectrometry of ω -phenylalkylamines: Role of radical cation- π interaction
No full textLinear ω-phenylalkylamines of increasing alkyl chain length have been investigated employing synchrotron radiation in the photon energy range from 7 to 15 eV. These molecules have received considerable interest because they bear the skeleton of biologically relevant compounds including neurotransmitters and because of the possible interaction between the amino moiety and the phenyl ring. Recently, the contribution of this interaction has been assayed in both neutral and protonated species, pointing to a role of the polymethylene chain length. In this work, the ionization energy (IE) values of benzylamine (BA), 2-phenylethylamine (2-PEA), 3-phenylpropylamine (3-PPA), and 4-phenylbutylamine (4-PBA) were investigated in order to ascertain the impact of the different alkyl chain lengths and to verify an amino radical cation-π interaction. The IEs obtained experimentally, 8.54, 8.37, 8.29, and 8.31 eV for BA, 2-PEA, 3-PPA and 4-PBA, respectively, show a decreasing trend that is discussed employing calculations at the CBS-QB3 level. Moreover, the appearance energy values for major fragments produced by the photofragmentation process are reported. © 2018 Author(s)
Monosolvation of R-1-phenyl-2,2,2-trifluoroethanol with amines: Configurational effects on the excitation, ionization, and fragmentation of diastereomeric complexes
No full textWavelength and mass selected resonant two-photon ionization spectra of molecular clusters between R-1-phenyl-2,2,2-trifluoroethanol (FER) and methylamine (M) or the enantiomers of 2-aminobutane (A(R) and A(S)) were recorded after supersonic molecular beam expansion and analyzed with the aid of ab initio molecular orbital calculations. The experimental results agree with theoretical calculations pointing to the predominance of the two most stable conformers of monosolvated FER whose CF3 group establishes intense NH center dot center dot center dot F interactions with the selected amines so as to orient them away from the aromatic ring. This reduces the enantio selectivity of FER toward the 2-aminobutane enantiomers as compared to that exhibited by the R-1-phenylethanol (E-R) analogue, where obviously NH center dot center dot center dot F interactions are absent
On the Morphology of Nanostructured TiO2 for Energy Applications: The Shape of the Ubiquitous Nanomaterial
No full textNanostructured titania is one of the most commonly encountered constituents of nanotechnology devices for use in energy-related applications, due to its intrinsic functional properties as a semiconductor and to other favorable characteristics such as ease of production, low toxicity and chemical stability, among others. Notwithstanding this diffusion, the quest for improved understanding of the physical and chemical mechanisms governing the material properties and thus its performance in devices is still active, as testified by the large number of dedicated papers that continue to be published. In this framework, we consider and analyze here the effects of the material morphology and structure in determining the energy transport phenomena as cross-cutting properties in some of the most important nanophase titania applications in the energy field, namely photovoltaic conversion, hydrogen generation by photoelectrochemical water splitting and thermal management by nanofluids. For these applications, charge transport, light transport (or propagation) and thermal transport are limiting factors for the attainable performances, whose dependence on the material structural properties is reviewed here on its own. This work aims to fill the gap existing among the many studies dealing with the separate applications in the hope of stimulating novel cross-fertilization approaches in this research field
Crescita via CVD di nanofili di silicio su substrati di acciaio funzionalizzati con oro, argento e rame
No full textIn questo rapporto si presenta uno studio sulla sintesi e caratterizzazione di materiali nanostrutturati nella forma di strutture nanometriche filiformi di silicio da utilizzare come materiali anodici in batterie litio-ione. In particolare si presentano dei campioni ottenuti mediante la tecnica di Chemical Vapor Deposition (CVD) utilizzando come metallo catalizzatore, oro, argento e rame.This report shows a study on the synthesis and characterization of nanostructured materials in the form of silicon nanowires for applications in lithium-ion batteries. In particular, the growth by Chemical Vapor Deposition (CVD) of Si nanowires on stainless steel substrates by using gold, silver and copper as the catalyst will be reported
Thermal diffusivity enhancement in nanofluids based on pyrolytic titania nanopowders: Importance of aggregate morphology
No full textNanoparticle suspensions (nanofluids) have been subject of several investigations in the last few years for their potential applications, one of them being the improvement of heat transfer characteristics of thermal fluids. However, notwithstanding the numerous studies on thermal properties of nanofluids containing various kinds of nanoparticles, in many cases, the results are conflicting, and the models proposed to explain the experiments are controversial. In this paper, the thermal conductivity enhancement in dispersions, based on titania nanoparticles produced by laser-assisted pyrolysis, is investigated using an optical technique known as forced Rayleigh light scattering. Measurements on these systems exhibited a linear relation between the thermal conductivity enhancement and the nanoparticle volume fractions with a maximum increase of 6% with respect to the base fluid ethanol at 0.6% titania content. Comparison of the experimental data with the aggregation model developed by Prasher et al. highlighted the crucial role of the morphology of pyrolytic nanoparticle aggregates on determining the thermal conductivity enhancement. Copyright © 2014 John Wiley & Sons, Ltd
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