Recursos de colección

Estudo Geral - Universidade de Coimbra (19.630 recursos)

ESTUDO GERAL é a designação do repositório digital da produção científica da Universidade de Coimbra, com o objectivo de divulgar conteúdos digitais de natureza científica de autores ligados à Universidade de Coimbra. A sua criação insere-se no movimento de Acesso Livre à Literatura científica (Open Access), ao qual o Conselho de Reitores das Universidades Portuguesas aderiu em 2006 e que a Universidade de Coimbra subscreveu. À semelhança de outras grandes universidades nacionais e internacionais, a UC tem o maior interesse em aumentar a sua presença na rede informática mundial, sendo cada vez mais - e também por essa via - um emissor de conhecimento e cultura.

FCTUC Química - Artigos em Revistas Internacionais

Mostrando recursos 1 - 20 de 796

  1. Energy Transfer from Fluorene Based Conjugated Polyelectrolytes to Onchain and Self-Assembled Porphyrin Units

    Marques, Ana T.; Pinto, Sara M. A.; Monteiro, Carlos J. P.; Melo, J. Sérgio Seixas de; Burrows, Hugh D.; Scherf, Ullrich; Calvete, Mário J. F.; Pereira, Mariette M.
    A new water soluble fluorene-based polyelectrolyte containing on-chain porphyrin units has been synthesized via Suzuki coupling, for use in optoelectronic devices. The material consist of a random copolymer of poly{1,4-phenylene-[9,9-bis(4-phenoxy butylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) and a 5,15-diphenylporphyrin (DPP). The energy transfer process between the PBS-PFP units and the porphyrin has been investigated through steady state and time-resolved measurements. The copolymer PBS-PFP-DPP displays two different emissions one located in the blue region of the spectra, corresponding to the fluorene part and another in the red due to fluorescent DPP units either formed directly or by exciton transfer. However, relatively inefficient energy transfer from...

  2. Intermolecular potentials for simulations of collisions of SiNCS+ and (CH3)2SiNCS+ ions with fluorinated self-assembled monolayers

    José Nogueira, Juan; Sánchez-Coronilla, Antonio; Marques, Jorge M. C.; Hase, William L.; Martínez-Núñez, Emilio; Vázquez, Saulo A.
    Analytical potential energy functions were developed for interactions of SiNCS+ and (CH3)2SiNCS+ ions with perfluorinated self-assembled monolayer (F-SAM) surfaces. Two model compounds were used to represent an F-SAM: CF4 and nine chains of perfluorobutane forming a miniSAM structure. Density functional theory plus dispersion (DFT-D) calculations were carried out to compute intermolecular potential energy curves (IPECs) for these systems. The applied DFT-D method (specifically, B97-D) was successfully tested against high-level wavefunction calculations performed on the smallest system investigated. The IPECs calculated at the B97-D level were fitted to analytical potentials of the Buckingham type. The calculations show that the parameters obtained...

  3. Palladium-catalysed reactions of 8-hydroxy- and 8-benzyloxy-5,7-diiodoquinoline under aminocarbonylation conditions

    Takács, Attila; Szilágyi, Antal; Ács, Péter; Márk, László; Peixoto, Andreia F.; Pereira, Mariette M.; Kollár, László
    Various 5-carboxamido-7-iodo-8-benzyloxyquinolines were synthesised via selective aminocarbonylation of 5,7-diiodo-8-benzyloxyquinoline in the presence of ‘in situ’ generated palladium(0) catalysts. Under similar conditions (50 °C, 80 bar CO), 5,7-bis(N-tert-butyl-glyoxylamido)-8-hydroxyquinoline was obtained using tert-butylamine as N-nucleophile. The unprotected 5,7-diiodo-8-hydroxyquinoline underwent dehydroiodination resulting in 8-hydroxyquinoline as the major product.

  4. Homogeneous length scale of shear-induced multilamellar vesicles studied by diffusion NMR

    Åslund, Ingrid; Medronho, Bruno; Topgaard, Daniel; Söderman, Olle; Schmidt, Claudia
    A recently developed protocol for pulsed gradient spin echo (PGSE) NMR is applied for the size determination of multilamellar vesicles (MLVs). By monitoring the self-diffusion behavior of water, the technique yields an estimate of the homogeneous length scale λhom, i.e. the maximum length scale at which there is local structural heterogeneity in a globally homogeneous material. A cross-over between local non-Gaussian to global Gaussian diffusion is observed by varying the experimentally defined length- and time-scales. Occasional observation of a weak Bragg peak in the PGSE signal attenuation curves permits the direct estimation of the MLV radius in favorable cases, thus...

  5. Application of Room Temperature Ionic Liquids to the Development of Electrochemical Lipase Biosensing Systems for Water-Insoluble Analytes

    Pauliukaite, Rasa; Doherty, Andrew P.; Murnaghan, Kevin D.; Brett, Christopher M. A.
    Biosensors have been prepared by modification of glassy carbon electrodes with functionalised multiwalled carbon nanotubes (MWCNT) dispersed in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonimide (BmimNTF2) and with lipase cross-linked with glutaraldehyde. The biosensor was applied to the determination of olive oil triglycerides by cyclic voltammetry. A phosphate buffer (pH 7.0) / BmimNO3 mixture is a better electrolyte than aqueous buffer alone. The response signal in the buffer-BmimNO3 mixture was found to increase with the number of cycles until a constant current was achieved. The calibration curve obtained exhibited a sigmoid-shape and a fourparameter model was used to fit the data which gave a limit of detection of...

  6. Nitrogen-Bridged Heterocycles via Cycloaddition of Non-Classical Heterocyclicfused-[c]thiazoles

    Soares, Maria I. L.; Gomes, Clara S. B.; Melo, Teresa M. V. D. Pinho e
    The [4π+2π] cycloaddition of non-classical heterocyclic-fused-[c]thiazoles was explored allowing the synthesis of a range of new nitrogen-bridged bi-, tri- and tetracyclic heterocyclic compounds namely, pyrazolo[1,5-a]pyridines, thiazolo[2,3,4-cd]pyrrolizines and indolizines. For the first time, one non-classical pyrrolo[1,2-c]thiazole was isolated and its structure determined by X-ray crystallography.

  7. Phenol And Para-Substituted Phenols Electrochemical Oxidation Pathways

    Enache, Teodor Adrian; Oliveira-Brett, Ana Maria
    The electrochemical behaviour of phenol, catechol, hydroquinone, resorcinol, dopamine, and para-substituted phenolic compounds, 4-ethylphenol, tyrosine, and tyramine, was studied over a wide pH range using a glassy carbon electrode. The oxidation of phenol is pH dependent and irreversible, occurring in one step, and followed by hydrolyse in ortho- and para- positions, leading to two oxidation products, catechol and hydroquinone. The oxidation of phenol oxidation products, ortho-phenol and para-phenol, is reversible and pH dependent. The oxidation potential of parasubstituted phenols varies slightly due to their substituent group in position C4, and occurs in one oxidation step corresponding to the oxidation of phenol. The oxidation products of this group of para-substituted...

  8. UV-induced lsomerization of (€)-Crotonic Acid. Combined Matrix-isolated IR and ab hitio MO Study

    Fausto, Rui; Kulbida, Anatoly; Schrems, Otto
    The results of a combined vibrational and structural study of the (€)-crotonic acid monomer undertaken by matrix-isolated low-temperature IR spectroscopy and ab initio SCF-MO calculations are presented. It is shown that in both argon and krypton matrices monomeric (€)-crotonic acid exists as a mixture of two conformers of similar energies, differing by the relative orientation of the C=C-C=O axis (the scis and s-trans forms, having a C=C-C=O dihedral angle equal to 0" and 180", respectively). Upon UV-irradiation in the 240-250 nm region by a xenon lamp, photoisomerization reactions about both C,-C and C=C bonds occur leading, respectively, to scis + s-trans (€)-crotonic acid rotamerization and (€)-crotonic...

  9. Investigation of the Structure of the Columnar Liquid-crystalline Phase of Copper(l1) Carboxylates. An FTlR Spectroscopic Study

    Moita, Maria F. Ramos; Duarte, Maria Leonor T. S.; Fausto, Rui
    The FTlR spectra of a series of anhydrous copper(it) carboxylates of general formula Cu,[CH,(CH,),CO,], (n = 4-8, 10, 12, 14 and 16) have been studied as a function of temperature. The spectra show notable changes associated with the phase transition from the crystalline to the columnar liquid-crystalline phase which indicate that the coordination of the carboxylate groups to the bimetallic centre changes from bridging bidentate to chelating bidentate. The observed change in the type of coordination is induced by the conformational disordering to the carbon chains and does not affect strongly the chemical environment around the copper atoms (in particular the Cu-Cu and Cu-0 distances), in...

  10. Rotational Isomerism in Acrylic Acid: A Combined Matrix-isolated IR, Raman and ab initio Molecular Orbital Study

    Kulbida, Anatoly; Ramos, Mozart N.; Rasanen, Markku; Nieminen, Janne; Schrems, Otto; Fausto, Rui
    The results of a combined vibrational and structural study of the acrylic acid monomer undertaken by matrixisolated low-temperature IR spectroscopy and ab initio SCF-HF and MP2 MO calculations are presented. in addition, both Raman and IR spectra of liquid acrylic acid and the Raman spectrum of the crystal are also reported and interpreted. It is shown that in both argon and krypton matrices acrylic acid monomer exists as a mixture of two conformers of similar energies, differing by the relative orientation of the C=C-C=O axis. Upon irradiation at 1 = 243 nm by a xenon lamp, the scis form (C-C-C-0 dihedral angle equal to 0"),...

  11. Structures and Vibrational Spectra of CH,OCH,CH,OH : The

    Gil, Francisco P. S. C.; Fausto, R.; Costa, A. M. Amorim da; Teixeira-Dias, J. J. C.
    Ab initio calculations at the MP2/6-31G* and MP2/6-31G*//6-31G* levels have been carried out for the monomer of 2-methoxyethanol (CH,OCH,CH,OH). The MP2/6-31G* results indicate that the two more stable conformers (tGg’ and gGg‘) display intramolecular hydrogen bonds directed from the hydroxy H atom to one of the lone pairs of the ether 0 atom, and that the tGg’ conformer is 6.3 kJ mol-’ more stable than the gGg’ conformer. As the MP2/6-31G* and MP2/6-3lG*//6-31G* calculations do not yield results differing by more than a few tenths of a kJ mol-‘, it is concluded that the structure-sensitive and the dynamic correlation corrections are far from being additive. While...

  12. Rotational Isomerism in CH3CH2-C(=S)SR (R = CH3, CH2CH3): A Combined Vibrational

    Fausto, R.; Martins, A. Gabriela; Teixeira-Dias, J. J. C.; Tonge, P. J.; Carey, P. R.
    The vibrational and conformational properties exhibited by CHsCH2C(=S)SCHs and CH~CH~C(=S)SCHZCH3 were studied by Raman and infrared spectroscopies for the liquid and solid phases, and by ab initio calculations for the isolated molecule. It is shown that these molecules tend to adopt nonsymmetricconformations near the C(=S)S group, in contrast to their oxygen analogues whose most stable conformers correspond to structures having a planar skeleton. For the conformers differing by internal rotation about the C,-C bond, the most stable conformer-the skew form having the CC-C=S dihedral angle equal to flOOO-is more stable than the symmetric syn conformer (CC-C=S equal to Oo) by ca. 1.0 kJ mol-'. In the...

  13. Conformers, Vibrational Spectra and Infrared-induced

    Kulbida, Anatoly; Fausto, Rui
    The results of a combined study of dichloroacetic acid monomer undertaken by matrix-isolated low-temperature infrared spectroscopy in both argon and krypton matrices and ab initio SCFMO calculations are presented. Two s-cis (C-0) conformations of CCI,HCO,H differing by internal rotation about the C-C bond and one s-trans form were found in the matrices. Their spectra are reported within the range 4000-400 cm-' and interpreted. The skewlscis form (H-C-C-0 dihedral angle equal to ca. 144") converts to the most stable syn/scis conformer (H-C-C-0: 0") upon irradiation in the v(0-H) region. The temperature dependences (annealing) of the isolated matrix vibrational spectra of the studied molecule before and after irradiation...

  14. Conformers, Vibrational Spectra and Laser-induced Rotamerization

    Fausto, Rui; Teixeira-Dias, Jose J. C.; Gil, Francisco P. S. C.
    A6 initio molecular orbital (MO) calculations with a 6-31G* basis set have been carried out for CH,CICOOH. Structures and energies of relevant conformations have been determined by gradient geometry refinement and some conformationally dependent geometry trends discussed. The results agree with reported experimental data and provide a good insight for the intramolecular interactions which determine the relative stability of the various conformers. The normal mode analysis based on the 6-31G* harmonic force field is used to review previous assignments of the vibrational spectra of the various conformers and to interpret previous experimental findings (A. Kulbida and A. Nosov, J. Mol. Struct., 1992, 265, 17) on the...

  15. s-cis and s-trans Conformers of Formic, Thioformic and

    Fausto, R.; Carvalho, L. A. E. Batista de; Teixeira-Dias, J. J. C.; Ramos, M. N.
    Ab initio SCF-MO calculations have been carried out for formic, thioformic and dithioformic acids using the 6-31G* basis set. Fully optimized geometries, atomic charges, relative stabilities and harmonic force fields for s-cis and s-trans conformers of these molecules have been determined and the effects of oxygen-by-sulphur substitution analysed. A realistic description of the molecular charge distribution can be reached by introducing a quantum-mechanical correction to the Mulliken atomic charges, derived from the 'charge'-'charge flux'-'overlap' (CCFO) model. Unlike reported theoretical results, the present ab initio calculations yield relative stabilities of the thioformic acid conformers in agreement with experiment [s-cis (thiol) > s-trans(thio1) > s-cis(thione) > s-trans(thione)]. The success of these ab initio...

  16. Raman spectroscopy and molecular mechanics in

    Teixeira-Dias, J.J.C.; Fausto, R.
    Molecular mechanics is used to provide information on the stable conformations of the monochloroacetic acid molecule. In the most stable form, the C-C1 is in the COO plane (syn), while in the other form (gauche) the C1-C-C=O dihedral angle is in the 131O region. Thus in the syn form, the bond dipoles associated with the C-C1 and CEO bonds are approximately aligned, leading to a C=O bond stronger than in the less stable conformer and, by consequence, to a larger C-0 stretching frequency for this form. The Raman spectra of Ca, Sr, Ba and Cu(I1) monochloroacetates in crystalline phases have been recorded and interpreted to assess the relative importance of the mutual interactions...

  17. A molecular mechanics force field for conformational analysis of aliphatic acyclic amines

    Carvalho, L A. E. Batista de; Teixeira-Dias, J. J. C.; Fausto, R.
    An improved molecular mechanics force field for conformational and vibrational studies of aliphatic acyclic amines is developed. The resulting force field reproduces molecular structures adequately and provides a good fit for energy differences between conformers and barriers to internal rotation for a large number of amines. In addition, vibrational frequencies are calculated in good agreement with available experimental data. When compared with existent force fields for amines, the present force field is considerably more simple and gives rise to calculated properties in closer agreement with experiment.

  18. FTIR spectroscopy structural analysis of the interaction between Lactobacillus kefir S-layers and metal ions

    Gerbino, E.; Mobili, P.; Tymczyszyn, E.; Fausto, R.; Gómez-Zavaglia, A.
    FTIR spectroscopy was used to structurally characterize the interaction of S-layer proteins extracted from two strains of Lactobacillus kefir (the aggregating CIDCA 8348 and the non-aggregating JCM 5818) with metal ions (Cd+2, Zn+2, Pb+2 and Ni+2). The infrared spectra indicate that the metal/protein interaction occurs mainly through the carboxylate groups of the side chains of Asp and Glut residues, with some contribution of the NH groups belonging to the peptide backbone. The frequency separation between the νCOO− anti-symmetric and symmetric stretching vibrations in the spectra of the S-layers in presence of the metal ions was found to be ca. 190...

  19. Five isomers of monomeric cytosine and their interconversions induced

    Lapinski, Leszek; Igor Reva, Igor Reva; Nowak, Maciej J.; Fausto, Rui
    Photoisomerization processes involving five isomers of cytosine were induced by narrowband tunable UV irradiation of matrix-isolated monomers of the compound. Irradiation of an argon matrix containing cytosine monomers with UV l = 313 nm laser light resulted in syn2anti photoisomerizations between the two imino–oxo forms, whereas the substantially more populated amino–hydroxy and amino–oxo forms stayed intact. Subsequent irradiation with shorter-wavelength UV l = 311 nm laser light led to two concomitant phototautomeric processes consuming the amino–oxo isomer: (i) an oxo - hydroxy hydrogen-atom transfer photoprocess converting the amino–oxo form into the amino–hydroxy tautomer; (ii) amino - imino hydrogen-atom transfer converting the amino–oxo form into the imino–oxo isomers. The...

  20. Study of Nα-benzoyl-L-argininate ethyl ester chloride, a model compound for poly(ester amide) precursors: X-ray diffraction, infrared and Raman spectroscopies, and quantum chemistry calculations

    Fonseca, A. C.; Jarmelo, S.; Silva, M. Ramos; Beja, A. M. Matos; Fausto, R.; Gil, M. H.; Simões, P. N.
    Poly(ester amide)s (PEAs) are lacking in structural and spectroscopic information. This paper reports a structural and spectroscopic characterization of Nα-benzoyl-L-argininate ethyl ester chloride (BAEEH+·Cl−), an important amino acid derivative and an adequate PEAs’ model compound. Crystals of BAEEH+·Cl− obtained by slow evaporation in an ethanol/water mixture were studied by different complementary techniques. X-ray analysis shows that BAEEH+·Cl− crystallizes in the chiral space group P21. There are two symmetry independent cations (and anions) in the unit cell. The two cations have different conformations: in one of them, the angle between the least-squares planes of the phenyl ring and the guanidyl group...

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