Browsing by Author "Fausto, R."
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- 4-hydroxyquinolin-2(1H)-one isolated in cryogenic argon and xenon matrices: tautomers and photochemistryPublication . Secrieru, Alina; Lopes, S.; Nikitin, T.; Cristiano, Maria de Lurdes; Fausto, R.4-Hydroxyquinolin-2(1H)-one (4HQ2O) was synthesized, isolated in cryogenic matrices (argon and xenon), and studied by infrared spectroscopy. Quantum chemical calculations carried out at the DFT(B3LYP)/6-311++G (3df,3pd) level of theory were used to determine the conformational and tautomeric properties of the molecule. Two tautomeric forms were identified in the as-deposited matrices with the help of the theoretical data. To investigate the photochemistry of the compound, in situ broadband ultraviolet (lambda > 283 nm) irradiation of the asdeposited argon matrix was performed. This irradiation led to the generation of an additional tautomer, together with the products of fragmentation of the heterocyclic ring of the molecule, specifically isocyanic acid and carbon monoxide. Photoproducts such as 1,3-dihydro-2H-indol-2-one and cyclohepta-1,2,4,6-tetraene were also observed in the photolyzed argon matrix. A comprehensive assignment of the infrared spectra of all the species observed experimentally is presented.
- Amino-Imino Tautomerization upon in Vacuo Sublimation of 2-Methyltetrazole-Saccharinate as Probed by Matrix Isolation Infrared SpectroscopyPublication . Ismael, Amin; Gómez-Zavaglia, A.; Borba, A.; Cristiano, Maria Lurdes Santos; Fausto, R.The amino−imino tautomerization of the nitrogen-linked conjugate 2-methyltetrazole-saccharinate (2MTS) was observed upon sublimation of the compound in vacuo. As shown previously by X-ray diffraction [Ismael, A.; Paixão, J. A.; Fausto, R.; Cristiano, M. L. S. J. Mol. Struct., 2011, 1023, 128−142], in the crystalline phase the compound exists in an amino-bridged tautomeric form. Infrared spectroscopic investigation of a cryogenic matrix prepared after sublimation of a crystalline sample of 2MTS and deposition of the sublimate together with argon (in ∼1:1000 molar ratio) onto an IR-transparent cold (15 K) substrate, revealed that the form of 2MTS present in the matrix corresponds to the theoretically predicted most stable imino-bridged tautomer. In this tautomer, the labile hydrogen atom is connected to the saccharine nitrogen, and the two heterocyclic fragments are linked by an imino moiety in which the double-bond is established with the carbon atom belonging to the saccharyl fragment. The observed isomeric form of this tautomer is characterized by a zusammen (Z) arrangement of the two rings around the CN bond of the bridging group and an intramolecular NH···N hydrogen bond. The experimental IR spectrum of the matrix-isolated 2MTS has been fully assigned based on the calculated spectra for the two most stable conformers of this tautomer. A mechanism for the conversion of the tautomeric form existing in the crystal into that present in the gas phase is proposed. As a basis for the interpretation of the experimental results, a detailed theoretical [at the DFT(B3LYP) level of approximation with the 6-31+ +G(d,p) and 6-311++G(3df,3pd)] study of the potential energy surface of the compound was performed.
- Conformational and structural analysis of 2-allyl-1,2-benzisothiazol-3(2H)-one 1,1-dioxide as probed by matrix-isolation spectroscopy and quantum chemical calculationsPublication . Gómez-Zavaglia, A.; Kaczor, A.; Coelho, Daniela; Cristiano, Maria Lurdes Santos; Fausto, R.2-Allyl-1,2-benzisothiazol-3(2H)-one 1,1-dioxide (ABIOD) has been studied by matrix-isolation infrared spectroscopy and quantum chemical calculations. A conformational search on the B3LYP/6-311++G(3df,3pd) potential energy surface of the molecule demonstrated the existence of three conformers, Sk, Sk0 and C, with similar energies, differing in the orientation of the allyl group. The calculations predicted the Sk form as the most stable in the gaseous phase, whereas the Sk0 and C conformers have calculated relative energies of ca. 0.6 and 0.8–3.0 kJ mol 1, respectively (depending on the level of theory). In agreement with the relatively large (>6 kJ mol 1) calculated barriers for conformational interconversion, the three conformers could be efficiently trapped in an argon matrix at 10 K, the experimental infrared spectrum of the as-deposited matrix fitting well the simulated spectrum built from the calculated spectra for individual conformers scaled by their predicted populations at the temperature of the vapour of the compound prior to matrix deposition. Upon annealing the matrix at 24 K, however, both Sk and Sk0 conformers were found to convert to the more polar C conformer, indicating that this latter form becomes the most stable ABIOD conformer in the argon matrix.
- Exploring saccharinate-tetrazoles as selective Cu(II) ligands: structure, magnetic properties and cytotoxicity of copper(II) complexes based on 5-(3-aminosaccharyl)-tetrazolesPublication . Ismael, Amin; Henriques, M. S. C.; Marques, C.; Rodrigues, M.; Barreira, Luísa; Paixao, J. A.; Fausto, R.; Cristiano, M. Lurdes S.The role of copper in the proliferation of cancer cells is under investigation and has been explored in the context of cancer chemotherapy. The evidence that proliferation of cancer cells requires a higher abundance of Cu(II) than their normal counterparts has prompted the development of new copper chelators that can avidly bind copper ions, forming redox active metal complexes that ultimately lead to harmful reactive oxygen species (ROS) in neoplasms. In this context, the mandatory properties of the chelators for medical applications are safety (neglectable cytotoxicity), high binding affinity and selectivity towards Cu(II). We report the synthesis, structure (calculations and single crystal X-ray diffraction), spectroscopic (IR; UV-Vis) and magnetic properties of two novel copper(II) complexes based on 5-(3-aminosaccharyl)-tetrazoles (TS and 2MTS), as well as their in vitro cytotoxicity against the human hepatic carcinoma cell line HepG2. Quite interestingly, we found that the saccharinate-tetrazoles tested exhibit strong binding selectivity to Cu(II), over Fe(II) and Ca(II). Additionally, the corresponding copper complexes have shown a huge increase in the in vitro cytotoxicity against tumoral cells, compared to the corresponding nontoxic ligands. Thus, the new ligands may be viewed as potential precursors of selective cytotoxic agents, acting as non-cytotoxic pro-drugs that can be activated inside neoplastic cells, known to be richer in Cu(II) than the corresponding normal cells.
- First observation of Chapman rearrangement of a pseudosaccharyl ether in the solid state: the thermal isomerization of 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide revisitedPublication . Almeida, R.; Gómez-Zavaglia, A.; Kaczor, A.; Cristiano, Maria Lurdes Santos; Eusébio, M. E. S.; Maria, T. M. R.; Fausto, R.3-(Methoxy)-1,2-benzisothiazole 1,1-dioxide, a pseudosaccharyl ether, was long ago known to undergo a thermal Chapman-like [1,30]-isomerization to the corresponding N-methyl pseudosaccharin at temperatures above its melting point (ca. 184 C) [Hettler H., Tetrahedron Lett. 1968, 15, 1793]. In the present study, it is shown that this rearrangement can also take place in the solid state, at temperatures as low as 150 C. This was the first observation of a Chapman-like [1,30]-isomerization in pseudosaccharyl ethers in the solid state. The study has been carried out by a multidisciplinary approach using temperature dependent infrared spectroscopy, differential scanning calorimetry (DSC), and polarized light thermomicroscopy, complemented by theoretical methods.
- Genesis of rare molecules using light-induced reactions of Matrix-Isolated TetrazolesPublication . Frija, L.; Cristiano, Maria Lurdes Santos; Gómez-Zavaglia, A.; Reva, I. D.; Fausto, R.Tetrazoles exhibit a very rich photochemistry, strongly influenced by the nature of substituents in the tetrazolic ring. Photolysis of representative tetrazoles trapped in a rigid environment of solidified noble gases at cryogenic temperatures (usually argon at 10–15 K) results in photofragmentation of the monomeric species with a wide range of exit channels. Since the obtained fragments are generally confined to the matrix cage where they are formed, no subsequent cross-reactions involving species resulting from photolysis can occur, strongly reducing the number of possible photoproducts in comparison with gas phase or solution studies. These conditions introduce a useful simplification for the interpretation of the reaction mechanisms and enable spectroscopic characterization of novel and/or highly reactive molecules. In this review, we provide an updated report on the photolysis of matrix-isolated tetrazoles, focusing on the scope and usefulness of this methodology for generation of rare molecules and investigation of photocleavage pathways. Special emphasis is placed on mechanistic interpretations and characterization of rare molecules and on the relevance of conformation and tautomerism on the photochemistry of the studied compounds.
- Infrared spectrum and UV-induced photochemistry of matrix-isolated 5-methoxy-1-phenyl-1H-tetrazolePublication . Gómez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, Maria Lurdes Santos; Fausto, R.The molecular structure, vibrational spectra and photochemistry of 5-methoxy-1-phenyl-1H-tetrazole (5MPT) were studied by matrix isolation infrared spectroscopy and DFT(B3LYP)/6-311++G(d,p) calculations. According to the calculations, only one double degenerated-by-symmetry minimum exists in the ground state PES of the compound. In this structure, the dihedral angle between the two rings (phenyl and tetrazole) is ca. 30◦, whereas the methoxyl group stays nearly in the plane of the tetrazole ring. In consonance with the theoretical predictions, only one molecular species was experimentally observed in the as-deposited argon matrices. Theoretical calculations were also used to help in assignment of the experimental spectrum of the compound, the calculated spectrum showing a very good agreement with the experimental data. In situ UVirradiation (λ > 235 nm) of the matrix-isolated 5MPT induced unimolecular decomposition of the compound, which led mainly to production of methylcyanate and phenylazide, this latter further reacting to yield, as final product, 1-aza-1,2,4,6-cycloheptatetraene. 3-Methoxy-1-phenyl-1Hdiazirene was also observed experimentally as minor product, resulting from direct elimination of molecular nitrogen from 5MPT.
- Low temperature matrix-isolation and solid state vibrational spectra of 5-chlorotetrazolePublication . Bugalho, Susana C. S.; Serra, Arménio C.; Lapinski, L.; Cristiano, Maria Lurdes Santos; Fausto, R.The vibrational spectra of 5-chlorotetrazole (CN4HCl) isolated in an argon matrix (T ¼ 8.5 K) and in the solid state (at room temperature) were studied. The infrared spectrum of monomers of 5-chlorotetrazole isolated in an argon matrix agrees well with the spectrum predicted theoretically (DFT(B3LYP)/6-31G*) for the 2Htautomer of the compound. The bands assigned to the 1H-tautomer appear in the experimental spectrum as very low intensity features. Based on the relative intensities of the bands in the spectra of the 1H- and 2Htautomers, the relative amount of the first tautomer in this matrix can be estimated as 1%. Three matrixes were deposited with different nozzle temperatures and the enthalpy difference between the tautomers DH ¼ 8.0 kJ mol 1 was estimated using the Van’t Hoff relation. The internal energy difference between the two tautomers was predicted theoretically (DFT B3LYP/6-31G*) as 12.6 kJ mol 1. This is in reasonable agreement with experimental observations. In the crystalline phase, this compound exists in its 1H-tautomeric form. Accordingly, the IR spectrum of polycrystalline 5-chlorotetrazole is well reproduced by the spectrum predicted theoretically for the 1H- tautomer.
- Low temperature matrix-isolation and solid state vibrational spectra of tetrazolePublication . Bugalho, Susana C. S.; Maçôas, Ermelinda M. S.; Cristiano, Maria Lurdes Santos; Fausto, R.Infrared spectra of tetrazole (CN isolated in an argon matrix (T \10 K) and in the solid state (at room 4H2) temperature) were investigated. In the crystalline phase, tetrazole exists in its 1H-tautomeric form and new assignments of the vibrational spectra (both infrared and Raman) of this phase are presented. The infrared spectrum of the matrix-isolated monomeric form of tetrazole is now reported and assigned for the Ðrst time, showing essentially the expected signature of the 2H-tetrazole tautomer. From relative intensities of the infrared bands ascribable to the two tautomers, the amount of the 1H-tautomer in the argon matrix was estimated to be ca. 10% of the most stable tautomer. Assuming that gas-phase relative populations of the two tautomers could be efficiently trapped in the argon matrix during deposition, the energy di erence between 1H- and 2H-tetrazole (*E was then obtained. The experimental value, kJ mol~1, 1Hh2H) *E1Hh2H\6.95^1.50 now determined for the Ðrst time, compares fairly well with the theoretical predictions for the molecule in vacuum (e.g., the zero point vibrational energy corrected energy di erence obtained at the B3LYP/6È31G* level of theory is 9.96 kJ mol~1).
- Matrix-isolation FTIR, theoretical structural analysis and reactivity of amino-saccharins: N-(1,1-dioxo-1,2-benzisothiazol-3-yl)-N-methyl amine and -N,N-dimethyl aminePublication . Almeida, R.; Gómez-Zavaglia, A.; Kaczor, A.; Ismael, Amin; Cristiano, Maria Lurdes Santos; Fausto, R.In this work, two novel amino-substituted derivatives of saccharin, N-(1,1-dioxo-1,2-benzisothiazol-3-yl)-N-methyl amine (MBAD) and N-(1,1-dioxo-1,2-benzisothiazol-3-yl)-N,N-dimethyl amine (DMBAD), were synthesized and characterized, and their molecular structure and vibrational properties were investigated by matrix-isolation FTIR spectroscopy and theoretical calculations undertaken using different levels of approximation. The calculations predicted the existence of two conformers of MBAD. The lowest energy form was predicted to be considerably more stable than the second conformer (DE > ca. 20 kJ mol 1) and was the sole form contributing to the infrared spectrum of the compound isolated in solid xenon. Both conformers have planar amine moieties. In the case of DMBAD, only one doubly-degenerated-by-symmetry conformer exists, with the amine nitrogen atom considerably pyramidalized. The effect of the electron-withdrawing saccharyl ring on the C–N bond lengths is discussed. The different structural preferences around the amine nitrogen atom in the two molecules were explained in terms of repulsive interactions involving the additional methyl group of DMBAD. Observed structural features are correlated with the reactivity exhibited by the two compounds towards nucleophiles. The experimentally obtained spectra of the matrix-isolated monomers of MBAD and DMBAD were fully assigned by comparison with the corresponding calculated spectra.
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