Browsing by Author "Reva, I. D."
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- 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.
- Molecular structure, vibrational spectra and photochemistry of 2-methyl-2 h -tetrazol-5-amine in solid argonPublication . Gómez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, Maria Lurdes Santos; Fausto, R.In this work, the molecular structure, infrared spectrum and UV photochemistry of 2-methyl-2H-tetrazol-5-amine (2MTA) isolated in solid argon (10 K) were investigated. The experimental studies were supported by extensive DFT(B3LYP)/6-311++G(d,p) calculations. The infrared spectrum of matrix-isolated 2MTA was fully assigned and correlated with structural properties. Taking into consideration the observed frequency of the NH2 wagging mode, it is suggested that, in the matrixes, the amine group becomes slightly more planar than in the gas phase, due to matrix-packing effects. In situ UV irradiation (ì > 235 nm) of the matrixisolated 2MTA monomer is shown to induce three main primary photochemical processes: (1) tautomerization to mesoionic 3-methyl-1H-tetrazol-3-ium-5-aminide; (2) nitrogen elimination, with production of 1-methyl-1H-diazirene-3-amine; (3) ring cleavage leading to production of methyl azide and cyanamide. Following the primary photoproducts, secondary reactions were observed, leading to spectroscopic observation of methylenimine and isocyanidric acid.
- Molecular structure, vibrational spectra and photochemistry of 5-mercapto-1-methyltetrazolePublication . Gómez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, Maria Lurdes Santos; Fausto, R.In this work, 5-mercapto-1-methyltetrazole was studied by low temperature matrix-isolation and solid-state infrared spectroscopy, DFT(B3LYP)/6-311CCG(d,p) calculations and photochemical methods. In the low temperature neat solid phase and isolated in an argon matrix, the compound was found to exist in the 1-methyl-1,4-dihydro-5H-tetrazole-5-thione tautomeric form. The infrared spectra of the compound were fully assigned and correlated with structural properties. In situ UV-irradiation (lO235 nm) of the matrix-isolated monomer is shown to induce different photochemical processes, all of them involving cleavage of the tetrazole ring: e.g. (1) molecular nitrogen expulsion, with production of 1-methyl-1H-diazirene-3-thiol, which is produced in two different conformers; (2) ring cleavage leading to production of methyl isothiocyanate and azide; (3) simultaneous elimination of nitrogen and sulphur with production of N-methylcarbodiimide. Following these photoprocesses, subsequent reactions occur, leading to production of methyl diazene, carbon monosulphide and nitrogen hydride. Spectroscopic evidence of the production of the above-mentioned chemical species is provided.
- Photochemical ring-opening and intramolecular hydrogen shift reactions in sulfur analogues of a-pyronePublication . Breda, S.; Reva, I. D.; Lapinski, L.; Cristiano, Maria Lurdes Santos; Frija, L.; Fausto, R.A combined matrix isolation FTIR and theoretical DFT(B3LYP)/6-311++G(d,p) study of the photochemistry of sulfur analogues of R-pyrone [2H-thiopyran-2-one (TP) and 2H-pyran-2-thione (PT)] was carried out. The vibrational spectra of monomers of the compounds isolated in low-temperature argon matrixes were studied experimentally and assigned completely on the basis of theoretical calculations. UV irradiation (ì > 337 nm) of the studied compounds isolated in low-temperature matrixes results mainly in the ring-opening reaction by means of the cleavage of the R-bond. Other photoprocesses, not involving the R-bond-cleavage step (such as generation of Dewar valence isomer), correspond to the minor reaction channels in both studied compounds. The ring-opening photoreaction in PT represents the first reported case of an R-bond cleavage in a compound with a CdS group attached to a six-membered ring, in which the internal strain practically does not exist, whereas the corresponding reaction in TP (a cleavage of a C-S bond in the R position with respect to a carbonyl group) is now reported for the first time. Following the ring-opening reactions, isomerization processes and intramolecular hydrogen shift reactions were observed, enabling production of TP from PT and vice versa. A detailed study of such processes was undertaken, and kinetical and mechanistical data are presented and discussed.
- Photochemistry and vibrational vpectra of matrix-isolated 5-ethoxy-1-phenyl-1 H -tetrazolePublication . Frija, L.; Reva, I. D.; Gómez-Zavaglia, A.; Cristiano, Maria Lurdes Santos; Fausto, R.A combined matrix isolation FT-IR and theoretical DFT(B3LYP)/6-311++G(d,p) study of the molecular structure and photochemistry of 5-ethoxy-1-phenyl-1H-tetrazole (5EPT) was performed. A new method of synthesis of the compound is described. Calculations show three minima, very close in energy and separated by low-energy barriers (less than 4 kJ mol-1), in the ground-state potential energy profile of the molecule. The method of matrix isolation enabled the reduction of the number of populated conformational states in the experiment at low temperature due to the effect known as conformational cooling. As a result, the spectrum of the as-deposited matrix of 5EPT closely matches that of the most stable conformer predicted theoretically, pointing to the existence of only this conformer in the low-temperature matrixes. In this structure, the dihedral angle between the two rings, phenyl and tetrazole, is ca. 30°, whereas the ethyl group stays nearly in the plane of the tetrazole ring and is as far as possible from the phenyl group. In situ UV irradiation (ì > 235 nm) of the matrix-isolated 5EPT induced unimolecular decomposition, which led mainly to production of ethylcyanate and phenylazide, this later compound further reacting to yield, as final product, 1-aza-1,2,4,6- cycloheptatetraene. Anti-aromatic 3-ethoxy-1-phenyl-1H-diazirene was also observed experimentally as minor photoproduct, resulting from direct extrusion of molecular nitrogen from 5EPT. This species has not been described before and is now characterized by infrared spectroscopy for the first time.
- Photochemistry of 1-phenyl-tetrazolone isolated in solid argonPublication . Gómez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, Maria Lurdes Santos; Fausto, R.The molecular structure, vibrational spectra, tautomerism and photochemistry of the derivative of tetrazole, 1-phenyl-tetrazolone (C7H6N4O; PT) have been studied by FT-IR matrix isolation spectroscopy and DFT/B3LYP/6-311++G(d,p) calculations. Among the five structures in which PT could be expected to exist (two keto tautomers, one mesoionic olate-form and two different conformers of the hydroxyl tautomer), only the most stable species, 1-phenyl-1,4-dihydro-5H-tetrazol-5-one, could be experimentally observed in low temperature argon matrices. Monomers of this tautomer give rise to an IR spectrum that fits nicely the calculated spectrum obtained at the DFT/B3LYP/6-311++G(d,p) level of theory. In situ UV irradiation (λ > 235 nm) of the matrix-isolated PT induces three main photochemical processes, all of them involving cleavage of the tetrazole ring: e.g. (1) molecular nitrogen loss, with production of 1-phenyl-diaziridin-3-one; this compound reacts subsequently to form 1-aza-1,2,4,6-cycloheptatetraene and isocyanic acid (eventually, also to form CO plus phenyldiazene); (2) cleavage of the C(5)–N(1) and N(3)–N(4) tetrazole-ring bonds, with production of phenylazide and isocyanic acid, with phenylazide then losingN2 to yield as final product 1-aza-1,2,4,6-cycloheptatetraene; and (3) cleavage of the N(1)–N(2) and N(4)–C(5) tetrazole-ring bonds, to yield phenylisocyanate and azide. The observed photochemical processes are distinct from the preferred thermal fragmentation channel, where CO is produced together with a weak IR absorbant species.
- Photochemistry of 5-allyloxy-tetrazoles: steady-state and laser flash photolysis studyPublication . Frija, L.; Khmelinskii, Igor; Serpa, C.; Reva, I. D.; Fausto, R.; Cristiano, Maria Lurdes SantosThe photochemistry of three 5-allyloxy-tetrazoles, in methanol, acetonitrile and cyclohexane was studied by product analysis and laser flash photolysis. The exclusive primary photochemical process identified was molecular nitrogen elimination, with formation of 1,3-oxazines. These compounds were isolated in reasonable yields by column chromatography on silica gel and were fully characterized. DFT(B3LYP)/6-31G(d,p)calculations predict that these 1,3-oxazines can adopt two tautomeric forms (i) with the NH group acting as a bridge connecting the oxazine and phenyl rings and (ii) with the –N=bridge and the proton shifted to the oxazine ring. Both tautomeric forms are relevant in the photolysis of oxazines in solution. Secondary reactions were observed, leading to the production of phenyl vinyl-hydrazines, enamines, aniline and phenyl-isocyanate. Transient absorption, detected by laser flash photolysis, is attributed to the formation of triplet 1,3-biradicals generated from the excited 5-allyloxy-tetrazoles. The 1,3-biradicals are converted to 1,6-biradicals by proton transfer, which, after intersystem crossing, decay to generate the products. Solvent effects on the photoproduct distribution and rate of decomposition are negligible.
- Photoisomerization of saccharinPublication . Duarte, L.; Reva, I. D.; Cristiano, Maria Lurdes Santos; Fausto, R.Most known applications of saccharin and saccharyl derivatives and their potential for new uses rely on the thermal and photochemical stability of the saccharyl system. Here, we show that saccharin undergoes structural rearrangement when subjected to a narrow-band ultraviolet irradiation. Monomeric saccharin was isolated in low-temperature argon matrices and its photochemistry was characterized by means of infrared spectroscopy and DFT calculations. Among several DFT methods used, the O3LYP/6-311++G(3df,3pd) level gave the best match with the experimental spectra. Irradiation of matrix-isolated saccharin, with a narrow-band source (290 nm), generates a so far unknown isomer that we call iso-saccharin. The structures of the conjugate bases of saccharin and iso-saccharin were also computed theoretically. Their free energies and dipole moments suggest that both anions may be relevant in systems where saccharin participates, as is the case of the recently proposed saccharin-based ionic liquids.
- Sigmatropic rearrangements in 5-allyloxytetrazolesPublication . Frija, L.; Reva, I. D.; Ismael, Amin; Coelho, Daniela; Fausto, R.; Cristiano, Maria Lurdes SantosMechanisms of thermal isomerization of allyl tetrazolyl ethers derived from the carbocyclic allylic alcohols cyclohex-2-enol and 3-methylcyclohex-2-enol and from the natural terpene alcohol nerol were investigated. In the process of the syntheses of the three 1-aryl-5-allyloxytetrazoles, their rapid isomerization to the corresponding 1-aryl-4-allyltetrazol-5-ones occurred. The experiments showed that the imidates rearrange exclusively through a [3,3¢]-sigmatropic migration of the allylic system from O to N, with inversion. Mechanistic proposals are based on product analysis and extensive quantum chemical calculations at the DFT(B3LYP) and MP2 levels, on O-allyl and N-allyl isomers and on putative transition state structures for [1,3¢]- and [3,3¢]-sigmatropic migrations. The experimental observations could be only explained on the basis of the MP2/6-31G(d,p) calculations that favoured the [3,3¢]-sigmatropic migrations, yielding lower energies both for the transition states and for the final isomerization products.