Browsing by Author "Borba, A."
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- 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.
- Structure and photochemistry of a novel tetrazole-saccharyl conjugate isolated in solid argonPublication . Ismael, Amin; Borba, A.; Duarte, L.; Giuliano, B. M.; Gómez-Zavaglia, A.; Cristiano, Maria Lurdes SantosA combined matrix isolation FTIR and theoretical DFT/B3LYP/6-311++G(3df,3pd) study of the novel synthesised tetrazole-saccharyl conjugate 2-[1-(1H-tetrazol-5-yl)ethyl]-1,2-benzisothiazol-3(2H)-one 1,1-dioxide [1-TE-BZT] was performed. In the gas phase, at room temperature, the compound exists as a mixture of six isomeric forms (four conformers of 1H tautomer and two conformers of 2H tautomer). According to theoretical calculations, conformers 1H were the most stable and the relative energies among the three most stable forms are lower than 4 kJ mol 1. These conformers benefit from stabilising intramolecular hydrogen bonds-like interactions involving the 1H of the tetrazole ring and the carbonyl oxygen of the saccharyl moiety. The photochemistry of 1-TE-BZT in solid argon was investigated and theoretical DFT/B3LYP/6-311++G(3df,3pd) calculations also helped in assignment of the experimental bands. A quick consumption of the compound occurred after irradiation of the matrix with UV laser light at k = 275 nm. Three photofragmentation pathways were proposed, one leading to 2-[1-(1H-diaziren-3-yl)ethyl]-1,2-benzisothiazol-3(2H)-one 1,1-dioxide and molecular nitrogen, a second one giving 2-(1,1-dioxide-3-oxo-1,2-benzisothiazol-2(3H)-yl)propanenitrile and azide, and a third one involving loss of azide from the tetrazole ring and decarbonylation of the saccharyl ring of 1-TE-BZT to give acrylonitrile and 7-thia-8- azabicyclo[4.2.0] octa-1,3,5-triene 7,7 dioxide. The comparison of the relative intensities of the bands of the photoproducts obtained from the three channels allowed us to consider the latter pathway, involving an unprecedented photocleavage of the benzisothiazole (saccharyl) ring, as the preferred photodegradation channel of 1-TE-BZT.