Browsing by Author "Kaczor, A."
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- 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.
- 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.
- 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.
- Molecular structure and infrared spectra of the monomeric 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide (methyl pseudosaccharyl ether)Publication . Kaczor, A.; Almeida, R.; Gómez-Zavaglia, A.; Cristiano, Maria Lurdes Santos; Fausto, R.The computational description of saccharin (1,2-benzisothiazol-3(2H)-one-1,1-dioxide) and its derivatives is difficult due to the presence of hypervalent S@O bonds in their structures. Therefore, in this investigation, the HF, DFT/B3LYP and MP2 methods were used to predict the geometry and the infrared spectrum of the saccharyl derivative 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide (MBID). Their relative predictive capabilities were then evaluated by comparing the obtained results with experimentally available data, namely the newly obtained IR spectra of MBID isolated in low-temperature inert matrices. For each method, different basis sets [6-31++G(d,p), 6-31++G(3df,3pd), 6-311++G(d,p), 6-311++G(2df,2pd), 6-311++G(3df,3pd), aug-cc-pVDZ and aug-cc-pVTZ] were considered. The best overall agreement has been achieved at the B3LYP/6-311++G(3df,3pd) and B3LYP/6-31++G(3df,3pd) levels of theory, showing the adequacy of the B3LYP functional to describe the investigated properties in this type of compounds and stressing the relevance of including high-order polarization functions in the basis set. The chosen level of theory [B3LYP/6-311++G(3df,3pd)] was applied to analyze the vibrational spectra and the geometry of the title molecule. In agreement with the experiment, the CAOAC linkage in MBID is predicted by these calculations to exhibit considerably short (1.320A ˚ ) and long (1.442A ˚ ) (N@)CAO and (H3)CAO bonds, respectively, and a hybridization of the central oxygen atom close to sp2 (the CAOAC angle is predicted to be ca. 117 ). This CAOAC bonding pattern fits the well-known high reactivity of MBID upon thermal rearrangement, which has been shown to result in easy selective [1,30]-isomerization of the compound.
- Structural investigation of nitrogen-linked saccharinate–tetrazolePublication . Gómez-Zavaglia, A.; Ismael, Amin; Cabral, Lília; Kaczor, A.; Paixão, J. A.; Fausto, R.; Cristiano, Maria Lurdes SantosThe molecular structure of nitrogen-linked saccharinate–tetrazole, N-(1,1-dioxo-1,2-benzisothiazol-3-yl)-amine-1H-tetrazole (BAT), was investigated in the crystalline state using X-ray crystallography and infrared and Raman spectroscopies, and isolated in argon matrix by infrared spectroscopy. Interpretation of the experimental results was supported by quantum chemical calculations undertaken at the DFT(B3LYP)/6-311++G(3df,3pd) level of theory. In the neat crystalline solid (space group C2/c, a = 21.7493(3) Å, b = 8.85940(10) Å, c = 10.76900(10) Å, b = 103.3300(10) deg; Z = 8), BAT units exist in the (1H)-tetrazole aminosaccharin tautomeric form, with the NH spacer establishing a hydrogen bond to the nitrogen in position-4 of the tetrazole group of a neighbour molecule, and the NH group of the tetrazole fragment forming a bifurcated H-bond to the saccharyl nitrogen of the same molecule and to one of the oxygen atoms of a second neighbour molecule. On the other hand, according to both the matrix isolation infrared studies and the theoretical calculations, the isolated BAT molecule exists preferentially as the (1H)-tetrazole iminosaccharin tautomer, where the main stabilizing interaction is the intramolecular H-bond established between the NH group of the saccharyl ring and the tetrazole nitrogen atom in position 4. A detailed conformational analysis of the studied molecule and full assignment of the vibrational spectra for both the matrix-isolated compound and crystalline sample were undertaken.
- The chapman-type rearrangement in pseudosaccharins: the case of 3-(methoxy)-1,2-benzisothiazole 1,1-dioxidePublication . Kaczor, A.; Proniewicz, L. M.; Almeida, R.; Gómez-Zavaglia, A.; Cristiano, Maria Lurdes Santos; Matos Beja, A. M.; Ramos Silva, M.; Fausto, R.The thermal Chapman-type rearrangement of the pseudosaccharin 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide (MBID) into 2-methyl-1,2-benzisothiazol-3(2H)-one 1,1-dioxide (MBIOD) was investigated on the basis of computational models and knowledge of the structure of the reactant and product in the isolated and solid phases. X-ray diffraction was used to obtain the structure of the substrate in the crystalline phase, providing fundamental structural data for the development of the theoretical models used to investigate the reaction mechanism in the condensed phase. The intra- and different intermolecular mechanisms were compared on energetic grounds, based on the various developed theoretical models of the rearrangement reactions. The energetic preference (ca. 3.2 kJ mol 1, B3LYP/6-31+G(d,p)) of interover intramolecular transfer of the methyl group is predicted for the ‘‘quasi-simultaneous” transfer of the methyl groups model, explaining the potential of MBID towards [1,30]-isomerization to MBIOD in the condensed phases. The predicted lower energy of MBIOD relative to MBID (ca. 60 kJ mol 1), due to the lower steric hindrance in the MBIOD molecule, acts as a molecular motor for the observed thermal rearrangement.
- Thermally induced sigmatropic isomerization of pseudosaccharyl allylic etherPublication . Gómez-Zavaglia, A.; Kaczor, A.; Almeida, R.; Cristiano, Maria Lurdes Santos; Eusébio, M. E. S.; Maria, T. M. R.; Mobili, P.; Fausto, R.The thermally induced sigmatropic isomerization of the pseudosaccharyl allylic ether [3-(allyloxy)-1,2-benzisothiazole 1,1-dioxide; ABID] has been investigated by a multidisciplinary approach using temperature dependent infrared spectroscopy, differential scanning calorimetry, and polarized light thermomicroscopy, complemented by theoretical methods. Migration of the allylic system from O to N occurs in the melted ABID, and the thermally obtained 2-allyl-1,2-benzisothiazol-3(2H)-one 1,1-dioxide (ABIOD) starts to be produced at ca. 150 °C, in a process with an activation energy of ∼92 kJ mol-1. From kinetic data, a concerted [3,3′] sigmatropic mechanism is proposed. In the temperature range investigated, ABIOD was found to exhibit polymorphism. Cooling of the molten compound leads to the production of a metastable crystalline form, which upon annealing at room temperature might be transformed to the stable crystalline phase. ABID shows a single crystalline variety. Assignments were proposed for the infrared spectra of the observed neat condensed phases of the two compounds.