12 research outputs found
Carboxylic Group and Its Tetrazolyl Isostere in One Molecule. Matrix Isolation FTIR and DFT Studies on Thermal Decomposition and Photochemistry of (Tetrazol-5-yl)acetic Acid
The title compound
(tetrazol-5-yl)acetic acid (TA) is an interesting
molecule that contains both a carboxylic group and its isostere tetrazolyl
group. Out of nine theoretically predicted stable structures of TA,
three appeared to be present in solid argon. Thermal decomposition
of the species aided by water molecules was studied in detail both
experimentally using FTIR matrix isolation technique and theoretically
at the B3LYP/6-311++G(2d,2p) level. Experimentally, it was found that
the decarboxylation process appeared at the presence of water traces
in the system. Theoretically, it was shown that the energy barrier
of the water assisted process was lower by ca. 30 kJ mol<sup>–1</sup> comparing to the process without water participation. The UV photolysis
of the TA/Ar system was studied using both broad-band and narrow-band
sources. The main photoproducts appeared to be carbodiimidylacetic
acid and (1<i>H</i>-diaziren-3-yl)acetic acid. The progress
of the reactions induced was followed by FTIR spectroscopy, whereas
interpretation of the results was supported by quantum chemical calculations
(DFT, TD-DFT)
Conformational Behavior and Tautomer Selective Photochemistry in Low Temperature Matrices: The Case of 5-(1H-Tetrazol-1-yl)-1,2,4-triazole
The conformational properties and the photolysis behavior of one of the simplest N-C bonded bicyclic azoles, 5-(1H-tetrazol-1-yl)-1,2,4-triazole (T), were studied in argon and xenon matrices by infrared spectroscopy. Analysis of the experimental results was supported by extensive theoretical calculations carried out at the B3LYP/6-311++G(2d,2p) level of approximation. Out of the eight T minima located on the potential energy surface, the three most stable species were detected in low temperature matrices, namely, 5-(1H-tetrazol-1-yl)-1H-1,2,4-triazole (T1) and two conformers of 5-(1H-tetrazol-1-yl)-2H-1,2,4-triazole (T2a and T2b). With increase of the substrate temperature either during deposition of the matrices or during annealing the T2b -> T2a conversion took place, in agreement with the predicted low energy barrier for this transformation (5.38 kJ mol(-1)). Both broad band and narrow band laser UV irradiations of T isolated in Xe and Ar matrices induce unimolecular decomposition involving cleavage of the tetrazole ring of T1 and T2a (T2b) that leads to the production of 1H-1,2,4-triazol-5-yl carbodiimide (P1) and 1H-1,2,4-triazol-3-yl carbodiimide (P2), respectively. When the laser is used, in addition to the main P1 and P2 photoproducts, several minor products could be successfully identified in the matrices: N-cyanocarbodiimide HNCNCN (detected for the first time) associated with nitrilimine HNNCH and HCN. An interesting phenomenon of tautomer-selective photochemistry was observed for the matrix-isolated compound. It could be explained by the different LUMO-HOMO energy gaps estimated for T1, T2a, and T2b, connected with different threshold energies necessary to start the photolysis of T1 and T2a (T2b)
Polymorphism and conformational equilibrium of nitro-acetophenone in solid state and under matrix conditions
Conformational and polymorphic states in the nitro-derivative of o-hydroxy acetophenone have been studied by experimental and theoretical methods. The potential energy curves for the rotation of the nitro group and isomerization of the hydroxyl group have been calculated by density functional theory (DFT) to estimate the barriers of the conformational changes. Two polymorphic forms of the studied compound were obtained by the slow and fast evaporation of polar and non-polar solutions, respectively. Both of the polymorphs were investigated by Infrared-Red (IR) and Raman spectroscopy, Incoherent Inelastic Neutron Scattering (IINS), X-ray diffraction, nuclear quadrupole resonance spectroscopy (NQR), differential scanning calorimetry (DSC) and density functional theory (DFT) methods. In one of the polymorphs, the existence of a phase transition was shown. The position of the nitro group and its impact on the crystal cell of the studied compound were analyzed. The conformational equilibrium determined by the reorientation of the hydroxyl group was observed under argon matrix isolation. An analysis of vibrational spectra was achieved for the interpretation of conformational equilibrium. The infrared spectra were measured in a wide temperature range to reveal the spectral bands that were the most sensitive to the phase transition and conformational equilibrium. The results showed the interrelations between intramolecular processes and macroscopic phenomena in the studied compound