Identification of tautomeric intermediates of a novel thiazolylazonaphthol dye - A density functional theory study.

The recently synthesized thiazolylazo dye, 1-[5-benzyl-1,3-thiazol-2-yl)diazenyl]naphthalene-2-ol called shortly BnTAN, is studied by density functional theory (DFT) in three tautomeric forms in order to explain the available 1H NMR, UV-Vis and FTIR spectra. An experimentally observed IR band at 1678 cm-1, assigned to the CO bond stretching vibration, supports the notion that BnTAN retains in the less stable keto-form even in the solid state due to an ultrafast single-coordinate intramolecular proton transfer. This finding is also in a good agreement with an X-ray crystallography analysis which indicates an intermediate position of the proton between the -OH and -N=N- groups. Calculations also show that some experimentally observed 1H NMR signals could be considered as being averaged values between theoretically calculated chemical shifts for the corresponding protons in the keto- and enol-tautomers. At the same time the UV-Vis spectra are almost insensitive to the tautomerization processes as the main single band absorption at 500 nm is present in all tautomers according to our TD DFT simulations. The minor differences in spectral features of the long-wavelength visible region are also noted and discussed with respect to the manifestation of the less stable tautomer form.

Aromaticity of the planar hetero[8]circulenes and their doubly charged ions: NICS and GIMIC characterization.

A series of planar hetero[8]circulenes and their doubly charged ions are studied by the NICS and GIMIC methods to interpret the aromatic properties of these high symmetry species. In accordance with the performed calculations all studied hetero[8]circulenes are found to be nonaromatic compounds because paratropic and diatropic ring-currents are completely canceled yielding almost zero net current. In great contrast, the dicationic and dianionic hetero[8]circulenes demonstrate the predominant contribution of diatropic ring currents resulting in the total aromatic character of the studied doubly charged ions. This fact allows us to predict the high stability of dianionic hetero[8]circulenes and explains the extremely high stability of dicationic species observed in the mass-spectra.