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

N N Karaush; V A Minaeva; G V Baryshnikov; B F Minaev; H Ågren

doi:10.1016/j.saa.2018.05.096

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

Spectrochim Acta A Mol Biomol Spectrosc. 2018 Oct 5:203:324-332. doi: 10.1016/j.saa.2018.05.096. Epub 2018 May 29.

Authors

N N Karaush¹, V A Minaeva², G V Baryshnikov³, B F Minaev³, H Ågren⁴

Affiliations

¹ Department of Chemistry and Nanomaterials Science, Bohdan Khmelnytsky National University, 18031, Cherkasy, Ukraine. Electronic address: karaush22@ukr.net.
² Department of Chemistry and Nanomaterials Science, Bohdan Khmelnytsky National University, 18031, Cherkasy, Ukraine.
³ Department of Chemistry and Nanomaterials Science, Bohdan Khmelnytsky National University, 18031, Cherkasy, Ukraine; Division of Theoretical Chemistry and Biology, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, 10691 Stockholm, Sweden.
⁴ Division of Theoretical Chemistry and Biology, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, 10691 Stockholm, Sweden; Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.

PMID: 29879648
DOI: 10.1016/j.saa.2018.05.096

Abstract

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.

Keywords: DFT calculations; FTIR spectra; NMR spectra; Tautomers; UV–vis spectra.