4-Carboxyl-2,6-dinitrophenylazohydroxynaphthalenes tautomerism NMR re-explained and other methods verified

Abstract

In two consecutive studies the tautomerism in 4-((2-hydroxynaphthalen-1-yl)diazenyl)-3,5dinitrobenzoic acid and the structurally similar 1-((2-nitrophenyl)diazenyl)naphthalen-2-ol has been considered from viewpoint of theoretical chemistry, UV-Vis spectroscopy and NMR.Although the theoretical data (at M062X level) show that both compounds exist only as a keto tautomer, the experiment proves existence of the enol form. The difference in the results obtained by UV-Vis spectroscopy and NMR requires a deeper consideration and verification of the NMR approach, which is based on using model compounds to provide the NMR signal pattern of individual tautomers.

Tautomerism in azo dyes: Border cases of azo and hydrazo tautomers as possible NMR reference compounds

“…The NMR chemical shieldings of selected tautomeric forms of the studied compounds were calculated using the GIAO approximation [33] at the B3LYP/6-311 + G(2d,p) level of theory. This level of theory was recommended in the pioneering work by Cheeseman et al [34] focused on the comparison of different models for calculating nuclear magnetic resonance shielding tensors and shows very good results in predicting the NMR spectra of azo-hydrazone tautomerism [35]. For comparison purposes the values obtained by using M06-2X/TZVP are presented as well.…”

Isomerization and aggregation of 2-(2-(2-hydroxy-4-nitrophenyl)hydrazono)-1-phenylbutane-1,3-dione: Recent evidences from theory and experiment

“…The NMR chemical shieldings of selected tautomeric forms of the studied compounds were calculated using the GIAO approximation [19] at the B3LYP/6-311+G(2d,p) level of theory. This level of theory was recommended in the pioneering work by Cheeseman et al [20], focused on the comparison of different models for calculating nuclear magnetic resonance shielding tensors and shows very good results in predicting the NMR spectra of azo-hydrazone tautomerism [21]. The calculated absolute shieldings were transformed to chemical shifts using the reference compound tetramethylsilane, Si(CH 3 ) 4 , for hydrogen, carbon and nitromethane, CH 3 NO 2 , for nitrogen atoms: Fig.…”

A detailed UV–Vis spectral investigation of six azo dyes derived from benzoic- and cinnamic acids: experimental and theoretical insight

“…The strength of these specific and nonspecific interactions is a limiting factor that influences further solvent-tautomeric proton interactions, and there-fore, the prototropy. On the other hand, there is a drawback in this field of research regarding the inability of spectroscopic characterization techniques, such as UV-Vis, FT-IR and NMR spectroscopy, to identify individual tautomeric species of certain molecules in solution [1,5]. In order to overcome the lack of information given by spectroscopic methods for a pure, single tautomer, various mathematical approximations provided by quantum-chemical models are often employed in the research [3].…”