Aggregation of o, o′-dihydroxyazo dyes II. Interaction of 2-hydroxy-4-nitrophenylazoresorcinol in DMSO and DMF

Tautomeric behaviour and isotopic multiplets in the 13C NMR spectra of partially deuterated 5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones and 5‐arylazo‐2‐thioxo‐pyrimidine (1H,3H,5H)‐4,6‐diones—evidence for elucidation of tautomeric forms

“…All dyes show azo-hydrazone tautomerism (Scheme 1). [24,25] Dyes 5a-7a have bifurcated intramolecular hydrogen bond and are examples of hydrogen bonds involving one proton and two acceptors [26,27] (Scheme 2). Two NH protons on pyrimidine-2,4,6-trione ring moiety in 5a-g have different chemical shifts, except 5b, and tautomeric behavior of dyes 7a-g is similar to that of 5a-g, except in the case of 7d.…”

Azocoupling products. V. Electronic spectroscopy study of two azocoupling products of 1-(4-hydroxy-6-methyl-pyrimidin-2-yl)-3-methyl-pyrazolin-5-one

“…Consequently, also by the effect of the concentration variation on the spectra of dyes 7 and 8 should be involved the above mentioned ionization-and eventually azoehydrazone equilibria. Otherwise, both the dilution and the increase of pH favors also in the case of other dyes that theoretically may be involved in azoe hydrazone tautomerism the similar shifts of ionization equilibrium towards ionized forms [3b,4b,6] and of azoe hydrazone equilibrium towards azo-form [6,8]. We mention however that the determined effect on the spectra of the dye 7 in EtOH solution by simple concentration variation (Fig.…”

“…The above situation is compatible with a normal ionization step corresponding to pH# values where quite well definite isosbestic points were obtained and a non-simple ionization step (cf. [8]) at lower pH# values with distorted isosbestic points (e.g. Fig.…”

“…6a, 8a, 9a, 12a 1, a 2 ) may correspond to acide base equilibria (cf. [3b,4b,5a, 8,13,22]) (see also Section 2.2.2). On the other hand, the existence of two ionization steps is confirmed by the plots absorbance vs. pH# at a certain analytical wavelength for each dye 7 or 8 because there were obtained sigmoidal curves in two pH# ranges (see e.g.…”

“…1a, 4a, 6) equilibria (Scheme 1). It must be underlined that such interconnected equilibria, as above [3,4,5a] or involving either in addition, [5b,6] or instead of acide base equilibrium, [7,8] an aggregation equilibrium, were formulated also in the quoted studies [3e8] concerning other azocoupling products that theoretically may be involved in azoehydrazone tautomerism. Therefore, the aim of this paper is to find new data in support of the proposed [1,2] interconnected equilibria and eventually even for the aggregation process.…”