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The disperse dye red 2C regarded as 1-amino-4-hydroxy-9,10-anthraquinone is not a 9,10-anthraquinone derivative. It is characterized by a keto-enol and amino-imine tautomerism, and it consists of equilibrium mixtures of tautomers and conformers containing 4-amino-9-hydroxy-1,10-, 9 amino-4-hydroxy-1,10-, 9 amino-10-hydroxy-1,4-anthraquinones, 9 hydroxy-1,10-anthraquinone 10-imine, and also their conformers with a single intramolecular hydrogen bond. The significant differences in the absorption spectra known for this dye originate from the dissimilar isomeric composition of the samples of this dye prepared or purified by various procedures [ABSTRACT FROM AUTHOR]

Fine structure of πl,π* absorption of mono- and dications derived from 1,5- and 1,8-dihydroxyanthraquinones originates from their existence as dynamic equilibrium mixtures of isomers differing by position of the positive charge, double bond distribution, and number of intramolecular hydrogen bonds. Protonation is accompanied by displacement of isomeric equilibria. Isomeric transformations of protonated dihydroxyanthraquinones involve mainly excited states of their molecules. [ABSTRACT FROM AUTHOR]

The compound widely known as 1,4-diamino-9,10-anthraquinone is in fact an equilibrium mixture of 4,9-diamino-1,10-anthraquinone and tautomeric imino forms, 10-amino-9-hydroxy-1,4-anthraquinone 1-imine and its conformer, and 4-amino-1-hydroxy-9,10-anthraquinone 9-imine or 4,9-dihydroxy-1,10-anthraquinone diimine. Amino-imino tautomerism and rotational isomerism are responsible for fine structure of the πl,π*-absorption of the title compound. [ABSTRACT FROM AUTHOR]

Fine structure of quinizarine π l ,π*-absorption in sulfuric acid is defined by isomerism of its protonated products. The monocation is an isomeric mixture, the isomers differ by the charge localization on various carbon atom: 1,4,9-trihydroxy-10-oxo-10 H-anthracen-1-onium, 1,9,10-trihydroxy-4-oxo-4 H-anthracen-1-onium, and 4,9,10-trihydroxy-10-oxo-10 H-anthracen-1-onium. For the dication the structures found were exclusively the conformers of 1,4,9,10-tetrahydroxyanthrqacene 1,10-dication containing 0 to 2 intramolecular hydrogen bonds. The protonation leads to a shift of the tautomeric and conformational equilibria characteristic of quinizarine. [ABSTRACT FROM AUTHOR]

Tautomerism of β-mono-, β,β′-dihydroxyanthraquinones, and their anions was studied for the first time by quantum-chemical and correlation methods. 2-Hydroxyanthraquinone exists exclusively in 9,10-quinoid form, and its ionization involves a tautomeric transformation into 10-oxido-2,9-anthraquinone. β,β′-Dihydroxyanthraquinones can exist as the corresponding 9,10-, 2,9-, 2,6-, and 2,3-quinoid tautomers, and the most characteristic forms of their anions are 2,9-quinoid structures. The considerable difference in the known spectra of the same compound is due to the shifts of the tautomeric equilibria. [ABSTRACT FROM AUTHOR]

Quinalizarin and anions derived therefrom exist as equilibrium mixtures of different tautomers and conformers, whose structure depends on the conditions. Quinalizarin was shown to have 9,10-, 1,10-, 1,4-, 1,5-, 1,7-, and 2,9-quinoid structures, but not 1,2-quinoid structure; and its anions in ethanol media were identified as 9,10-, 1,10-, 2,9-, and 1,5-quinoid tautomers. Interactions with solvents and ionization could give rise to displacement of tautomeric and conformational equilibria, leading to considerable change in the number and position of π l ,π* bands in the electronic absorption spectra, which are responsible for the color. [ABSTRACT FROM AUTHOR]

1,5-Dihydroxyanthraquinone and its substituted derivatives are capable of existence in the states structurally described as 9,10-, 1,10-, and 1,5-quinoid tautomerism, and as rotational isomerism involving a cleavage of intramolecular hydrogen bond. 1,5-Quinoid tautomers are characteristic only of substituted derivatives, and also appear in some metal complexes. The considerable color changes on introducing into the 1,5-dihydroxy-anthraquinone methyl, methoxy, and sulfo groups are caused by the shift in tautomeric and conformer equilibria. [ABSTRACT FROM AUTHOR]

Hydroxy-substituted anthraquinones, naphthoquinones, and naphthacenequinones exist as equilibrium mixtures of different tautomers and rotational isomers which give rise to several π1,π* bands in their electronic absorption spectra. Each π1,π* band can be assigned to a particular tautomer and conformer on the basis of the substituent constants σA. [ABSTRACT FROM AUTHOR]

1-Hydroxyanthraquinone and its substituted derivatives exist as equilibrium mixtures of four tautomers and rotational isomers. Their anions have 9,10-and 1,10-quinoid structures. Each tautomer or conformer is characterized by a single π1,π* band in the electronic absorption spectrum. [ABSTRACT FROM AUTHOR]

The 9,10–1,10-anthraquinoid tautomer was found to be characteristic of physcion and emodin and their analogs in solutions using spectrophotometric, quantum-chemical, and correlation methods. Ionization of these compounds was accompanied by a shift in tautomeric equilibrium. In alkaline solutions 1,10-anthraquinoid anions with a single α-hydroxy that were stabilized by an intramolecular H-bond were formed. Tautomerism occurred in both the ground and excited states of the molecules. [ABSTRACT FROM AUTHOR]

Existence of chrysophanol, aloe-emodin, and rhein in solutions as primarily the 9,10-anthraquinoid forms was established by quantum-chemical and correlation methods. However, the 1,10-anthraquinoid tautomers are observed in certain media. As a rule, these compounds in alkaline solutions form 1,10-anthraquinoid anions that are stabilized by intramolecular H-bonds and have a single α-hydroxy group. Tautomers exist in both the ground and excited states of the molecule. Quantitative rules that control changes in the ionization and tautomerization parameters of chrysophanol were found. [ABSTRACT FROM AUTHOR]

According to the data of quantum-chemical calculations and correlation analysis, the experimental electronic absorption spectra of quinizarin and its substituted derivatives contain bands corresponding to 9,10-, 1,10-, and 1,4-anthraquinoid tautomers. Analysis of the absorption spectra revealed specific features of the tautomeric structure of substituted quinizarins. [ABSTRACT FROM AUTHOR]