Quantum-chemical and correlation study of the tautomerism and ionization of 1,2,3-Trihydroxy-9,10-anthraquinone

1,2,3-Trihydroxy-9,10-anthraquinone (anthragallol) exists as an equilibrium mixture of the 9,10-, 2,9-, 1,2-, and 2,3-quinoid tautomers. Its anion was detected in the 9,10-, 1,10-, 2,9-, and 2,3-quinoid forms, and its metal complexes, the 9,10-and 2,9-quinoid forms. Ionization and complexation of anthragallol can shift the tautomeric equilibrium.     

Tautomerism of anthraquinones: IV. 1-Hydroxy-9,10-anthraquinone and its substituted derivatives

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.     

Anthraquinones tautomerism: VII. Hydroxy-substituted anthraquinones

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.     

Quantum-chemical and correlation study of deprotonation and complexation of 1-amino-4-hydroxyanthraquinone

Existing views on the deprotonation and complexation of 1-amino-4-hydroxyanthraquinone are wrong. This compound, its anions, and complexes with metals are not individual substances, but they form a dynamic equilibrium mixture of keto-enol (keto-oxide) and amino-imine tautomers. Different samples of the same compound differ by the tautomeric composition, the respective information is contained in their electron absorption spectra. In weak alkaline solutions the deprotonation occurs exclusively at the hydroxy group. Most typical structure of 1-amino-4-hydroxyanthraquinone anions is 1,10-quinoid, its metal complexes have 9,10-and 1,10-quinoid structures. The ground states of molecules are more responsible for the tautomeric transformations than the excited states. Quantum-chemical calculations of tautomeric anthraquinones by semiempirical PPP methods are more reliable than modern ab initio calculations.     

Tautomerism of anthraquinones: V. 1,5-Dihydroxy-9,10-anthraquinone and its substituted derivatives

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.     

Tautomerism and ionization of carminic acid

Carminic acid exists as an equilibrium mixture of 9,10-, 1,4-, 1,10-, 2,9-, and 1,7-anthraquinoid tautomers. Its anions have 9,10-, 1,4-, 1,10-, and 2,9-anthraquinoid structures. No conformational equilibria were detected for carminic acid anions. Variation of the solvent and pH and ionization are accompanied by displacements of tautomeric equilibria. Shifts of the long-wave absorption maxima due to tautomeric transformations are determined mainly by change in the energy of the ground rather than excited states of molecules.     

Protonation of 1,4,5-tri- and 1,4,5,8-tetrahydroxyanthraquinones in sulfuric acid: Multistep reaction involving tautomers and conformers

The fine structure of the ??_l,??*-absorption of hydroxyanthraquinones solutions in sulfuric acid arises due to the existence of the protonated forms as equilibrium mixtures of tautomers and conformers distinguished by the positions of the ??-bonds, charges, and the number of intramolecular hydrogen bonds. Using quantum-chemical calculations and correlation analysis of the absorption spectra tautomers were identified of mono- and dications of 1,4,5-trihydroxyanthraquinone of 9,10-, 1,4-, and 1,10-isomeric structure. For 1,4,5,8-tetrahydroxyanthraquinone 9,10-, 1,4-, 1,10-, and 1,5-isomeric mono- and dications and their conformers were found.     

New stage in the development of anthraquinone chemistry and the structure of alizarin

Chemistry of 9,10-anthraquinones is considered as chemistry of isomeric anthraquinones existing in dynamic equilibrium with each other. Diversity of electronic absorption spectra of the same compounds is determined by tautomeric transformations. Alizarin exists as equilibrium mixtures of tautomers and conformers having 9,10- and 2,9-quinoid structures. Qualitatively different compositions are inherent not only to samples of alizarin prepared or purified by different methods but also to different solvates of the same sample.     

Tautomerism of Anthraquinones: I. Purpurin and Anions Derived Therefrom

A procedure was proposed for quantitative analysis of tautomeric equilibria of organic compounds. Purpurin was found to exist mainly as 9,10-, 1,4-, and 1,10-anthraquinoid tautomers, its monoanion, as 1,10-anthraquinoid tautomer, the dianion, as 1,10- and 2,9-anthraquinoid tautomers, and the trianion, as 1,10-, 1,4-, and 2,9-anthraquinoid tautomers. Tautomeric transformations occur both in the ground and in the excited states, and the corresponding changes of quantum-chemical parameters in these states are essentially different. The excited states are more sensitive to tautomeric transformations than the ground states.__________Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 1, 2005, pp. 43–50.Original Russian Text Copyright © 2005 by Fain, Zaitsev, Ryabov.     

Metal Complexes with Alizarin Complexone AC: Electronic Absorption Spectra and Ligand Structure

The character of the electronic absorption spectra of metal complexes with alizarin complexone AC is determined by the ionization degree of the ligand and the ratio between its excited states with different contributions of tautomeric 9,10-, 1,10-, 2,9-, and 1,2-anthraquinoid resonance structures. It was found by the spectrophotometric, quantum-chemical, and correlation methods that the ligand in metal complexes can exist in three forms, namely, neutral and two ionized forms (containing one or two deprotonated hydroxy groups). For each of the latter two forms, four excited states with the dominating contribution of the 9,10-, 1,10-, 2,9-, or 1,2-anthraquinoid resonance structures are possible. The formation of red monometallic complexes involves the peri- or ortho-hydroxycarbonyl group in anthraquinoid tautomers (mostly, 1,2- and 2,9-structures). The color of bimetallic complexes is determined by four anthraquinoid structures of the ligand (from red 9,10- to blue 1,10-anthraquinones). Fluorine-containing complexes exist only as 1,2- and 1,10-anthraquinoid structures, which are responsible for their blue color. The known metal complexes with Alizarin Complexone AS were classified by their structures.     

Quantum-chemical and correlation study on the tautomerism and ionization of 1,4,5,8-tetrahydroxy-9,10-anthraquinone and its alkyl-substituted derivatives

1,4,5,8-Tetrahydroxy-9,10-anthraquinone and its alkyl derivatives exist as equilibrium mixtures of prototropic tautomers and rotational isomers differing in the mode of intramolecular hydrogen bonding. Their electronic absorption spectra contain π_l,π* bands corresponding to 9,10-, 1,10-, 1,4-, and (more rarely) 1,5-anthraquinoid structures. Introduction of substituents, solvation, ionization, and complex formation lead to displacement of tautomeric and conformational equilibria, which are responsible for the observed diversity of their absorption spectra.     

Metal complexes with 1-hydroxyanthraquinone and its derivatives: Electronic absorption spectra and ligand structures

Reactions of metal salts with 1-hydroxyanthraquinone and its derivatives gave tautomeric 9,10-and 1,10-quinoid complexes and compounds containing no C=0 → M coordination bond. Each form is characterized by a single π₁,π*-band. The absorption bands were assigned by using correlations with the σ^A-constants of the hydroxy and oxido groups for tautomeric anthraquinones. Complexes with nonionized ligands have particularly the 9,10-anthraquinoid structures; complexes with ionized ligands can form both 9,10-and 1,10-quinoid structures.     

Quantum-chemical and correlation study of ionization of Alizarin

According to the results of quantum-chemical and correlation studies, ionized states of Alizarin and Alizarin Red are contributed mainly by tautomeric anthraquinoid structures. The probability of their formation increases due to solvation. The experimental electronic absorption spectra of the corresponding anions contain bands typical of the 2,9-, 1,10-, and 1,2-anthraquinoid tautomers. The 9,10-anthraquinoid structure of the anions is less probable.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 10, 2004, pp. 1681–1686.Original Russian Text Copyright © 2004 by Fain, Zaitsev, Ryabov.This revised version was published online in April 2005 with a corrected cover date.     

Tautomeric composition and tautomeric transformation sequence of 1,4-bis(alkylamino)anthraquinones

Compounds widely known as 1,4-bis(alkylamino)-9,10-anthraquinones are in fact neither individual substances nor substituted 9,10-anthraquinones but equilibrium mixtures of tautomers. Their aminoimine tautomeric transformations follow the sequence 4,9-bis(alkylamino)-1,10-anthraquinones ? 9-alkylamino-4-(alkylimino)-10-hydroxy-1,4-dihydroanthracen-1-ones ? N ¹,N ¹⁰-dialkyl-4,9-dihydroxy-1,10-dihydroanthracene-1,10-diimines.     

Competing tautomeric transformations and the structure of 1-(alkyl,aryl)amino-4-hydroxyanthraquinones

Keto-enol and amine-imine tautomerism and equilibria with trans-conformers are characteristic of 1-(alkyl,aryl)amino-4-hydroxy-9,10-anthraquinones. Amino forms possess 1,10- and 1,4-, but not 9,10-quinoid structure. Various tautomeric and conformeric transformations are in competition. The outcome of this competition may be studied by the correlation analysis of electron absorption spectra but it would be possible to understand the causes of changes in the direction of the competing transformation only in the case when each action on the substance would be accompanied with establishing the corresponding alterations in its tautomeric composition.     

A Quantum-Chemical and Correlation Study of the Ionization of Purpurin

Anthraquinoid tautomers participate in the ionization of purpurin. The tautomerism takes place in both ground and excited states of the molecules; the excited state is more sensitive to the tautomerism. The solvation and ionization shift the tautomeric equilibria. In the experimental absorption spectra of purpurin, the major bands correspond to the 9,10-, 1,4-, and 1,10-anthraquinoid tautomers; anions with the 9,10-anthraquinoid structure are not detected. The position and intensity of the π,π* bands, and also the quantum-chemical parameters linearly correlate with the degree of ionization of purpurin.     

Role of tautomerism and rotational isomerism in the interaction of α-hydroxyanthraquinones with boric acid

The products of reaction of α-hydroxyanthraquinones with boric acid are mixtures of 9,10-, 1,10 -, 1,4- and 1,5-quinoid tautomeric complexes of boric acid and borate esters differing by the coordination bonds with carbonyl groups existing in the dynamic equilibrium. The deepening of the reagents color in the presence of boron does not a result only of the complexation, but in the accompanying shift of the tautomeric equilibria.     

Anthraquinones tautomerism: VI. Substituted 1,4,5-trihydroxyanthraquinones

1,4,5-Trihydroxy-9,10-anthraquinone and its substituted derivatives exist in equilibrium of structures distinguished by quinoid tautomerism and rotational isomerism. Their electron absorption spectra contain π₁, π*-bands corresponding to 9,10-and 1,10-, more seldom to 1,5-and 1,4-anthraquinoid structures. Of three isomeric 1,10-anthraquinones only 4,8,9-trihydroxy-1,10-antraquinones were found. All tautomer may exist as conformers with contiguous CO and OH groups not bound by an intramolecular hydrogen bond. The considerable difference in color of structurally similar substituted compounds is due to tautomerism and conformer transformations.     

Reaction of Perfluorobenzocyclobutene with Excess Pentafluorobenzene in SbF<Subscript>5</Subscript>

The reaction of perfluorobenzocyclobutene with excess pentafluorobenzene in SbF₅, followed by hydrolysis, gave a mixture of perfluoro-1,3,3-triphenyl-1,3-dihydro-2-benzofuran-1-ol, perfluoro-1,1,2-triphenylbenzocyclobuten- 5-one, and perfluoro-4-(2,2-diphenylbenzocyclobuten-1-ylidene)cyclohexa-2,5-dien-1- one. When the reaction mixture was treated for a long time with Olah’s reagent (HF–pyridine), isomeric perfluoro-9,10-diphenyl-1,4-, -1,10-, -2,9-, and -9,10-dihydroanthracenes were formed and were converted to perfluoro-9,10-diphenylanthracene by the action of SbF₅.     

Metal Complexes with 1,5- and 1,8-Dihydroxy-9,10-Anthraquinones: Electronic Absorption Spectra and Structure of Ligands

Metal complexes with 1,5-dihydroxy-9,10-anthraquinone are studied by the spectrophotometric, quantum-chemical, and correlation methods. It was established that the ligand in these complexes can occur in seven excited states that differ not only in the ionization degree but also in the prevailing contribution of the tautomeric 9,10-, 1,10-, and 1,5-anthraquinoid structures. In all known complexes with 1,8-dihydroxy-9,10-anthraquinone and singly ionized ligand, this ligand has the 1,10-anthraquinoid structure; in complexes with the doubly ionized ligand, the latter ligand most often has the 9,10-anthraquinoid structure. The known complexes are classified according to the ligand structures.