Abstract

Equilibrium geometries have been determined at the STO-3G RHF level for the ground electronic states of 1,4-, 1,5-, and 1,8-dihydroxyanthraquinone (DHAQ) and 1-aminoanthraquinone. Calculated bond lengths agree well with the limited available experimental data. The structure around the intramolecular hydrogen bonds is very similar in all three dihydroxyanthraquinones, including 1,8-DHAQ where both hydrogen bonds are to the same oxygen atom. The form arising from double intramolecular proton transfer in 1,4-DHAQ (quinizarin) is 10.4 kcal/mol higher in energy. Even though the hydrogen bond in 1-aminoanthraquinone is weak by the usual spectroscopic criteria, it appears to play a major role in organizing the molecule's structure. The oxygen and nitrogen atoms of the NH…OC hydrogen bond are moved in opposite directions out of the mean molecular plane. This, along with rotation about the CN bond, brings the proton of the hydrogen bond close to the plane in which the carbonyl oxygen lone pair electrons are expected to lie.

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