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 NH…OC hydrogen bond are moved in opposite directions out of the mean molecular plane. This, along with rotation about the CN 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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