Ground-state intramolecular proton transfer and observation of high energy tautomer in 1,4-Dihydroxyanthraquinone

Abstract

Temperature dependent steady state UV/vis absorption technique is combined with ab initio calculations to investigate the ground-state intramolecular proton transfer (GSIPT) process in 1,4-dihydroxy-9,10-anthraquinone (quinizarin). The relative energy for various tautomers is calculated at MP2/6-311++G(d,p) level of theory. The next higher energy tautomer for quinizarin is identified as 9,10-dihydroxy-1,4-anthraquinone which is around 7.7 kcal/mol higher in energy. The experimental Gibbs free energy of the equilibrium between two tautomers is evaluated to be 4.5±1.5 kcal/mol. The transition state for GSIPT process leading to the higher energy tautomer is theoretically characterized which lies ~11.5 kcal/mol above the ground state of quinizarin.