Quantum chemical study of the structure and properties of citrinin

Detailed structures and electronic properties of three tautomeric forms of the toxin citrinin were investigated using several quantum calculation methods. Energetic preference of the predominant p- and o-quinone methide tautomeric forms is dependent on the method of calculation. A previously unstudied carboxylic acid enol tautomer was calculated to be surprisingly stable in vacuo, being within 2.5 kcal mol(-1) at the B3LYP/6-311++G(2d,2p) level of theory. Despite differences in bond nature and connectivity of tautomers, the natural bond orbital analysis revealed that tautomeric forms share similar natural charges and natural electron configurations. Calculated bond lengths corresponded with experimentally observed values and assignments for the calculated infrared vibrational frequencies are reported.