Keto-enol tautomerism in linear and cyclic beta-diketones: A DFT study in vacuo and in solution

DFT geometry optimizations have been performed at the B3LYP/6-31G* level in the gas phase and at the IEF-PCM/B3LYP/6-31G* level in tetrahydrofuran (THF) and aqueous solutions using scaled Bondi radii for the diketo and ketoenol forms of acetylacetone and cyclohexanedione. To evaluate basis set effects, starting from the aforementioned minima, the 6-311++G** optimized structures have been obtained. A number of complexes of both systems including one explicit water molecule have been considered up to the B3LYP/6-311++G** level, for cyclohexanedione taking into account the B3LYP/6-31G* basis set superposition errors as well. The diketo-ketoenol interconversion mechanisms have been investigated at the B3LYP/6-31G* level in vacuo. Interestingly, the geometric constraint due to the presence of the ring facilitates the description of the reaction mechanism in cyclohexanedione. Despite the very different flexibility of the two systems that in the case of acetylacetone prevents a straightforward interconversion of the diketo to the most stable of its ketoenol forms, both reactions occur with a very high barrier (about 62-63 kcal/mol), unaffected by continuum solvents, that decreases by 2.5-3.5 kcal/mol after the inclusion of thermal corrections. The barriers are almost halved, becoming ~31-35 kcal/mol, for the addition of a single water molecule according to various model reaction paths. Thermal corrections are limited (0.8-1.6 kcal/mol) for those adducts. The formation of a 1,1-diol, explored in the case of acetylacetone, might facilitate the obtainment of the most stable diketo conformation, featuring the carbonyl groups in distinct orientations. Inclusion of dispersion and basis set effects via the G2MP2 protocol does not alter the relative stability of both system tautomers. In contrast, the G2MP2 interconversion barriers for the isolated systems in vacuo are close to the B3LYP ones, whereas they turn out to be somewhat higher than the free energy based B3LYP barriers in the presence of a catalytic water molecule.