Keto-enol tautomerism of the 4,5-dimethyl-2-(2′-hydroxyphenyl)imidazole in water solution: Modeling equilibrium between neutral forms and accurate assignment of the absorption bands

Tautomerizarion between keto, cis- and trans-enol forms of the 4,5-dimethyl-2-(2 '-hydroxyphenyl)imidazole (DMHI) is studied using molecular dynamics simulations with the quantum mechanics/molecular mechanics (QM/MM) potentials. We applied the live solvent selection approach that presumes division of water molecules to either QM or MM subsystems on the fly depending on their distances from the chromophore. This allowed us to treat a water shell around the chromophore at the QM level and to describe properly all chromophore-water interactions. According to the calculated Gibbs energy profiles the most stable is a keto form and a cis-enol form is similar to 1 kcal/mol higher. The trans-enol form is much higher in energy and should not be observed in experiment. Vertical S-0,S-min - S-1 excitation energies of all neutral tautomers were calculated at the XMCQDPT2 level: both enol forms have similar values similar to 4 eV and keto form is red-shifted, with the excitation energy of 3.3 eV. Thus, we conclude that two absorption bands in experimental absorption spectrum of DMHI in water solution are due to the presence of cis-enol and keto forms of the chromophore.

Automated summary

Using QM/MM potentials, molecular dynamics simulations were conducted to study the tautomerization between keto, cis- and trans-enol forms of DMHI. It was found that the keto form is the most stable, followed by the cis-enol form, while the trans-enol form is higher in energy. The presence of two absorption bands in the experimental absorption spectrum of DMHI in water solution was attributed to the cis-enol and keto forms of the chromophore based on calculated excitation energies.