Tautomeric contributions to the absorption spectrum of [2,2?-bipyridyl]-3,3?-diol in water unveiled by molecular dynamics with accurate quantum mechanically derived force-fields

In this work, we provide a realistic description of the tautomers of [2,2?-bipyridyl]-3,3?-diol (BPOH2) and their relative populations in water solution, eventually validating the computational predictions through the comparison of the simulated absorption spectrum with the one experimentally measured at ambient conditions. To this end, we present an original computational protocol, which exploits accurate and specific quantum mechanically derived force-fields in a MD/QM sequential procedure, allowing for a first principle characterization of all BPOH2 tautomers, including their thermalized conformational dynamics and relative Boltzmann weights. To our knowledge, this represents an unprecedented result demonstrating that the lower energy bands, appearing in water, are due to the absorption of the diketo tautomer of BPOH2, which is stabilized by a strong and specific hydrogen bond network. Considering its general applicability, the proposed computational procedure paves the way to future application on more complex systems of biological or technological relevance, as for instance protic solvents micro-confined in large organic aggregates.