Cooperative and stabilization effects in hydrogen-bonded chains of microhydrated thymine: a QTAIM and TD-DFT study

We investigate the thresholds of the cooperative effects in hydrogen-bonded chains formed by thymine with 1 to 6 explicit water molecules. Using Density Functional Theory (DFT), combined with Quantum Theory of Atoms in Molecules (QTAIM) and Time-Dependent DFT (TD-DFT), we analyze the evolution of the electronic density at H-bond critical points (ρ H-bond) and its influence on the excited states. Our results indicate that the cooperative effect is stronger with the first water molecules, followed by weaker contributions beyond four water molecules. TD-DFT calculations reveal corresponding shifts in electronic transitions, linking H-bond topology with spectral changes. These findings contribute to a quantitative understanding of hydration effects in nucleobases, with implications for DNA stability and photochemistry.