Microsolvation effect, hydrogen-bonding pattern, and electron affinity of the uracil-water complexes U-(H2O)n (n=1, 2, 3)

To achieve a systematic understanding of the influence of microsolvation on the electron accepting behaviors of nucleobases, the reliable theoretical method (B3LYP/DZP++) has been applied to a comprehensive conformational investigation on the uracil-water complexes U-(H2O)(n) (n = 1, 2, 3) in both neutral and anionic forms. For the neutral complexes, the conformers of hydration on the 02 of uracil are energetically favored. However, hydration on the O4 atom of uracil is more stable for the radical anions. The electron structure analysis for the H-bonding patterns reveal that the CH center dot center dot center dot OH2 type H-bond exists only for di- and trihydrated uracil complexes in which a water dimer or trimer is involved. The electron density structure analysis and the atoms-in-molecules (AIM) analysis for U-(H2O)(n) suggest a threshold value of the bond critical point (BCP) density to justify the CH center dot center dot center dot OH2 type H-bond; that is, CH center dot center dot center dot OH2 could be considered to be a H-bond only when its BCP density value is equal to or larger than 0.010 au. The positive adiabatic electron affinity (AEA) and vertical detachment energy (VIDE) values for the uracil-water complexes suggest that these hydrated uracil anions are stable. Moreover, the average AEA and VIDE of U-(H2O)(n) increase as the number of the hydration waters increases.