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 O2 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 CHaaaOH2 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 CHaaaOH2 type H-bond; that is, CHaaaOH2 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 (VDE) values for the uracil-water complexes suggest
that these hydrated uracil anions are stable. Moreover, the average AEA and VDE of U-(H2O)n increase as
the number of the hydration waters increases.