Structural and Electronic Properties of U5M+ and T5M+ (U = Uracil, T = Thymine, M = Ag and Au) Cluster Cations

The geometric and electronic structures and the bonding of U5M+ and T5M+ (U = uracil, T = thymine, M = Ag, Au) cluster cations have been investigated with density functional theory methods. They have a perfectly planar structure with C5h symmetry and significant stability, containing self-complementary N-H···O hydrogen bonds and five Au-O or Ag-O contacts. The energy gap between the LUMO and HOMO in the U5Ag+ cluster is 4.2 eV, which is twice as large as the HOMO-LUMO gap observed in the U5Au+ cluster. This notable difference clearly indicates that the U5Ag+ cluster possesses substantially greater stability compared to the U5Au+ cluster. This finding is consistent with the results from the energy decomposition analyses, which show that the total interaction energy of U5Ag+ is significantly higher than that of U5Au+. The same trend is observed in T5M+ as well. The interaction between the metal atoms, whether gold (Au) or silver (Ag), and the nucleobase is not predominantly controlled by electrostatic forces, as initially believed. Instead, it is primarily characterized by pronounced covalent bonding effects.