Excess Electron Attachment Induces Barrier-Free Proton Transfer in Binary Complexes of Uracil with H2Se and H2S but Not with H2O

Abstract

The photoelectron spectrum of the uracil−H2S anionic complex (UH2S)- has been recorded with 2.540 eV photons. Unlike the (uracil−H2O)- spectrum, which displays a broad feature with maximum at about 0.9 eV, the (UH2S)- spectrum reveals a broad feature with a maximum between 1.7 and 2.1 eV. The latter vertical detachment energy value is too large to be attributed to an (UH2S)- complex in which an intact uracil anion is solvated by H2S. The effects of electron attachment to the UH2A complexes (A = Se, S, O) have been studied at the density functional theory level with the B3LYP and MPW1K exchange correlation functionals as well as at the second-order Møller−Plesset perturbation theory level. The three acids cover a broad range of acidity with calculated gas-phase deprotonation enthalpies being equal to 14.8, 15.1, and 16.9 eV for H2Se, H2S, and H2O, respectively. In the case of H2Se and H2S, electron attachment is predicted to induce a barrier-free proton transfer (BFPT) from the acid to the O8 atom of uracil, with the product being the radical of hydrogenated uracil bound to AH-. No BFPT is predicted for the anion of uracil with H2O. Critical factors for the occurrence of BFPT have been analyzed, and the role of the stabilizing interaction between the hydrogenated uracil and the deprotonated acid has been discussed. Four structures have been considered for every UH2A complex, and their relative stabilities are different for the neutral and anionic species. The increased stabilities of anionic complexes that undergo BFPT can be related to the properties of the second hydrogen bond (C5H···A or N1(3)H···A). In comparison with the case of neutral structures, this bond is weakened for anionic structures without BFPT and strengthened for those with BFPT.