Effect of Hydrogen Bonding on Barrier‐Free Proton Transfer in Anionic Complexes of Uracil with Weak Acids: (U…HCN)− versus (U…H2S)−

Abstract

AbstractA photoelectron spectrum is reported for an anionic complex of uracil (U) with HCN. The effects of electron attachment to a complex of U with HA (A = CN, NC) have been studied at the density functional theory level with 6–31++G** basis sets and the B3LYP and MPW1K exchange correlation functionals. Critical anionic structures have been reexamined at the MP2/6–31++G** level. The B3LYP gas‐phase deprotonation enthalpies are equal to 14.56, 15.13, and 15.12 eV for HNC, HCN, and H2S, respectively. The experimental deprotonation enthalpies are 15.217 ± 0.009 and 15.212 ± 0.126 eV for HCN and H2S, respectively. Hence, HCN and H2S have very indeed similar deprotonation enthalpies. The photoelectron spectra of anionic complexes of uracil with HCN and H2S are, however, very different. The (UHCN)− spectrum reveals a broad feature with a maximum between 1.2–1.4 eV, whereas the main feature of the (UH2S)− spectrum has a maximum between 1.7 and 2.1 eV. We suggest that barrier‐free proton transfer (BFPT) occurs in the (UH2S)−complex, but not in (UHCN)−. Critical factors for the occurrence of BFPT have been analyzed. The difference between the (UHCN)− and (UH2S)− complexes is attributed to differences in hydrogen bonds formed by HCN and H2S with uracil.