Hydrogen Atom or Proton Transfer in Neutral and Single Positive Ions of Salicylic Acid and Related Compounds

The nature of the process “hydrogen atom” or “proton” transfer reaction in neutral and single positive ions of salicylic acid, o -hydroxybenzaldehyde, and salicylaldimine has been studied using the B3LYP density functional method. Single-point calculations at the CCSD(T) level have also been performed. For neutral systems, the reaction corresponds to a proton transfer process coupled with an important electronic transfer, which leads to a compound with nonzwitterionic character. A similar behavior is observed for ionized π radical cations. In ionized σ states, however, the process is that of a hydrogen transfer. In all cases, π ionization strengthens the initial H-bond and favors the reaction. For some of the systems the proton transfer occurs spontaneously.