Effects of positive and negative ionization for 2-aminopyrimidine in the gas phase and in water solution

Quantum-chemical calculations were performed for neutral 2-aminopyrimidine (2APM) and for its ionized forms (2APM − e → 2APM+ and 2APM + e → 2APM−) in the gas phase {DFT(B3LYP)/6-311+G(d,p)} and in water solution {PCM(water)//DFT(B3LYP)/6-311+G(d,p)}. For calculations, the complete tautomeric mixture containing the amino and imino forms was considered. Geometric isomerism of the exoNH group for the imino forms was also taken into account. There is a good correlation between prototropy and electron delocalization for the neutral isomers of 2APM. The aromatic amine NH2 tautomer is favored for neutral and ionized 2APM in the gas phase and in water solution. Positive and negative ionization increase the stability of the imine NH isomers containing the labile proton at the endo N-aza atom that their contribution cannot be neglected in the positively and negatively ionized tautomeric mixture. Ionization influences also electron delocalization. For the major and minor forms of 2APM, the exo N atom may lose preferentially one of the non-bonding electrons, and the pyrimidine ring may gain one excess electron. The adiabatic ionization potential and the adiabatic electron affinity change when going from the gas phase to water solution by ca. 2 eV.