Ionization Energy Thresholds of Microhydrated Adenine and Its Tautomers

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In the present work the vertical and adiabatic ionization energy thresholds (IET) of adenine, and its amino and imino tautomers complexed with 1-3 water molecules are presented. The vertical and adiabatic IETs have been calculated at the B3LYP and P3 levels of theory, using the standard 6-31++G(d,p) basis set. The results show that there is hardly any effect of microhydration on the vertical ΔIET of adenine, which is at odds with the experimental values determined by Kim et al, (J. Phys. Chem. 1996, 100, 7933). In an attempt to assign the experimental ΔIET values, calculations have been performed on the microhydrated amino and imino tautomers of adenine. Vertical ΔIET calculations and adiabatic ΔIET calculations on adenine N7H tautomers complexed with water are in better agreement with the experimental results than are calculations involving the canonical (N9H) form of adenine.