Effects of ionization and proton-transfer on bond length alternation in favored and rare isomers of isocytosine

摘要: Abstract For favored and rare prototropic tautomers of isocytosine (iC), geometric consequences ionization, one-electron loss (iC − e → iC+ ) gain (iC + e → iC− ), have been studied. Effects protonation deprotonation on geometry neutral (iC + H+ → iCH+ iC − H+ → iC−H−) as well its charged radicals (iC−  + H+ → iCH (iC+  − H+ → iC−H also examined for selected isomers. Bond length alternation the ground states individual isomers, investigated in two extreme environments at B3LYP/6-311+G(d,p) PCM(water)//B3LYP/6-311+G(d,p) levels, quantitatively described using geometry-based HOMED (harmonic oscillator model electron delocalization) index. Generally, loss, gain, proton-transfer reactions change delocalization consequently values. However, indices calculated structures optimized DFT level vary analogously to those with PCM. When compared other pyrimidine bases, cytosine (C) uracil (U), exhibiting analogous tautomeric equilibria, rotational and/or isomerism exo groups ( NH2/ NH OH/ O), linear trends can be distinguished between values negatively ionized isomers , C− U− ). positively C+ U+ relations seem more complex. This indicates some similarities mechanisms differences bases. Hence, descriptor considered kind ‘marker index’ ionization mechanism series π-electron systems.