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

Publication date: Available online 15 December 2018Source: Computational and Theoretical ChemistryAuthor(s): Ewa D. Raczyńska, Weronika JurasAbstractFor favored and rare prototropic tautomers of isocytosine (iC), geometric consequences of ionization, one-electron loss (iC – e → iC+•) and one-electron gain (iC + e → iC-•), have been studied. Effects of protonation and deprotonation on geometry of neutral isocytosine (iC + H+ → iCH+ and iC – H+ → iC-H-) as well as of its charged radicals (iC-• + H+ → iCH• and (iC+• – H+ → iC-H•) have also been examined for selected isomers. Bond length alternation for the ground states of individual isomers, investigated in two extreme environments at the B3LYP/6-311+G(d,p) and PCM(water)//B3LYP/6-311+G(d,p) levels, have been quantitatively described using the geometry-based HOMED (harmonic oscillator model of electron delocalization) index. Generally, one-electron loss, one-electron gain, and proton-transfer reactions change electron delocalization in isocytosine isomers, and consequently the HOMED values. However, the HOMED indices calculated for structures optimized at the DFT level vary analogously to those optimized with PCM. When compared to other pyrimidine bases, cytosine (C) and uracil (U), exhibiting analogous tautomeric equilibria, rotational and/or geometric isomerism of exo groups (−NH2/=NH and/or −OH/=O), linear trends can be distinguished between the HOMED values of negatively ionized isomers (i...
Source: Computational and Theoretical Chemistry - Category: Chemistry Source Type: research