Prototropy and π-electron delocalization for purine and its radical ions - DFT studies

• Published in: Journal of physical organic chemistry Vol. 23; no. 9; pp. 828 - 835
• Main Authors: Raczyńska, Ewa D., Kamińska, Beata
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01-09-2010
• Subjects: complete tautomeric equilibria; DFT; neutral forms and charged radicals; purine; π-electron delocalization
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.1668

Complete tautomeric equilibria and π‐electron delocalization were studied at the B3LYP/6‐311+G** level for neutral purine (P) and its charged radicals (P+• and P−•). All possible nine tautomers (four NH and five CH forms) and all possible 36 tautomeric equilibria (six NiH → NkH, twenty NH → CH, and ten CiH → CkH conversions) were considered. The greatest variations of the tautomeric equilibrium constants (as pKT) were observed for the NH → CH conversions when proceeding from neutral to reduced purine (P + e → P−•). These variations completely change the tautomeric preferences. One‐electron oxidation (P − e → P+•) has considerably smaller effect on the pKT values and does not change the tautomeric preferences. π‐Electron delocalization depends on the position of the moving proton and on the type of the electron transfer. For individual tautomers, some linear relations between the relative stabilities and the HOMA (harmonic oscillator model of aromaticity) indices occur for neutral and oxidized purine. For reduced purine, a scatter plot is found. Copyright © 2010 John Wiley & Sons, Ltd.
Complete tautomeric equilibria and π‐electron delocalization were studied at the B3LYP/6‐311+G** level for neutral purine (P) and its charged radicals (P+• and P−•). For individual tautomers, linear relations between the relative stabilities (ΔG) and the HOMA (harmonic oscillator model of aromaticity) indices occur for neutral and oxidized purine. For reduced purine, a scatter plot is found.