B3LYP and MP2 levels of QM theory Journal Article Research Support, Non-U.S. Gov't cooperativity of the H-bonds the double proton transfer 5Z93L87A1R 8J337D1HZY 9007-49-2 Adenine CH···N H-bond Cytosine DNA Guanine JAC85A2161 NBO parameters along the IRC NH···HC dihydrogen bond Protons QR26YLT7LT QTAIM analysis QTAIM analysis ·Grunenberg's compliance constant Thymine adenine·guanine (A·G) DNA base mispair amino and imino tautomers of cytosine amino and imino tautomers of the adenine electron-topological geometric keto and enol tautomers of the guanine keto and enol tautomers of thymine mutagenic tautomerization polar