Substituent Electron Push-Pull Interaction in Intermolecular Resonance-Assisted Hydrogen Bonds : Thymine/Adenine Base Pair and Their Complexes with Carboxylic Acids

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<jats:title>Abstract</jats:title> <jats:p>Theoretical studies of substituent R-effects on intermolecular resonance-assisted hydrogen bonds (RAHBs) of R-thymine/R-COOH, R-adenine/R-COOH complexes, and R-thymine/R-adenine base pairs have been made via Taft–Topsom treatment and natural bond orbital analysis, using density functional theory at the M06-2X/aug-cc-pVDZ level. Dimerization energy arising from constituent double H-bonds is essentially dependent on the nature of π-resonance of a substituent, i.e. π-electron-donating and -accepting properties. For monosubstituted base complexes with reference formic acid, π-donor (−R) substitution enhances π-delocalization of conjugated amide/amidine frames and activates the corresponding electron-donor site whereas π-acceptor (+R) substitution quenches the system via the same site, depending on their potentialities |σR+| and |σR−| of π-resonance effect. When both π-donor and π-acceptor substituents coexist in the complexes and base-pairs, they are activated under the support of mutual π-potentialities from counter molecules. Accordingly, one H-bond is significantly enhanced through cooperative electron push–pull interaction by substituents whereas the other being quenched due to the π-resonance effects in the opposite direction. Since bare thymine and adenine bases are actually found π-donating in nature against the amide/amidine frames of RAHBs, π-acceptor substitution of the counter molecule causes cooperative electron push–pull interaction on the corresponding H-bond and actually influences the dimerization energy depending on relative contribution of the constituent H-bonds.</jats:p>

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