Outer valence orbital response to proton positions in prototropic tautomers of adenine

Orbital response to proton positions in the prototropic tautomers of adenine (Ade-N1, Ade-N3, Ade-N7 and Ade-N9) has been studied in position space and momentum space using dual space analysis (DSA). Based on the electronic structures of our previous density functional theory (DFT-BP86/TZ2P and BP86/QZ4P) study of adenine tautomers (J. Phys. Chem. A, 110(2006)4012), variations in properties such as ring perimeters, dipole moments, Hirshfeld charges, vertical ionization spectra and orbital theoretical momentum distributions (MDs) of these tautomers are compared, in order to understand the impact of the mobile proton positions in the purine ring. It is found that the proton relocation causes only small perturbations in isotropic properties such as geometries and vertical ionization energies in the outer valence space of adenine. Molecular polarity and dipole moments differentiate the tautomers. Hirshfeld charges divide the nitrogen sites of the tautomers into amino (single bonds) and imino (at least one double bond) nitrogen sites. Adenine tautomerization is essentially a σ-bonding phenomenon with little perturbation to the π-bonding framework. That is, the π (or a'') orbitals, including the frontier orbitals such as the highest occupied molecular orbital (HOMO), 6a'', and the third HOMO (HOMO-2), 5a'', do not respond apparently to the proton relocation (note that the next HOMO (HOMO-1) is 29a', a σ orbital). Only relevant σ or a' orbitals residing within the purine plane, such as 21a'–24a' and orbital 27a', respond significantly to the proton positions. The present study demonstrates that the tautomer electronic structures depend not only on three dimensional geometries but also on the electron density distributions