Theoretical exploration of structures and electronic properties of double-electron oxidized guanine-cytosine base pairs with intriguing radical-radical interactions.

We present a computational study of the double-electron oxidized guanine-cytosine base pair as well as its deprotonated derivatives, focusing on their structural and electronic properties. Some novel electromagnetic characteristics are found. A hydrazine-like (N-N) cross-linked structure between the G and C radical moieties is the lowest-energy one for the [GC](2+) complexes. Double-electron oxidation can considerably destabilize the GC unit and leads to a barrier-hindered dissociation channel with negative dissociation energy. This channel is governed by a balance between electrostatic repulsion and attractive hydrogen-bonding interaction co-existing between G˙(+) and C˙(+). The proton/electron transfer reactions in the double-electron oxidized Watson-Crick base pair occur through a proton transfer induced charge migration mechanism. For the deprotonated [GC](2+) derivative, the [G(-H(+))C](+) series prefers to accompany by transfer of an electron from the G to C moiety when the G(+) is deprotonated, and its highest-doubly occupied molecular orbital mainly localizes over the C moiety with a π-bonding character. For the diradical G˙(+)C(-H)˙ series in which the C moiety is deprotonated, the two unpaired electrons reside one on each moiety in the π system. The diradical base pairs possess open-shell broken symmetry singlet states, and their magnetic coupling interactions are controlled by both intra- and inter-molecular interactions. The double-electron oxidized Watson-Crick base pair shows strong antiferromagnetic coupling, whereas the magnetic interactions of other diradical derivatives are relatively weak. This study highlights the crucial role of H-bonding in determining the magnetic interactions.