Rational magnetic modification of N,N-dioxidized pyrazine ring expanded adenine and thymine: a diradical character induced by base pairing and double protonation

Rational modification of biomolecules especially DNA base pairs for the theoretical design of molecular magnets has attracted extensive interest. In this work, we report a modification strategy for adenine/thymine-based magnets through introducing a N,N-dioxidized pyrazine ring to either adenine or thymine to form ring-expanded bases (noA/noT) based on their experimentally synthesized derivatives. Further functionalization is conducted by double protonation and pairing with a normal complementary base (nohA-T/nohT-A), respectively. The diversity of protonation sites in noA generates totally six nohA-Ts, together with nohT-A forming seven two-step modified topic base pairs. DFT calculations are performed to characterize the magnetic properties and the diradical character, which indicate three diamagnetic (DM) nohA-Ts and three antiferromagnetic (AFM) nohA-Ts with extremely large magnetic coupling constants J ranging from -1279.7 to -2807.4 cm-1, while a relatively mild AFM nohT-A with a J of -194.6 cm-1. The electron separation effect induced by attraction of positive charges originating from protonation is proposed to explain the diradicalization, which is different from the traditional radical-coupler-radical coupling mode. In...Continue Reading