Observation of spin‐correlated radical pairs via two‐dimensional electron paramagnetic resonance nutation spectroscopy

Two‐dimensional (2D) Fourier‐transform (FT) electron paramagnetic resonance (EPR) nutation experiments were done in order to verify anomalous flip‐angle dependence of a FT–EPR spectrum of a photochemically generated spin‐correlated radical pair (SCRP). It was demonstrated by both the experiment and theory that the signals of SCRP have additionally higher and lower nutation frequencies than that of a free radical. This spectroscopy provides a direct spectroscopic evidence of spin‐correlation within SCRP, and enables one to partially separate SCRP signals from free radical signals in the 2D EPR spectrum.