Phys. Chem. Chem. Phys., 14, pp.9137-9148 (2012)DOI:10.1039/c2cp40778g
Weakly exchange-coupled biradicals have attracted much attention in terms of their DNP application in NMR spectroscopy for biological systems or the use of synthetic electron-spin qubits. Pulse-ESR based electron spin nutation (ESN) spectroscopy applied to biradicals is generally treated as transition moment spectroscopy from the theoretical side, illustrating that it is a powerful and facile tool to determine relatively short distances between weakly exchange-coupled electron spins. The nutation frequency as a function of the microwave irradiation strength ω1 (angular frequency) for any cases of weakly exchange-coupled systems can be classified into three categories; D12 (spin dipolar interaction)-driven, Δg-driven and ω1-driven nutation behaviour with the increasing strength of ω1. For hetero-spin biradicals, Δg effects can be a dominating characteristic in the biradical nutation spectroscopy. Two-dimensional pulse-based electron spin nutation (2D-ESN) spectroscopy operating at the X-band can afford to determine small values of D12 in weakly exchange-coupled biradicals in rigid glasses.