Intermolecular contributions, filtration effects and composition of the SIFTER signal

Abstract. To characterise structure and order in the nanometer range, distances between electron spins and their distributions can be measured via dipolar spin-spin interactions by different pulsed electron paramagnetic resonance experiments. Here, for the single frequency technique for refocusing dipolar couplings (SIFTER), the buildup of dipolar modulation signal and intermolecular contributions is analysed for a uniform random distribution of monoradicals and biradicals in frozen glassy solvent by using the product operator formalism for electron spin S = 1/2. A dipolar oscillation artefact appearing at both ends of the SIFTER time trace is predicted, which originates from the weak coherence transfer between biradicals. The relative intensity of this artefact is predicted to be temperature independent, but to increase with the spin concentration in the sample. Different compositions of intermolecular backgrounds are predicted in the case of biradicals and in the case of monoradicals. We compare these predictions to experimental SIFTER traces for nitroxide and trityl monoradicals and biradicals. Our analysis demonstrates a good qualitative match with the proposed theoretical description. The resulting perspectives of quantitative analysis of SIFTER data are discussed.