Three-spin correlations in double electron-electron resonance

Distance measurements by pulse electron paramagnetic resonance techniques are increasingly applied to systems that contain more than two spin labels. Under typical experimental conditions this leads to signal contributions from sum and difference combinations of dipolar frequencies. As these contributions are neglected in the expressions used in data analysis they cause artifacts in distance distributions. It is shown that the artifacts lead to substantial broadening of the distance distribution. By measuring double electron-electron resonance (DEER) data with variable inversion efficiency of the pump pulse and fitting the data at each point in time by appropriate polynomials the pair contribution can be separated from the three-spin contribution. Experiments on a conformationally invariant triradical with equilateral triangle geometry and the corresponding biradical with the same interspin distance demonstrate that this approach leads to a considerable improvement in the distance distribution. The three-spin contribution is shown to provide information on angles between spin-spin vectors. Data for a series of triradicals with equilateral, isosceles, and scalene triangle geometry are analysed by fitting a general triangle model simultaneously to the dipolar spectra of the pair and three-spin contribution. The agreement with the expected side lengths of the triangles is satisfying.