Two-Pulse Electron Spin Echo as a Method for Studying Distances in Dipolar EPR Spectroscopy: A Case of Triarylmethyl Radicals

Pulsed dipolar spectroscopy (PDS) is an electron paramagnetic resonance (EPR) technique for measuring distances between spin labels in the important nanoscale range. The most commonly used its variant is double electron-electron resonance (DEER, also known as PELDOR), which employs EPR excitation at two different microwave frequencies. The application of this method encounters difficulties when studying species with narrow EPR spectra, for which the excitation profiles of the two frequencies may overlap. Existing single-frequency PDS approaches require complex theoretical consideration in some cases. In addition, data interpretation in known PDS approaches can be challenging in the presence of multiple dipolar pathways. Here, for bis-triarylmethyl biradical, we show how simple two-pulse electron spin echo (2p ESE) spectroscopy, despite the well-known its problems of dead time, interference of electron-nuclear interactions, and fast signal decay, can provide reliable distance information. Besides the simplicity of the experiment and theoretical description, the advantage of the method is the absence of artifacts associated with additional dipolar pathways. This method can serve as a useful complement to the existing PDS approaches.