The effect of microwave pulse duration on the distance distribution function between spin labels obtained by PELDOR data analysis

A method is proposed for obtaining a distance distribution function between spin labels in spin-labeled molecules on the basis of the data of pulsed electron-electron double resonance with regard to the finite duration of microwave (mw) pulses. It is shown that taking into account the finite duration of mw pulses makes it possible to extend the range of the studied distances between spin labels to the region of short distance in which the magnitude of the dipole-dipole interaction becomes comparable to or exceeds the spectrum widths of exciting mw pulses. With frozen glassy solutions of biradicals as model systems with a length between spins of less than 2 nm, the shape for the distance distribution function between labels was obtained, and the value and sign of the exchange interaction were estimated. It is demonstrated that the analysis of the dipole-dipole interaction neglecting the duration of mw pulses can lead to substantial distortions in the shape of the distance distribution function.