PELDOR at S- and X-band frequencies and the separation of exchange coupling from dipolar coupling

A pulsed electron double resonance (PELDOR) setup working at S-band frequencies is introduced and its performance compared with an X-band setup. Furthermore, to verify experimentally that it is possible to disentangle the dipolar coupling v_Dip from the exchange coupling J by PELDOR we synthesized and investigated four bisnitroxide radicals. They exhibit in pairs the same distances r_AB between the nitroxide moieties but only one of each pair possesses a non-zero J. The experimental values for r_AB match the ones from molecular modeling very well for the molecules without exchange coupling. For one bisnitroxide it was possible to separate v_Dip from J and to ascertain the magnitude and sign of J to +11 MHz (antiferromagnetic spin-spin coupling).