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 nu(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 nu(Dip) from J and to ascertain the magnitude and sign of J to +11 MHz (antiferromagnetic spin-spin coupling).