Frequency-Swept Electron–Electron Double Resonance: DPPH in Liquid and Frozen Solution

Instrumentation for frequency-swept electron–electron double resonance (ELDOR) at X band is described. Reliable control of the sweeping (pump) microwave frequency to 1 part in 106 (10 kHz) has been achieved. The technique has been applied to liquid and frozen solutions of DPPH in toluene. The isotropic nitrogen coupling constants are Aα = 7.933 ± 0.007 and Aβ = 9.739 ± 0.014 G including second-order corrections. The ratio Aα / Aβ = 0.815. In the solid phase the largest hyperfine coupling to the α-nitrogen | A + 2B |α and the largest hyperfine coupling to the β-nitrogen | A + 2B |β are each 20.1 G. The literature on this molecule is reviewed and the present experiments discussed in this context. Much of the paper is devoted to the physical basis of ELDOR spectroscopy. An analysis of ELDOR line shapes is given in an appendix.