Abstract. Double electron–electron resonance (DEER) is a pulse electron paramagnetic resonance (EPR) technique that measures distances between paramagnetic centres. It utilizes a four-pulse sequence based on the
refocused Hahn spin echo. The echo decays with increasing pulse sequence
length 2(τ1+τ2), where τ1 and τ2 are the
two time delays. In DEER, the value of τ2 is determined by the
longest inter-spin distance that needs to be resolved, and τ1 is
adjusted to maximize the echo amplitude and, thus, sensitivity. We show
experimentally that, for typical spin centres (nitroxyl, trityl, and Gd(III)) diluted in frozen protonated solvents, the largest refocused echo amplitude for a given τ2 is obtained neither at very short τ1 (which minimizes the pulse sequence length) nor at τ1=τ2 (which maximizes dynamic decoupling for a given total sequence length) but rather at τ1 values smaller than τ2. Large-scale spin dynamics simulations based on the coupled cluster expansion (CCE), including the
electron spin and several hundred neighbouring protons, reproduce the
experimentally observed behaviour almost quantitatively. They show that
electron spin dephasing is driven by solvent protons via the flip-flop
coupling among themselves and their hyperfine couplings to the electron
spin.
The decay of the refocused Hahn echo in double electron–electron resonance (DEER) experiments
