Double quantum coherence electron spin resonance on coupled Cu(II)–Cu(II) electron spins

We demonstrate for the first time the ability to generate double quantum coherences (DQCs) for the case of Cu(II). We show that small splittings (∼7 MHz) from the Cu(II)–Cu(II) electron–electron magnetic dipolar interaction can be reliably resolved even though the inhomogeneously broadened Cu(II) linewidth is ∼2 GHz. A Cu(II)–Cu(II) distance of 2.0 nm was measured on a model peptide system, thus, demonstrating that distances on the nanometer scale may be measured using DQC electron spin resonance (ESR).