Excitation of singlet–triplet coherences in pairs of nearly-equivalent spins†
We present approaches for an efficient excitation of singlet–triplet coherences in pairs of nearly-equivalent spins. Standard Nuclear Magnetic Resonance (NMR) pulse sequences do not excite these coherences at all or with very low efficiency. The single quantum singlet–triplet coherences, here termed the outer singlet–triplet coherences, correspond to lines of low intensity in the NMR spectrum of a strongly-coupled spin pair (they are sometimes referred to as “forbidden transitions”), whereas the zero-quantum coherences, here termed the inner singlet–triplet coherences, do not have a direct spectral manifestation. In the present study, we investigated singlet–triplet coherences in a pair of nearly-equivalent carbon spins of the 13C-isotopomer of a specially designed naphthalene derivative with optimized relaxation properties. We propose and compare several techniques to drive the singlet–triplet coherence in strongly coupled spin pairs. First, we study different methods for efficient excitation of the outer singlet–triplet coherences. The achieved conversion efficiency of magnetization to the coherences of interest is close to the theoretically allowed maximum. Second, we propose methods to convert the outer coherences into the inner singlet–triplet coherence. The inner singlet–triplet coherence is insensitive to field inhomogeneity and can be long-lived. By probing this coherence, we perform a very precise measurement of the spin–spin J-couplings. A remarkable property of this coherence is that it can be preserved even in absence of a spin-locking radiofrequency field. Consequently, it is possible to shuttle the sample between different magnetic fields preserving the coherence. This allows one to study the field dependence of the relaxation time, TIST, of the inner singlet–triplet coherence by performing field-cycling experiments. We observed dramatic changes of the ratio TIST/T1 from about 1 (in strong fields) up to 2.4 (in weak fields), which is the evidence of a significant influence of the chemical shift anisotropy on relaxation. We have detected a remarkably long lifetime of the inner singlet–triplet coherence of about 200 s at the magnetic field of 5 mT.