Long-lived nuclear singlet order in near-equivalent 13C spin pairs.

Molecules that support (13)C singlet states with lifetimes of over 10 min in solution have been designed and synthesized. The (13)C(2) spin pairs in the asymmetric alkyne derivatives are close to magnetic equivalence, so the (13)C long-lived singlet states are stable in high magnetic field and do not require maintenance by a radiofrequency spin-locking field. We suggest a model of singlet relaxation by fluctuating chemical shift anisotropy tensors combined with leakage associated with slightly broken magnetic equivalence. Theoretical estimates of singlet relaxation rates are compared with experimental values. Relaxation due to antisymmetric shielding tensor components is significant.