Long-Lived Nuclear Singlet Order in Near-Equivalent ¹³C Spin Pairs

Abstract

Molecules that support ¹³C singlet states with lifetimes of over 10 min in solution have been designed and synthesized. The ¹³C₂ spin pairs in the asymmetric alkyne derivatives are close to magnetic equivalence, so the ¹³C 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