Theory of long-lived nuclear spin states in solution nuclear magnetic resonance.I.Singlet states in low magnetic field

摘要

We have recently demonstrated the existence of exceptionally long-lived nuclear spin states in solution-state nuclear magnetic resonance.The lifetime of nuclear spin singlet states in systems containing coupled pairs of spins-1/2 may exceed the conventional relaxation time constant T_1 by more than an order of magnitude.These long lifetimes may be observed if the long-lived singlet states are prevented from mixing with rapidly relaxing triplet states.In this paper we provide thedetailed theory of an experiment which uses magnetic field cycling to observe slow singlet relaxation.An approximate expression is given for the magnetic field dependence of the singlet relaxation rate constant,using a model of ntramolecular dipole-dipole couplings and fluctuatingexternal random fields.