Molecular properties determined from the relaxation of long-lived spin states

The populations of long-lived spin states, in particular, populations of singlet states that are comprised of antisymmetric combinations of product states, $∣αIβS⟩−∣βIαS⟩$⁠, are characterized by very long lifetimes because the dipole-dipole interaction between the two “active” spins I and S that are involved in such states is inoperative as a relaxation mechanism. The relaxation rate constants of long-lived (singlet) states are therefore determined by the chemical shift anisotropy (CSA) of the active spins and by dipole-dipole interactions with passive spins. For a pair of coupled spins, the singlet-state relaxation rate constants strongly depend on the magnitudes and orientations of the CSA tensors. The relaxation properties of long-lived states therefore reveal new information about molecular symmetry and structure and about spectral density functions that characterize the dynamic behavior.