The accuracy of distance measurements in solid-state NMR

The accuracy with which distances can be measured using dipolar recoupling experiments in solid-state NMR is investigated. The relative precision of e xperiments in a three spin system versus an isolated spin pair is found to depend very strongly on the nature of the coupling Hamiltonian. The accurac y of distances measured in even the simplified three spin system is seen to be very poor for existing homonuclear recoupling Hamiltonians. This sugges ts that it would be difficult to exploit broadband homonuclear recoupling t o measure geometrical information reliably in complex spin systems. These c onclusions apply equally to both single-crystal studies and powder samples. In contrast, the presence of additional spins has marginal impact on the a ccuracy when the coupling Hamiltonians commute with each other, as in the c ase of heteronuclear recoupling. The possibility of creating such a Hamilto nian for homonuclear recoupling using a suitable rotor-synchronized pulse s equence is discussed. (C) 1999 Academic Press.