Measurement of dipolar cross correlation in scalar-coupled systems

In systems with dipolar relaxation in isotropic phase, it is possible to measure the extent of cross correlation of the fluctuations of two selected dipole-dipole interactions A-M and A-X by selectively exciting and spin-locking the transverse magnetization of spin A. If the system comprises only three spins A, M, and X, the conversion of in-phase magnetization IxA into doubly antiphase magnetization 4IxAIzMIzX during the spin-locking period occurs spontaneously through relaxation. The rate of this conversion is proportional to the spectral d. of the cross correlation of the random fluctuations of the dipolar A-M and A-X interactions. Larger systems, comprising at least a 4th spin K, were studied. The complexity of the situation is increased, since other forms of three-spin order such as 4IxAIzKIzX or 4IxAIzMIzK become accessible. Also, this paper addresses the role of scalar couplings, which are a prerequisite for making three-spin order observable, but which are also a source of perturbations, since scalar couplings can contribute significantly to the creation of various three-spin-order terms. If the spin-locking field is too weak compared to the width of the multiplet under study, residual scalar interactions give three-spin order. If the spin-locking field is too strong compared to the relative offsets of other passive spins, further complications occur. These can be avoided most effectively by using very high static magnetic fields. If coherent contributions to three-spin order can be suppressed or accounted for through simulations, the remaining buildup of three-spin-order terms arising from dipolar cross-correlation effects can be interpreted in terms of structural and motional parameters. [on SciFinder (R)]