Measurement of cross-correlation of fluctuations of dipolar couplings and anisotropic chemical shifts by selective spin locking

New exptl. methods are presented that allow one to measure cross-correlation between the fluctuations of dipolar interactions and chem. shift anisotropy (CSA) in scalar coupled systems with two and three spins I = 1/2 in isotropic soln. The methods involve selective excitation of the transverse magnetization IAx of a chosen spin A, followed by selective spin locking of this magnetization. If the spin locking radiofrequency (RF) field is exactly on-resonance for spin A while leaving the other spins M and X unaffected, there is no cross-relaxation (Overhauser effect) in the rotating frame, i.e., no transfer from IAx to IxM or IXx. By contrast, cross-correlation between different relaxation mechanisms may give rise to single-quantum coherence of higher rank. In particular, CSA-dipolar cross-correlation may lead to a partial conversion of IAx into 2IAxIMz or 2IAxIXz, while interference between two dipole-dipole interactions leads to the appearance of terms such as 3IAxIMzIXz. [on SciFinder (R)]