5924
J . Am. Chem. SOC.1990, 112, 5924-5946
spin-diffusion becomes meaningless as each pair of protons may exhibit either behavior or behavior between the extremes. The relative sign of a cross-peak will not necessarily accurately reflect the rate of global molecular reorientation. Extreme-narrowing (anti-phase with respect to the diagonal) cross-peaks observed in NOESY spectra of macromolecules should not be ignored or automatically assigned to artifacts. The simulations show that motional averaging about an angle in the range of 50' to 60' will produce this type of cross-peak. Indeed, confirmation of a negative cross-peak would yield a new structural constraint. A spatial constraint would define a range of angles for the orientation of motional averaging of a proton pair with respect to the major axis of global molecular reorientation. The calculations indicate that highly anisotropic molecular orientation will produce results that cannot be explained by calculations on the basis of the rigid isotropic assumptions. Oriented molecules in lipid bilayers and micelles or semiflexible polymers encountered in material sciences demand application
of the full spectral density functions. In extremely high magnetic fields ('H frequencies >500 MHz) molecules with large dipole moments are known to align along the direction of the static field. The results for highly anisotropic motion (7'
