Molecular reorientation from NMR relaxation times in planar molecules

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Abstract
The spectral density J(0) for the autocorrelation function referred to an arbitrarily chosen body-fixed coordinate system has been rederived for a planar asymmetric rotor undergoing rotational diffusive motions. The angular dependence of J(0) was found to be more complicated than in the case of coincidence of the body-fixed frame with the principal axes of rotation. Therefore, the orientation of the principal axes of the rotational diffusion tensor can be determined from four different J(0) or 1/T1 values. This result disagrees with previous conclusions based upon the assumption that the same angular dependence should be expected irrespective of the relative orientation of the diffusion tensor axes. Likewise, the values of the three diffusion coefficients can be in principle determined from more than two (or three) different J(0) values for molecules with (or without) a binary symmetry axis. In practice, however, only two relations between them can usually be obtained due to the uncertainty in T1 measurements and to the relative insensitivity of J(0) to some correlated changes in the diffusion coefficients for moderate anisotropic molecular rotations.