Effect of Molecular Shape on Nuclear Magnetic Relaxation

Abstract

The dependence of magnetic relaxation processes upon molecular shape is examined by regarding the molecule as a rigid ellipsoidal body which shows anisotropic rotational and translational Brownian motions. The shape effects are examined based on particular mechanisms of relaxation: (1) intramolecular dipole—dipole interactions, (2) intermolecular dipole—dipole interactions, (3) anisotropic spin—spin couplings, (4) interactions of the nuclear moments with the local magnetic fields due to anisotropic shielding of the external field, and (5) electric quadrupole interactions. The relaxations caused by the first and second mechanisms are discussed in detail by taking a prolate spheroid as an example; the correlation time and the relaxation time are obtained as functions of the axial ratio. Saturations of multiple NMR lines are discussed; then, by taking the multiple lines of the three vinyl protons of acrylic acid as an example, the shape effect on the saturation, as well as an application of the saturation effects of multiplet lines to the assignment of a complex NMR spectrum, is discussed.