molecular reorientation is to be considered [10]. However, for
intermolecular interactions both parameters are varied by a combina-
tion of molecular reorientation and translational diffusion. It is
usual to simplify the treatment of intermolecular interactions by
ignoring molecular reorientation as compared to translational diffu-
sion [5].
Wangsness and Bloch [11,12] give an approximate treatment of
intermolecular dipole-dipole interactions in which a single molecule
is isolated from the rest of the liquid. All spins outside that one
molecule are considered as belonging to the lattice. The spin relax-
ation of one molecule is considered as due to a randomly fluctuating
magnetic field caused by all the other spins in the liquid.
The basic theory of dipole-dipole interactions in magnetic
resonance was developed by Bloembergen, Purcell, and Pound [.l]
Briefly, the BPP theory considers a rigid lattice of dipoles for which
a general Hamiltonian is written. Those terms in the dipolar part of
the Hamiltonian are nicked out which will be effective in relaxation.
The local magnetic field produced at one nucleus by neighboring mag-
netic nuclei is spread out into a Fourier spectrum extending to fre-
quencies of the order of 1/T, where T is a correlation time associated
with local Brownian motion and closely related to the characteristic
time which occurs in Debye's theory of polar liquids [13]. Essentially,
the theory describes the rotational and translational motion of the
molecules by means of a classical diffusion equation. An expression