where I is the moment of inertia [34]. The change in orientation of
the molecule is assumed to be due to rotational isotropic Brownian
motion. These equations are based on Stokes' diffusional model for
spheroidal molecules.
The chief difficulty in calculating the various contributions
to the spin-lattice relaxation time from the derived equations is in
obtaining reliable correlation times. The BPP theory assumes that
dipole-dipole interactions are diffusionally controlled processes.
The correlation time is given by
T = 4n T] a3/kT (11)
For intermolecular dipolar interactions the BPP theory seems to give
an adequate representation. However, for intramolecular dipolar
contributions, the theory is adequate only for polar highly associated
liquids. In most cases, the relaxation times predicted are much
shorter than those observed [28,29,30,35]. Steele argues, in fact,
that there is no direct relationship between macroscopic viscosity
and molecular rotation [35].
Spernol and Wirtz [36] and Gierrer and Wirtz [37] suggested
that the macroscopic viscosity be replaced by a microviscosity coef-
ficient which reduces the Debye correlation time by a factor of six
for pure liquids. Steele has proposed an inertial model in which the
rotational motion is best described by classical equations of motion
for a rigid rotator. He calculated the rotational contribution to
the spin-lattice relaxation time in several hydrocarbons and derived