correlation time in BPP theory. A comparison of the experimental
and calculated times would give an indication of the applicability
of BPP theory to nonspherical molecules. The two variables besides
the temperature in equation (11) are the macroscopic viscosity and
the molecular radius of the sphere. Plots of the log of the macro-
scopic viscosity as a function of 1/T are given in Figure 7. Close
packing of spheres was assumed to calculate the molecular radius.
The bond distances needed in equation (4) were obtained from microwave
spectroscopy [73,74]. These calculations for o-, m-, and p-chloro-
fluorobenzene at various temperatures are given in Table 12. A com-
parison of the experimental and calculated values shows that the BPP
theory predicts too efficient a relaxation mechanism. That is, the
relaxation times are too short. The assumption of spherical symmetry
might not be expected to apply in these molecules. However, the BPP
theory does show the proper ordering in regard to relative magnitudes
with o > m > p as found experimentally. Agreement is better in the
para molecule than in the ortho or meta molecule presumably because of
the cylindrical symmetry.
Steele has suggested that the relations described above, between
macroscopic viscosity and molecular rotation, are not valid [35]. In
their place he proposed an inertial model of rotation. A correlation
time may be obtained from the equation of motion for a rigid rotator
and is given by
T j (18)