The spin-rotation contribution, TI sr follows the sequence
o < m < p over the entire temperature range. This result indicates
that the position of the chlorine atom on the aromatic ring has
a significant effect on the molecular motion of the molecule. From
equation (7), which is written for a spherical molecule, the spin-
rotation relaxation time, to a first approximation, should be inversely
proportional to the mean moment of inertia. The moments of inertia and
bond distances for o- and m-chlorofluorobenzene have been measured by
microwave spectroscopy [73,74]. However, there were no reported values
for p-chlorofluorobenzene. Since the bond distances were almost
exactly equal in the ortho and meta molecules, a set of values was
chosen, and the moments of inertia were calculated for p-chlorofluoro-
benzene. These values are summarized in Table 10. The mean moments
of inertia are in the sequence o < m < p, or opposite to that which
would explain the experimental results. It seems that the spherical
approximation is not a good one.
Assume that the major portion of the internal motion consists
of rotation about the axis in the plane of the ring coincident with the
C-F bond. The moment of inertia about this axis, IA, lies in the
sequence p < m < o which explains the experimental results. The moment
of inertia about the C-F bond axis for each of the three molecules is
significantly smaller than the moments of inertia about the other two
axes. As the chlorine is moved on the ring from the para to the ortho
position, deviation from cylindrical geometry causes more restricted
motion [31] and introduces tumbling of the molecule. Another