the spin-rotational correlation time. The modulus E may be written,
after substitution, as
3 t2 T, 1/2
12 T
1 sr
where I is the mean moment of inertia, and Td is the dipolar correla-
tion time. A relationship between TI intra d and Td may be obtained
by using equation (4). Substituting, one obtains
T = A(TI T1 d)-l/2 (23)
1 sr 1 intrad
where A is a parameter depending on the molecule for which the
calculations are being made. Powles argues that, for the Hubbard
relationship to hold, (T T d /2 should be independent
1 sr 1 intrad
of temperature [2]. He showed that this relationship is approxi-
-2 2
mately obeyed for fluorobenzene and calculated c to be 2.5 kHz in
good agreement with the molecular-beam values of Chan. Also, Hubbard
[23,34] has shown that equation (10) gives good agreement with calcu-
lated data for the spherical molecules, tetrafluoromethane and sulfur
hexafluoride. However, for o-, m-, and p-chlorofluorobenzene,
(T T 1/2 is not temperature independent. A mean value
1 sr 1 intra d
of the product (T T1 ) was taken over the temperature
1 sr 1 intra d
range from 0 to 100 degrees Centigrade. The values of c obtained in
this manner are 20.0, 13.5, and 15.8 kHz for o-, m-, and p-chloro-
fluorobenzene, respectively. The experimental values of the modulus
are somewhat high compared to those calculated from magnetic shielding