In the absence
of matrix effects, ΔS≠ would be negative for a
unimolecular decomposition reaction (often observed
for polymer degradation) and positive for a
bimolecular addition reaction. Both the sign and
magnitude of ΔS≠ provide useful information about
the structure of the activated complex. Both of these
parameters may be readily extracted if the rate
constant for reaction can be determined at several
temperatures. This is most reliably done using an
isothermal technique, i.e., by determining the rate
constant at several fixed, well-defined temperatures
[3]. Then a plot of lnk/T (where k is the rate constant
and T the absolute temperature) vs. l/T yields a slope
of –ΔH≠/R (where R is the gas constant,
1.9872 cal mol or equivalent). The activation
entropy, ΔS≠ may then be calculated at any given
temperature from the expression