Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

5.4 Advanced collision theory 130
(4) Arrhenius activation energy:
y
= 2 ln
(5.191)
= 0 + 1 (5.192)
2
c) Allowance for the “steric effect”
In order to account for the “structures” of the molecules which can undergo a reaction
only if the reaction partners have a specificrelativeorientation,weintroducea(generally
-dependent) steric factor ( ):
µ 12 µ
8
( )=( )
2 exp −
0
(5.193)
In ( ), we include all sorts of factors leading to deviations from the predicitions by simple
collision theory. In favorable cases, ( ) can be predicted by theory. Towards these
ends, we define a reaction probability ( ) (also called the opacity function,
andcalculatedbytheory)andintegrateaccordingto
( )=
Z
0 max
( )2 (5.194)
0
In other cases, it may be convenient to define an angle dependent reaction threshold
energy ∗ 0:
∗ 0 = 0 + 0 (1 − cos ) (5.195)
y
∗ 0 = 0 for =0
∗ (5.196)
0 0 for 6= 0
Such models are helpful for explaining steric factors in the range of 1 ≤ ≤ 0001.