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

5.4 Advanced collision theory 132
I Harpooning: The value of ≥ 1 for the steric factor of the K + Br 2 reaction can
be understood by the “harpooning mechanism”: An electron jumps from the K to the
Br 2 at a long intermolecular distance. The subsequent collision and reaction at closer
distance then occurs between a K + and Br − 2 . The collision between these two is strongly
affectedbytheCoulombattraction.
5.4.6 Long-range intermolecular interactions
To this point, we have neglected any interactions between the reacting molecules other
than the the collision itself. Thus, only straight line trajectories were allowed. In reality,
however, there will be attractive or repulsive interactions between the colliding molecules
(see Fig. 5.24). These will inevitably affect the collision dynamics. An extreme example
is the K + Br 2 reaction mentioned above.
We will investigate the potential energy hypersurfaces governing the reactions in Section
6. Here, we will consider only long-range intermolecular interactions which can be easily
implementedincollisiontheory.
I
Figure 5.24: Examples of attractive and repulsive trajectories for the collision between
two molecules.
a) Types of long-range interactions
The long-range interactions between molecules can be described using multipole expansions
of the potentials (charge: 1; dipole: 2; induced dipole: 3; quadrupole: 3 . . . In
general, the interaction potential between two multipoles of ranks and 0 is proportional
to − with = + 0 − 1:
µ
() =−
(5.197)