Reaction Rate Constants of Ion-Molecule Reactions

Reactions of ions (with denoting atomic or molecular ions) with neutrals B proceeds with reaction rate constants k, defined by 

How large, then, can values of k be for specific reactions Each neutral atom or molecule has a polarizability a, which means that an electric dipole is induced when the neutral particle is put into an electric field. 

Whenever an ion approaches a neutral (molecule or atom), its Coulomb field induces a dipole in this neutral, which results in an attractive force. This leads to the formation of an ion-neutral collision complex when the impact parameter is helow a critical value, as has been shown by Gioumousis and Stevenson (1958). The rate constant for formation of such complexes is independent of temperature and has the value 

In cases where neutral reactants already possess a permanent dipole moment /i, the capture rate constant is larger than ki. Su and Bowers (1979) have derived the expression 

Values of k are often similar to those of A doI often the two differ by less than 

---