MoP-02


WHAT IS THE CORRECT FORMULA TO USE IN CALCULATING THE RATE CONSTANT OF AN ION/DIPOLE REACTION?

Derek R. Smith

School of Chemistry, The University of New South Wales, Sydney, New South Wales, Australia 2052


In our studies on the reactions of metal ions (trapped in the ion trap region of an FT-ICR mass spectrometer) with organic molecules we frequently have recourse to the calculation (for comparison purposes) of the rate constants arising from the ion/dipole interaction. Traditionally1,2,3 such rate constants have been calculated using the formula arising from the Langevin ion/dipole theory4 :-

Formula

The derivation of this formula is here subject to critical review which reveals the origin of its apparent flaw, in that it predicts a rate constant value of zero if the 'ion' is uncharged or an infinitesimally small value if the molecule is not polarisable. An alternative derivation is suggested for a formula to calculate the rate constant of an ion/molecule reaction.

Comparisons of the calculated values obtained by both methods with experimental values will be made.

1. Eyring, H., Hirschfelder, J. O. and Taylor, H. S., J. Chem. Phys., 4, 479, (1936)
2. Su , T. and Bowers, M. T., J. Chem. Phys., 58, 3027, (1973)
3. Gioumousis, G. and Stevenson, G. P., J. Chem. Phys., 29, 294, (1958)
4. Langevin, P. M. Ann. Chem. Phys., 5, 245, (1905)