From my understanding, the steric factor accounts for the relative orientation required by reactants in order to react in collision theory. This implies that steric factors should probably always be below 1 and above 0, since the relative orientation logically should always be a limiting factor, but below 0 or equal to 0 would mean that the reaction cannot take place.

However, my textbook, as a thinking point, asked if there could be a case where the steric factor is more than 1. Specifically, the example it gave was the harpoon mechanism between potassium and the iodine molecule.

My question is, can the steric factor be above 1 or less than 0, and how does that apply to the harpoon mechanism?

  • $\begingroup$ Can you actually have a steric factor less than 0? That would imply a negative rate. $\endgroup$ – orthocresol Sep 29 '16 at 4:33
  • $\begingroup$ I dont know, thats what Im asking. $\endgroup$ – phi2k Sep 29 '16 at 4:34
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    $\begingroup$ Harpoon reactions have a steric factor greater than one. en.wikipedia.org/wiki/Harpoon_reaction $\endgroup$ – MaxW Sep 29 '16 at 5:04
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    $\begingroup$ A negative steric factor would imply a negative rate constant which is not possible. In a harpoon reaction a factor greater than 1 simply means that bigger cross-sections are found to be necessary, compared to those normally used, to fit experiment to theory. Usually there are so many approximations in transition state theory ( geometry of transitions state, activation energy, bond frequencies, friction due to solvent) that steric factors are added as a sort of loose way of fitting calculation to experiment when one has nothing more concrete. $\endgroup$ – porphyrin Oct 27 '16 at 20:41
  • $\begingroup$ I have also read (on Wikipedia https://en.wikipedia.org/wiki/Collision_theory#Steric_factor, but without reference indicated ) that a greater than 1 steric factor can happen due to the solvent, making so that "several collisions can take place in a single encounter". I don't know how this phenomenon should be balanced with the underestimated cross-section mentioned by @porphyrin. $\endgroup$ – The Quark Jan 27 '17 at 14:27

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