# Significance of Proton Tunneling

It has been repeatedly emphasized that proton transfers "involving" three or four membered ring transition states are not favorable and should not be drawn as the primary mechanism for any reaction.

However, with regard to a recent discussion involving $\ce{H3CSH}$ it was noted that the rate of proton tunneling in the anionic form $\ce{H3CS-}$ was likely extremely high.

Carbon-Sulfur Bond Lengths; Resonance Effects (Or Lack Thereof)

So:

a) Is this a valid rule of thumb: avoid intramolecular proton transfers unless the transition state has 5 or 6 members in its ring?

b) What's more significant with regard to proton transfer - proton tunneling or physical proton transfer? Or is the answer an "it depends"?

• 1) Was the discussion on Chem SE? If so, could you reference it? 2) I don't follow, in what sense is the proton in the anion tunneling, what is the proton doing? – ron Dec 6 '14 at 18:34
• – Dissenter Dec 6 '14 at 18:35
• @ron I'd even guess there's a significant H tunneling rate in the syn form (or an inversion from anti to syn followed by H shift). So I agree it would at least be tough. Unfortunately, my microwave expert retired a few years ago, so I can't ask. – Geoff Hutchison – Dissenter Dec 6 '14 at 18:36