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Generally $\ce{H^+}$ ions in water remain in the form of hydronium ions ($\ce{H3O+}$), and not as "free" aqueous protons, as far as I've been taught.

My question: is this always the case, even when considering dynamics at extremely short time scales? By this I am not meaning interconversion with the Zundel ($\ce{H5O2+}$) or other states. I mean to say, does bare $\ce{H+}(\mathrm{aq})$ ever exist on, say, a femtosecond time scale?

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  • $\begingroup$ I think this is one of those things where there probably is a single proton floating around somewhere in a solution, but the formation of this is so unlikely. It means that you need to have just a hydrogen atom somewhere in solution and some other molecule which will take that electron and form a radical, which is certainly not stabilizing. Then, the proton needs to somehow not react with anything for a femtosecond. My bet is that this just doesn't happen... $\endgroup$ – jheindel Feb 25 '16 at 2:26
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    $\begingroup$ This is probably more of a philosophical question - when is a H bonded to something, and when is it not? That's quite tricky to answer, particularly in the liquid state... $\endgroup$ – Gerhard Feb 25 '16 at 13:24
  • $\begingroup$ Relevant: chemistry.stackexchange.com/questions/32558/… $\endgroup$ – getafix Jan 1 '18 at 19:19
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According to Myths about the proton. The nature of H+ in condensed media Accounts of Chemical Research, vol. 46, pp 2567–2575:

But to put this into perspective, an electron-free proton has an estimated acidity $10^{56}$(!) times greater than 100% H2SO4, the threshold of superacidity.$^2$ Adding a bare proton to a molecule is a real event in the gas phase but only a solvated hydrogen ion can be transferred in condensed phases.

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No. The proton is always within some electron density, even when transitioning between the hydronium/water and whatever anion it was bonded with.

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    $\begingroup$ To expand on this answer: Say a proton moves from one water molecule to another. It does not jump off and become a bare proton, even for a moment. Rather the second water molecule begins to form a bond with the proton as the first water molecule begins to loosen, leading to a transition state where the proton is partially bonded to both water molecules. $\endgroup$ – Oscar Lanzi Mar 7 '16 at 0:49

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