[msk034]

Proton is acidic because of what? low bond charge density? Doesn't seem too realistic - it's still far not zero (bonding energy is on the order of 10-15% of atomic orbtal energies, hence change in charge density is probably on the same order), other electrons will be kept away of the proton inside bond cloud. So the proton needs to literally leap through the cloud. Tunneling? - Is it really the case?

[Irlanur]

you should really try to use your words. I don't really know what EXACTLY the question is and what you would expect as an answer.

[msk034]

Well, the question is about quantum tunneling in proton migration. Meaning comparison of tunneling with alternative migration routes and identification of relevant mechanisms and their contribution. And yes, these are my own english words.

[Irlanur]

ok, so your question is far from a question about acidity. acidity is thermodynamic question, whereas tunneling and proton transfer reactions are kinetic.

so what do you know about proton transfer reactions? where could tunneling be more relevant than alternative pathways etc. ? we discussed this here in the forum lately...

[msk034]

The reason for the question is as follows: consider a proton covered with electron cloud. The proton naturally sits around the minimum of its potential energy surface (PES). Consider next an external electrical field (EEF). Classically, when an EEF extracts a proton from its seat inside a negative charge distribution (NCD), it should first bring the proton on the verge of this NCD, in other words the proton will not jump inside-out of the electronic cloud, but rather move gradually to the boundary and therefrom get detached by the EEF. This is classically. If we now bring an external electronic cloud (EC) close to a shielded proton, we could expect that 1) either this external EC penetrates inside the proton's own EC and gets the proton over, or 2) the electric field generated by the external EC is strong enough to fully deshield the proton, as in classical picture, or 3) the proton leaps through its cloud by virtue of tunneling. The 1st route should be ruled out by QM [as I understand, not mixing electronic clouds cannot get overlapped in space], the second requires very strong electron accepting capacity of the proton counter particle (conjugate base), which cannot be taken for granted, so the 3d option seems the only probable.
This is all some background. The question is still there - if tunneling is fully ruled out, will we observe any acidity at all (well, yes, in the kinetic sence, i.e. happening within our lifetime )?

[Irlanur]

The proton naturally sits around the minimum of its potential energy surface (PES). Consider next an external electrical field (EEF). Classically, when an EEF extracts a proton from its seat inside a negative charge distribution (NCD), it should first bring the proton on the verge of this NCD, in other words the proton will not jump inside-out of the electronic cloud, but rather move gradually to the boundary and therefrom get detached by the EEF.

1)The EEF will change the PES extremely.
2) you forget the vibrational levels.

the second requires very strong electron accepting capacity of the proton counter particle (conjugate base), which cannot be taken for granted

that's what's all about with strong acids...