Take a look at the attached link for clues....

Article Photoinduced Electron and Proton Transfer in Phenol and Its ...

Thanks John, I'll read through that paper once I have finished reading through this one, which is of equal interest:

http://pubs.rsc.org/en/content/articlelanding/2012/cp/c2cp41842h#!divAbstract

Dear Anthony,

Yes. You need to run TS for protonation reaction as well as for a water dissociation. In most cases the activation energy is relatively low and this step is NOT the rate-limiting step of the process. However, it may be a rate limiting step in some processes.

I have published more than 30 papers on proton transfer reactions. You may visit my RG profile.

Hoping this will be helpful,

Rafik

Many thanks Rafik,

I'll look through some of your publications. I'm hoping to find a good illustration of a transition state along with the respective mechanism :) 

Many thanks

Anthony

 Unfortunately, I don't seem to be able to optimise two different states (reactant and product) using an H3O+ proton donor near my hydroxyl group. Even when I assign my charge seperately using atom groups and apply counterpoise I end up with the exact same geometry; the positive charge is always over the proton donor in BOTH situations.  

Dear Anthony,

Attached is one of my papers which may helpful.

Good luck,

Rafik

Dear Rafik,

Many thanks for that. I had looked through your publications earlier, but you have many and I wasn't sure which would be the best. 

I do have a could of questions, which should help my own study:

1) In Scheme2, the act of protonation from Int2 to Int4, which has this acid disassociation and protonation not have a transition step?

2) In Scheme2, I am guessing that the step Int2 -> TS5 -> Int3 is almost what I need. I understand what the arrangement of atoms in TS5 must look like, but there is no illustration of that mediating water in either Int2 or Int3. Could you please tell me what that would look like, in particular the charge and spin. 

Many thanks

Anthony

I'm still struggling with optimising the protonation of a hydroxyl on a carbon compound. The explicit water environment always takes the hydrogen (H+) back from the -OH group. 

Dear Anthony,

Since the barrier from INT2 to INT4 is very low (fast reaction) I have not drawn the structure of the TS.

Regarding the multiplicity and charge:

Multiplicity 1

Charge 0

INT2 should have interaction with one molecule of water to balance the reaction.

Hoping this will be helpful,

Rafik

Dear Rafik,

Many thanks for that information, it was extremely useful. If I am not mistaken, the protonation of a hydroxyl group by an acid (H+) is probably going to be very fast and have a very low activation barrier. I can check this out by just doing an Energy = protonated - not_protonated.

Thanks

Anthony

Absolutely Right.

Rafik

Dear Rafik, just one further question.

If that activation is very low, do you think DFT/ab-initio techniques would capture it in its transition state? Or would I resume my calculation using:

Re->TS1(nucleo_attack)->Int1(ringclosed)->Int2(Protonated)->TS2(-OH2+ leaving)->Pr

i.e., no transition state between Int1 and its protonated self?

Thanks

Anthony

Dear Anthony,

For sure, DFT and ab initio can capture the TS. If this step is so important you should search for the TS and calculate the activation energy.

Please note, that activation energies of 0.5- 1 kcal/mol have been calculated by ab initio and DFT.

Rafik

Dear Rafik,

I think every transition state will be important, even at low energies as you quoted. Many thanks for your help with this. Is H3O+ an ideal proton donor for the protonation of a C-OH to a C-OH2+ ?

Thanks

Anthony 

Yes. It is.

Rafik