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deprotonation transition state G03

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Member
Registered: Feb 2009
Posts: 3
Hello,

I am a Physics Master student, and I have recently started working with Gaussian03. I am trying to locate the transition structure for the deprotonation reaction (DQH+) ---> DQ + (H+). I'm having lots of trouble with it. I even tried DQ + (H3O+) ---> (DQH+) + H2O. I am using the QST3 method. The problem is that I am not a chemist, so I probably lack the knowledge required to thoroughly analyze reaction paths. Any sugestion would be very, very helpful.
Thank you very much in advance.
Member
Registered: Aug 2008
Posts: 43
Could you post your input file please and I'll take a look.
Member
Registered: Feb 2009
Posts: 3
Thank you very much for your reply.
This is my line of command:
# opt=(calcfc,qst3,maxcycle=1000) freq=noraman ub3lyp/6-31g(d) geom=connectivity;
Charge 1 Multilpicity1;
the molecule specification includes DQH(+) +H2O (the reactants), DQ +H3O(+) (the products) and a guess transition state where the bond length between DQ and H(+) is very large.
The transition state converges after all, but it is not the one I am looking for.
For me it would be much more helpful if I could find the transition state for the reaction DQH(+) --> DQ + H(+), without the water molecule. But, I saw comments on other G03 forums that it is very difficult to model H+ reactions.
Thank you very much in advance for your help.
Member
Registered: Aug 2008
Posts: 43
I have two suggestions for you, but first a couple of general notes. If a geometry opt doesn't converge in 50 iterations it is very unlikely to do so ever. You can always feed an almost optimized geometry into a second optimization job, so I'd say setting maxcycle=50 is much more reasonable. Second, you should probably include int(grid=ultrafine) into your line while using dft.

The first suggestion is easiest if you have the molecule specification in your input file. You do not want to do any frequency calculations here, nor do you want to use gdiis b/c it doesn't work with partial geometry optimizations. Then you specify,

opt(z-matrix,maxcycle=50)

and in the molecule specification you should have the O in the H3O+ connected to your DQ molecule and the other 3 H's should have their distance specified relative to the O. Then, for that O to DQ distance, I'll call it dqo1, put,

dqo1 = 1.1 s 20 0.05

This will do a partial geometry optimization at each point as the water molecule gets moved away from the DQ.


My second suggestion is that if you get the above to work, try copying the rought TS structure from above to a new file and specifying OPT(ts,calcfc,gdiis,maxcycle=50) or you can do a qst2 calc where you choose one geometry before the TS and one after the TS from your partial geometry optimization above and enter those as the 2 geometries for the qst2.

I hope that helps.
Benj
Member
Registered: Feb 2009
Posts: 3
Thank you very much. I will try your advice and get back to you.

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