When you set up the initial geometry of a TS in Gaussian, should I be modelling the charge and spin multiplicity of the TS itself or either the product/reactant? For example, in the case of the dissociation of acetic acid in aqueous solution (see link), I assume the H is between the H3O(+) and acetic acid. What charge would I give the water molecule as the third hydrogen is technically being shared with the carbonyl group for a fraction of time?
Many thanks
Anthony
http://upload.wikimedia.org/wikipedia/commons/9/96/Acetic-acid-dissociation-3D-balls.png
The charge should be the same in all three geometries as should the multiplicity. Gaussian won't allow either of these values to vary in a QST2/3 calculation. This doesn't mean that there aren't unpaired electrons but that the net multiplicity is always the same (i.e. no spin violation). In the acetic acid example you've used the charge and multiplicity would, I assume, be 0 and 1 respectively. However, H3O+ is not a good example of a solvated proton, a larger system involving multiple co-ordinated H2O molecules would allow for stabilization of the proton and more accurate enthalpy, free energy changes to be calculated
During the calculation of a reaction potential energy surface (Reactant -> TS -> Product) you should maintain same spin multiplicity throughout, unless there is a surface crossing in between. For your case of acetic acid dissociation, you should maintain a singlet throughout. About charge, you should care about the total charge of the system, and not the individual charge on an atom. Therefore, the overall charge of your TS calculation will be zero, as in the TS, the O is partially negative and leaving H is partially positive, yielding a overall zero. The same will hold for the "reactant complex" and the "product complex".
The previous comments are very precise and correct. To be completely informed visit the website http://www.gaussian.com/g_tech/1.htm
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