Is it a proton transfer process or only H+ addition? For H+ addition you might not get any TS. You can run a relaxed potential energy scan from the protonated species by increasing the X-H distance and see if there is any barrier or its a barrier less association. In every step of the scan also check the charges of X and H to make sure if the X-H bond cleavage is heterolytic (X- & H+) otherwise the scan will not be useful.
There isn't necessarily a transition state when a bond is formed and no bonds are broken. If you want a transition state, you may need to put in the entire acid, such as HCl or H3O+. I'll also second Debashish's answer about the relaxed potential energy surface scan for the X(-)--H(+) cleavage, as a way of demonstrating whether a transition state exists or not.
Mr. Visentin,
IRC is used to evaluate the energetic Saddle–points of a minimum energy path including TS ensemble interactions.
But toward a process of protonation, you could pay attention to an example of interacting particles {[OH-.NH3] + H+} (attachment). As you can see IRC predicts the direction of the protonation. But if you focus attention to BOMD-opt data (attachment), you shall see that the optimization of the system using BOMD expresses better protonation process with the final stationary state (SS). As we may expect the "TSs" (or, here, we are talking about an intermediate state (IM), in fact, because of it does not correspond to the maximum of the potential energy and the mechanism of the reaction can be assigned as a 'single-route linear (two-steps)' one) obtained by IRC and BOMD are almost identical (DE = /0.03/ a.u.). In addition, you could use ADMP as well as in studying protonation reactions.
Thanks a lot for your explanation. I found it really clear and able to solve every doubt.
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