I have tried both QST2 and QST3 methods to find the transition state of a radical reaction.(Addition of OH radical to adenine). My output files don't have an imaginary frequency.So I can't consider them as the actual transition state.Can someone tell me what I have to do to obtain one negative frequency.
I'm not an expert on Gaussian, so I guess you are not getting any reasonable structures in addition to your result. But for QST3 you would need a TS guess anyway, so what are the frequencies for that one?
Actually in my experience, if the TS mode is not too complex, the most useful approach is always to do a plain relaxed surface scan for the proposed reaction coordinate. Mostly that also leads to a quick convergence for the transition state. And even if not, you could still check the energy profile and test for imaginary frequencies for every structure later on.
That being said, are you even sure your reaction is not barrierless with the method used? Another handy outcome of the above approach is that the energy profile directly tells you if there is a maximum at all.
As an example for the barrierless reactions: I had some reactions without a TS if I used GGAs, but hybrid functionals seemed to work fine.
Mrs/Miss Senanayake,
Please pay attention to the discussion below. There are shown few examples of inputs for TS computations:
https://www.researchgate.net/post/Hello_How_to_build_a_structure_for_transition_state_for_a_single_electron_transfer_in_gauss_view
Dear Thorsten ,Carmen ,Bojidarka ,
Thank you for your valuable information. I think the problem was with my guess structure of the TS. I got an imaginary frequency after doing a few changes to my guess structure.Also I confirmed it using TS(berry) optimization and an IRC calculation.
Thanks again!
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