I have attached one of the reactant and its product.When used the qst2 method it gives an error.so I thought of using qst3 method for which I have to guess the TS.
If the assumed mechanism is OH addition to an sp2-hybridized carbon atom, then you have a tricky problem. In the gas phase there will be a van der Waals complex that rearranges through a barrier to the product. That barrier might be LOWER in energy than the separated reactants. See, for example, OH addition to ethylene done well (http://pubs.acs.org/doi/abs/10.1021/jp0566820). I believe the supporting information has all the structures you would want to see. Results may depend heavily on the method chosen, and the size of your molecule limits the methods you can use.
If you want reliable results, I strongly suggest testing your method on ethylene and/or a smaller analogue of your base.
Thank you Theodore. I read the article you have sent the link.And I read another article to which your article has referred.Those clearly give an idea about the Structure of the transition states.Thanks again!
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