I am trying to to find out the transition state for the reaction (attached as an image). The reaction has two reactants and one product.
1. I have optimized both the reactants separately using Gaussian.
2. I have also optimized the product separately.
I am unable to figure out how to create an input file to search for transition state. I tried appending optimized reactant2 to optimized reactant1 and then added product to the same file (add to the molecule group). I, then, used the connection editor to make sure they reactants and product have same atom numbering. When I tried to run a QST2, gaussian did not run the file. It gave the following error "Try using 3 structures as input for QST transition state search". Would appreciate any suggestions.
For QST to work you need a good guess of the structure of products just after the reaction took place (your product is a good guess) and reactants just before the reaction takes place (which you don't have, since you optimized the reactants separately). There are a few options that you can use.
1. Use your TS guess, but move the small molecule farther from the N-N bond and optimize it (OPT). There is a good chance, that it will converge to a structure with both molecules interacting with eachother but still as reactants (not products). If so, use it as the guess for reactants. If the reaction has a small barrier the optimization may proceed all the way to products, so next:
2. Do the same but try to freeze some N-C distance (N from one, big molecule and C fromt he other) at around 2-3 Angstroms and do the same. The optimized structure should be a good guess of reactants.
3. You already have a good guess of TS, so forget about QST and try TS optimization without specyfying reactants/products (opt=(ts,noeigentest)). Try it, perhpaps also with freezing some of the N-C and C-C distances at resonable lengths (like you would expect in ts - around 2.0 Angstroms).
I tried adding optimized reactants to one file but gaussian kept giving error. I will try this and post the results here. Thanks for these great suggestions.
You should observe carefully the both reactant and product structures and predict several ways to link them together. Based on this guess you can imagine the TS between them. Do it by your experience, however I must examine your structures first.
Hi Tam,
Thank you for sharing the file. Could you also share the input file please? All the methods I tried failed and I am not able to figure the exact reasons.
Tam,
If you had to prepare the input file for TS using optimized starting materials (reactants), how would you do that ?
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