I am trying to run a interaction studies between two molecule for Hydrogen abstraction (HAT mechanism). So when i put two molecules together in Gaussian at 3-21G basis set, the HAT is happening but when i put the same input file and start optimizing at higher basis set 6-31G(d) basis set from the start, it does not result in H atom abstraction. Can anayone suggest why it is so??
Dear Prerna Bansal ,
Can you provide a bit more of information about what you are trying to obtain? I understand you are trying to model som AH + B -> A + HB reaction, but I don't get which is your current problem with the larger basis set:
1) Is it that you want to optimize the transition state A---H---B, and you succeeded with the 3-21g but it fails with the 6-31g(d)? In this case, try to start the optimization not from your initial structure, but from the 3-21g optimized one, and if the optimization fails due to having more than one imaginary frequency, use the opt=(calcall,ts,noeigentest) option
2) Is it that you tried to optimize the initial AH + B complex, and while it landed directly to the A + HB product for the 3-21g basis set, it kept the H atom in A for the 6-31g(d)? Then, it seems that hydrogen migration may be a barrierless process for the inaccurate 3-21g basis set, but that there is an activation barrier with the 6-31g(d). Thus, what you need to do is to optimize separately the A + HB complex (try to manually move the hydrogen atom in the AH +B structure), and then the transition state to get the full reaction pathway. Relaxed scans are always of great help in these situations.
Hope you find it helpful
Whatever you calculated with the 3-21 basis set - forget about it. 3-21 is big enough only for generating the initial guess. For the H-abstraction you need a basis set with extra diffuse functions assigned to H atoms and the active site atoms. So, 6-311++, at least, or aug-cc-pVTZ which would be much better. Don't use 6-31++, since it would be ridiculously overloaded with diffuse functions while not having many enough functions for inner and valence electrons.
Dear Prerna Bansal,
Please take a look at links on topic.
Article Correlation of Hydrogen-Atom Abstraction Reaction Efficienci...
Article Mechanisms for Hydrogen-Atom Abstraction by Mononuclear Copp...
https://www.frontiersin.org/articles/10.3389/fchem.2018.00406/full
https://dugi-doc.udg.edu/bitstream/handle/10256/11165/023605.pdf?sequence=1&isAllowed=y
Best regards
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