I am doing a ts search for a reaction involving Ir catalyst. I have obtained optimized structure for reactant and product. This step involves rotation of a dihedral like in butane gauche anti rotation. While setting up qst2 job how to set options such that ts involves only that dihedral rotation. I have previously tried opt=ts but single imaginary frequency obtained doesn't correspond to dihedral rotation.
First of all, transition state searches are complicated and kind of an art form.
Your opt=ts job need to good guess of the transition state to converge because what it does is to minimize along all but the one mode with the lowest energy in the given structure. If you start from the product or educt, the lowest energy mode will be anything but not your reaction coordinate, and thus it won't converge to your transition state.
How to get a good guess is another question. One often successful way is to do a scan (constraint optimizations) along the coordinate (every 10 or 15°) and take the highest energy structure as a starting point for opt=ts. ORCA does this automatically with !ScanTS, I don't know if Gaussian has a similar option.
Yo can try a Scan calculation along the proper redundant coordinate (in this case, the dihedral angle that you are considering for your study). This method renders a potencial energy surface (PES) along which, you can search for intermediaries and posible transition states.
Thanks for the info. I have another question: in a different reaction mechanism reactant has +3 charge and product +1. Reaction step involves iridium complex dissocites into hydrogen gas and another intermediate. How do I model this using opt=ts. What charge and multiplicity do I take. A detailed answer will be most helpful.
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