Dear Olive,

The cis/trans isomerization demands rotation around an unsaturated bond, e.g. >C=C< which is strongly prevented. Consequently, such isomerization becomes a stepwise process in which the first step is associated with the breaking the pi-bond by a way leading to conversion C=C double bond to the C-C single bond. Such conversion can take place thermally and/or photochemically. A very simple example is given in the attached portrait. I hope it is found helpful.

Regards,

Saeed

Olive-

It depends on what it is you are wanting to understand and how much computational power that you have to spend.  Molecular mechanics with a reliable forcefield will likely get most of the bulk thermodynamic properties and large structural changes in the molecule, but if you want detail information about what is occurring with with the electronic structure then TD-DFT is probably a reasonable starting point.  It also might not hurt to review the literature and see what has already been done on similar projects, if anything, and go from there.  Best of luck. 

Jamin

Thank Mr. Emamiam!!

Your information was really helpful

Mr. Perry,

I have trying to understand through reviewing the literature. I am slowly getting to know the details regarding it.

Systematic study of cis-trans isomerization mechanism can be done with looking at evaluation of torsion angles along the intrinsic reaction coordinates. Particularly for study the Hula-twist mechanism:

First you need to identify the "interesting step" of the isomerization, i.e. the actual cis-trans even may not be the first step of the isomerization processes.

Then you need to locate the transition state of the "interesting step" and perform an IRC calculation, starting from the transition state in the both directions.

Finally, you should analyze the IRC-path, in order to identify the mechanism.

In the pure-rotation mechanism the only torsion angle that changed along the IRC path is the torsion angle of the double bond

(particularly the cis-trans isomerization happens via a pi2pi* excitation, so the double bond is not a double-bond anymore. You should control it with NTO analysis).

In the Hula-twist mechanism, in addition to the former torsion angle, the torsion angle of the neighbor bond is changed also substantially to the opposite direction. So if there are 3 torsion angles (similar to Azobenzene) , there are two Hula-twist paths. If all three torsion angles changing spontaneously, the mechanism is called "concerted Hula-twist".

In order to see more details, please see the L. Grimmelsmant et.al. J. Phys. Chem. Lett. 2017, 8(9), 1986-1992 .

Thank you so much sir!! It was very helpful!!