Dear all,
I have molecules with ethanol and ethyl chloroformate. I will simulate the reaction of two molecules that the oxygen atom of ethanol is approach to the carbon atom of ethyl chloroformate. (please see the picture that I upload) Also I use DFT 6-31G*.
So I set the distance of two atoms from 3 A to 1 A decreasing 0.1 A interval, and try the single point energy calculation of each case, but the value of energy is just getting bigger, that means I don't have idea when is the transition state. Also, the energy value is same as bonded two atoms and non-bonded two atoms.
In summary, my question is, how can I find transition state of this reaction with DFT???
Does anyone have good idea what can I DO for it?? :( I'm really confused about this problem..
Dear Lee Jieun,
Maybe it is a good idea to perform a relaxed surface scan, which corresponds to a constrained geometry optimization in which all degrees of freedom are optimized, except the one you are sampling. These geometry optimizations along the coordinate are important since both molecules can assume different conformations according to the intermolecular distance.
After finding the distance that corresponds to the energy maximum, you can perform a transition state optimization for the corresponding structure.
Please check this very interesting page, which explains how to perform these calculations in Orca:
https://sites.google.com/site/orcainputlibrary/geometry-optimizations/transition-state-optimizations
Best,
Felippe
Have you just been calculating points on the PES or have you been using one of these methods:
https://www.tau.ac.il/~ephraim/glab-4.pdf
The system you are calculating looks not that complicated, so opt=ts might be sufficient. If it doesn't work, try the QST3 method in which you provide the initial and final state and a guess for the transition state. For the initial/final products you can caluclate the corresponding van der Waals complexes by simply using the normal opt=EF version. Eventually, you may have to use Hessian updates, see here for parameters:
http://gaussian.com/opt/
Edit: I just saw you wrote "the energy value is same as bonded two atoms and non-bonded two atoms". Here's the thing: DFT doesn't "know" the concept of bonds, it only reads in the coordinates of the nuclei. The bonds are only relevant if you perform a preoptimization with a force field method.
Dear Lee Jieun,
Apart from the technical side of this problem, there is an issue with the chemistry itself. You are trying to get a transition state to form an adduct between the ethanol and the ethyl chloroformate molecules, which is quite unlikely to happen. You can check this by running a simple optimization calculation for the corresponding product (most probably, these two molecules fly away from each other).
Why is this happening? You are trying to make a trigonal planar C(in sp2 hybridization) to a tetrahedral C (in sp3 hybridization), with a formal negative charge on the carbonyl oxygen and a formal positive charge on the alcohol oxygen. Immediately, the system tries to minimize its energy by eliminating one group attached to tetrahedral carbon. In your case, ethyl alcohol is leaving from the tetrahedral carbon to make two stable neutral molecules (ethyl alcohol and ethyl chloroformate).
The reaction you are trying (not very clear to me, I am guessing) is the solvolysis of ethyl chloroformate, in which CH3CH2O- group substitute Cl- along with the elimination of one molecule of HCl. So, in reality, the reaction between these two molecules is proceeding through a four-membered transition state in which C-Cl and O-H bonds break and O-C and H-Cl bonds form.
I hope the explanation helps!
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