Hi everyone,
I synthesized a molecule which is the vanillinacetone via the aldol condensation of vanilla and acetone in excess.
I saw that the molecule has double bonds in it, and I would like to predict the color it might has.
I tried to use different formula like :
lamba = h*c/E
and E = bond dissociation energy/Avogadro number
I take the BDE from the Avogadro Software.
After the calculations, I'm not convinced of the result I found.
Can someone give me solutions?
Thank you
To predict a wavelength, the best thing to do is a full DFT calculation and then TD-DFT.
And so, thanks to these calculations, I can predict the color of the compound, that's right ?
Could you give me an example of calculation on an organic molecule, please ?
Thank you
Dear Kevin Untrau
Couldn't you just synthetize the molecule and make an UV spectrum ? Otherwise, it might be possible to predict transitions via quantum chemistry software. I don't know if you have an access to Gaussian 09 or Gaussian 16, but for example, an input file could be:
##########################
%NPROC=16
%MEM=300MB
%Chk=my_molecule
#P aug-cc-pVTZ OPT POP=FULL UCIS(NStates=6) GFINPUT IOP(6/7=3)
my_molecule
0 1
atom1 x_1 y_1 z_1
atom2 x_2 y_2 z_2
atom3 x_3 y_3 z_3
....
#########################
(Be careful, Researchgate doesn't accept the spaces in my input !!!)
This is not probably the best input but I'm sure it works. Note I asked for singlet excitation research, you could also ask for triplet excitation research. I also didn't use TD-DFT, but it is possible for you to choose this method. My basis set could also be changed, everything depending on the accuracy, size of your system you want to calculate and ressources you have. If you cannot have an access to Gaussian; Orca, Gamess, NWChem have the possibility to perform the calculation you need and they are free.
If you have Gaussian, I advise to take a look at the book "Exploring chemistry with electronic structure methods" by James B. Foresman and Aelen Frisch where I bet, you would find examples for finding excited-states of your molecule.
Always if you have an access to Gaussian, you may use this site where you will find useful information when reading your output: https://gaussian.com/uvvisplot/
Hope this could help!
Thank for your answer.
Sure, I can use UV spectrometry, but I want to hypothetise theoretically the absorption and predict the color.
I'm novice in this field, but I like to discover new things and learn by myself.
I'm trying to get the licence for Gaussian, but I can use the free software you send me, if they are good enough to give me the information I want.
Thank you very much for your help.
I agree with both previous answers.
If you want to make previsions do perform theoretical calculations.
To measure the wavelength ( I presume you mean lambda max) simply run an UV-Vis spectrum. If it has lambda max in the visible region, the value gives you the color, or look at the compound itself or in solution in the appropriate (colorless) solvent.
Dear Kevin Untrau
Don't hesitate to contact me if you need some help in your calculation or an access to any program I could have suggested you.
Good luck !
To determine the wavelength of an organic molecule measure the absorption at various wavelength keeping conc. constant say 1x10-4 m in UV-Vis region. Plot a graph of absorption vs Wavelength. The wave length at which there is maximum absorption is the wavelength of organic molecule
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