What is the relation between excitation energy and maximum absorption wavelength in the UV-vis spectrum?
for example, the attached picture is the UV-vis spectrum of CO2, visualized by Gaussview. The excitation energy is 7.37eV, but there is no absorption peak. I am confused about the concept of absorption.
What is the advantage of calculating the higher number of excited states?
Any help is really appreciated.
If that is the result you received, something probably went completely wrong with the calculation. If this was real, it would mean that the substance would not aBsorb (not aDsorb) soft UV light at all.
Hello Maryam,
I believe you mean absorption, not adsorption. The difference is fundamental. If you mean absorption or extinction, then look for spectra. I found, that for 7,3 eV the extinction is almost zero. For a maximum you need energies between 8eV and 10eV
Yours
R. Mitdank
The maximum absorption wavelength is the wavelength at which your sample absorbs the majority of electromagnetic radiation. At this wavelength, the transition is most likely to occur, and using the maximum absorption wavelength, you can calculate the excitation energy with the photon energy formula E = hc/λ, where c is the speed of light, h is Planck's constant, and λ is your maximum absorption wavelength. However, to use UV-VIS spectroscopy, you need a molecule with a dipole moment. For CO2, the dipole moment is 0, resulting in a transition probability of 0 for the excited state. Consequently, I believe that your picture doesn't show any transition, and the oscillator strength (a value related to the transition probability) is 0 for this reason. You may want to try a different spectrum, perhaps with ethanol with ethanol molecule you have see some transition in the UV-VIS
Ismael Vargas Rodriguez
1) This is not an experimental thread, these are theoretical calculations. Therefore "variation of the solvent" is not an approach to solving the problems.
2) Having a permanent dipole moment is the selection rule for microwave absorption, not for UV-Vis. Here you need a transition dipole moment which is proportional to the term "oscillator strength" which you find in the graph. Additionally, the energies shown here are rather high and at some point everything will absorb, the electrons will just not stay in the molecule but it will get ionized.
Thank you all for your precious comments.
As Jürgen mentioned correctly, I had made a mistake in the calculation. increasing the number of calculated excited states solved the problem.
Best regards.
In this video, we'll guide you through the process of performing UV-VIS spectra using Gaussian 09/16 with Model calculations, ensuring it's both informative and practical.
https://youtu.be/fsd55cP3ObA?si=yUH7KThg2pMY0yi_
- Badges
- Science topic
- Similar topics
- Methodology
- Research Methodology
- Research Papers