I ran a TDDFT calculation of a copper(II) complex to get the UV-Vis spectrum. The maximum wavelength of each band is basically the same for both spectra (calculated and experimental), but some of the bands don't have similar absorbance. Does anyone know how I can fix that?
You can play with the full width and half maximum (FWHM) of the calculated transitions so that it matches closely to the experimental one.
Thanks for the suggestion, Tuhin. I've already tried changing the width and half maximum but some absorbances are still not in good agreement with the experimental data. I think I'll try running with another DFT method (I used wB97XD so now I might try CAM-B3LYP).
You might not be aware of the fact (like most collegues with a chemical background) that calculating absorbance from quantum chemical calculations is a kind of lunacy, since you assume that electrons have wave properties whereas using absorbance means light is denied the same. Anyway, from which quantities and how did you calculate the absorbance and how did you "measure" absorbance (note that there is not a single method that really measures absorbance, therefore the quotation marks)?
dr Mayerhoefer
As a chemist I like to ask what the absorbance has to do with the nature of light?
In QM the transition moment is the measure for absorbance and can be calculated for instance for small molecules and polycyclic aromatic in at least quite a good a qualitiative way
We did this back in 1970 for triplet-triplet transitions in PAHs
Unfortunately absorbance has nothing to do with the nature of light! Quite often it is derived in connection with the derivation of Fermi's Golden Rule and I guess this is what you are referring to, right? There it is assumed that the electric field intensity (E2) can be completely derived from the intensity of light at the source (I0) and the absorption coefficient of something between light source and detector. Any interference effects are then killed by setting E2=I. There two scenarios where this works, namely when the source, the medium between the sample and the source and between sample and detector as well as the detector itself share the same optical constants or when you have a very diluted gas. The first scenario is certainly nonsense, even if it were possible, since then the absorption has to be very weak so that some light is able to reach the detector. By "accident" there is a third scenario which, however, does not work as described in the textbooks like e.g. the Atkins. This scenario is realized in spectrophotometry where you ratio the transmittance of a diluted solution to the transmittance to that of the pure solvent. Taking this ratio is absolutely essential, because this renders the so-calculated absorbance compatible to Maxwell's equations: Article Employing Theories Far beyond Their Limits-The Case of the (...
It is also essential that the cuvette is thick. Were its thickness like those in infrared spectroscopy, the same measurement scenario would make absorbance useless, because then interference takes place and E2 and by that absorption becomes strongly dependent on location and wavenumber. So this is a typical scenario were the wave nature of light takes over. Over the last year we checked nearly every measurement scenario in infrared spectroscopy and the only one which works more or less ok is the pressed pellet method if you record also a pellet without sample and ratio the transmittances as in spectrophotometry. Using absorbance in other scenarios, results in errors concerning intensities between 10 and 1000 %, where the errors strongly differ with sample thickness, oscillator strengths, wavenumber etc., that means even within one and the same spectrum. This is why I asked how absorbance was calculated and how it was measured...
P.S.: Certainly, quantum mechanical calculations can work very good, but when I see how from the calculated quantities spectra are calculated I have to say it is a wonder if calculation and measurement more or less coincide, in particular for polycrystalline materials...
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