I want to calculate the absorption spectrum of 3 dyes malachite green, brilliant green and ethyl green through a GAUSSIAN 09 calculation but I cannot find results close to the experimental values and the shift in the value of λmax is important λmax EXP is 618 nm but calculations the values are 513 nm I am looking for where the exact default and why this shift and how I realize but calculations thank you
Only in single atoms are there pure electronic transitions. Atoms do not rotate and vibrate and therefore, the spectral bandwidth is narrow in atomic absorption spectroscopy. In contrast to atoms molecules rotate, nuclei vibrate and transitions between electronic states are connected with simultaneous transitions between rotational and vibrational states. Therefore, absorption bands in molecular spectroscopy are broader than those encountered in atomic spectroscopy. The shapes of the former are mainly determined by the spacing and intensity distribution of the vibrational sub-bands. I think you calculate the so-called adiabatic transition energy, which has not theoretical background.
In addition, your results show that GAUSSIAN 09 is not suitable to calculate electronic spectra of organic molecules. Please read e. g. H. Mustroph, S. Ernst, B. Senns, A. D. Towns. Molecular electronic spectroscopy: from often neglected fundamental principles to limitations of state-of-the-art computational methods. Color. Technol. 131 (2015) 9.
Dear Djamel,
Assuming that you used TD-DFT for calculating the absorption spectra, the first point to consider is that different exchange-correlation functionals can yield very different results. Which criterion did you apply to select the functional you used? For this kind of molecules, the first functionals to try that come to my mind are M06-2X, CAM-B3LYP and LC-wPBE, they usually give good results for similar organic molecules.
The next question is about the basis set. How large is the one you used? Even if these systems are cations, the use of diffuse functions is often critical for a proper description of more delocalized excited states (such as charge transfer states, which may appear in these dyes). I would suggest using the Pople's 6-31+G(d,p) or Dunning's aug-cc-pVDZ basis sets at least (if you can afford it, you can also try a triple-zeta basis set for higher accuracy, although this double-zeta basis sets should be enough).
And finally, about the environment. I assume those experiments you referred are done in solution. Did you include solvent effects in the calculation? Solvatochromic effects can be crucial to replicate the spectrum. They can be dealt easily with continuum solvation models, for Gaussian09 you have at your disposition the PCM and SMD models, try both (once more, they can produce different results). And if you are not interested in the whole spectrum but in a particular transition, you may consider the option of state-specific solvation.
Other more computationally demanding options such as including the counterion or explicit solvent molecules should be considered only if everything else fails, but I can anticipate that they may have a minimal impact in the results.
Hope you find it helpful.
Dear Djamel,
Couple of years we have tried different functionals in describing the electronic spectra of organic dyes. As far as I can remember B3LYP performs well. M06-2X has good statistics, but the predicted positions of the bands are systematically blue shifted. You can read the paper and to decide what you want to do.
- Badges
- Science topic
- Similar topics
- Chemistry
- Biochemistry
- Biochemical Techniques
- Blotting Techniques
- Immunoblotting
- Dyes