Dear Oscar, according to the Hugh´s answer and a short search on RG , please read this link published on RG 3 years ago and the various answers belonging to the Johan´s question. On RG , you will find the answer(s) to your question. Please start RG search window in the future prior posting a question. Thanks.
Dear Oscar, according to the Hugh´s answer and a short search on RG , please read this link published on RG 3 years ago and the various answers belonging to the Johan´s question. On RG , you will find the answer(s) to your question. Please start RG search window in the future prior posting a question. Thanks.
Unless you have a very well-resolved UV-vis spectrum, you usually observe broad absorption bands corresponding to excitations of the type 00->1X type (numeric labels mean electronic state-vibrational state). That is, from the ground vibrational-electronic state to an excited state (in this case the first one) in any vibrational substate X. The higher the sublevel X, the higher the transition energy and thus the shorter the wavelength.
Thus, the 'danger' in considering lambda max as the HOMO-LUMO gap is that these frontier orbitals usually refer to the 00->10 transition, which may not be the most intense one in your absorption spectrum. Furthermore, similar molecules may have different 00->1X dominant transitions.
What to do? If you have well-resolved UV-Vis with clear vibronic structure, simply take the longest-wavelength sub-band from your absorption of interest. But if you don't (I'd say >95% of the typical UV-Vis spectra of organic dyes in solution), you can approach this by the onset of absorption from the red side of your band. This can be very uncertain, of course. But if you want to compare a series of similar molecules, you may have similar uncertainties.
If you want to compare your HOMO-LUMO with computed values using DFT, the treatment of electron correlation can be very poor. What to do? Run a TDDFT calculation instead to include some more correlation. Compare frontier orbitals shapes and energies vs those obtained from static calculation. Anyway, since vibronic coupling is neglected, there will be discrepancies with experimental values most of the time.
Unless you have a very well-resolved UV-vis spectrum, you usually observe broad absorption bands corresponding to excitations of the type 00->1X type (numeric labels mean electronic state-vibrational state). That is, from the ground vibrational-electronic state to an excited state (in this case the first one) in any vibrational substate X. The higher the sublevel X, the higher the transition energy and thus the shorter the wavelength.
Thus, the 'danger' in considering lambda max as the HOMO-LUMO gap is that these frontier orbitals usually refer to the 00->10 transition, which may not be the most intense one in your absorption spectrum. Furthermore, similar molecules may have different 00->1X dominant transitions.
What to do? If you have well-resolved UV-Vis with clear vibronic structure, simply take the longest-wavelength sub-band from your absorption of interest. But if you don't (I'd say >95% of the typical UV-Vis spectra of organic dyes in solution), you can approach this by the onset of absorption from the red side of your band. This can be very uncertain, of course. But if you want to compare a series of similar molecules, you may have similar uncertainties.
If you want to compare your HOMO-LUMO with computed values using DFT, the treatment of electron correlation can be very poor. What to do? Run a TDDFT calculation instead to include some more correlation. Compare frontier orbitals shapes and energies vs those obtained from static calculation. Anyway, since vibronic coupling is neglected, there will be discrepancies with experimental values most of the time.
here a link about LUMO/HOMO.
https://www.sciencedirect.com/topics/chemistry/homo-lumo-gap
here a link about LUMO/HOMO.
https://www.sciencedirect.com/topics/chemistry/homo-lumo-gap
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Dear Oscar,
according to the Hugh´s answer and a short search on RG , please read
this link published on RG 3 years ago and the various answers belonging to the Johan´s question.
On RG , you will find the answer(s) to your question.
Please start RG search window in the future prior posting a question. Thanks.
https://www.researchgate.net/post/In_a_UV-Vis_theoretical_calculation_what_does_the_HOMO-LUMO_gap_mean
JRG
Dear Oscar,
according to the Hugh´s answer and a short search on RG , please read
this link published on RG 3 years ago and the various answers belonging to the Johan´s question.
On RG , you will find the answer(s) to your question.
Please start RG search window in the future prior posting a question. Thanks.
https://www.researchgate.net/post/In_a_UV-Vis_theoretical_calculation_what_does_the_HOMO-LUMO_gap_mean
JRG
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here a link about LUMO/HOMO.
https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry_Textbook_Maps/Map%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)/Chapter_04%3A_Structure_Determination_I/4.4%3A_Ultraviolet_and_visible_spectroscopy
here a link about LUMO/HOMO.
https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry_Textbook_Maps/Map%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)/Chapter_04%3A_Structure_Determination_I/4.4%3A_Ultraviolet_and_visible_spectroscopy
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Check out the Tauc Representation of absorption.
https://en.wikipedia.org/wiki/Tauc_plot
Check out the Tauc Representation of absorption.
https://en.wikipedia.org/wiki/Tauc_plot
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Unexpected solvent effects on the UV/Vis absorption spectra of
royalsocietypublishing.org › doi ›
Unexpected solvent effects on the UV/Vis absorption spectra of
royalsocietypublishing.org › doi ›
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Dear Oscar,
Unless you have a very well-resolved UV-vis spectrum, you usually observe broad absorption bands corresponding to excitations of the type 00->1X type (numeric labels mean electronic state-vibrational state). That is, from the ground vibrational-electronic state to an excited state (in this case the first one) in any vibrational substate X. The higher the sublevel X, the higher the transition energy and thus the shorter the wavelength.
Thus, the 'danger' in considering lambda max as the HOMO-LUMO gap is that these frontier orbitals usually refer to the 00->10 transition, which may not be the most intense one in your absorption spectrum. Furthermore, similar molecules may have different 00->1X dominant transitions.
What to do? If you have well-resolved UV-Vis with clear vibronic structure, simply take the longest-wavelength sub-band from your absorption of interest. But if you don't (I'd say >95% of the typical UV-Vis spectra of organic dyes in solution), you can approach this by the onset of absorption from the red side of your band. This can be very uncertain, of course. But if you want to compare a series of similar molecules, you may have similar uncertainties.
If you want to compare your HOMO-LUMO with computed values using DFT, the treatment of electron correlation can be very poor. What to do? Run a TDDFT calculation instead to include some more correlation. Compare frontier orbitals shapes and energies vs those obtained from static calculation. Anyway, since vibronic coupling is neglected, there will be discrepancies with experimental values most of the time.
Hope this was helpful!
Besitos
Dear Oscar,
Unless you have a very well-resolved UV-vis spectrum, you usually observe broad absorption bands corresponding to excitations of the type 00->1X type (numeric labels mean electronic state-vibrational state). That is, from the ground vibrational-electronic state to an excited state (in this case the first one) in any vibrational substate X. The higher the sublevel X, the higher the transition energy and thus the shorter the wavelength.
Thus, the 'danger' in considering lambda max as the HOMO-LUMO gap is that these frontier orbitals usually refer to the 00->10 transition, which may not be the most intense one in your absorption spectrum. Furthermore, similar molecules may have different 00->1X dominant transitions.
What to do? If you have well-resolved UV-Vis with clear vibronic structure, simply take the longest-wavelength sub-band from your absorption of interest. But if you don't (I'd say >95% of the typical UV-Vis spectra of organic dyes in solution), you can approach this by the onset of absorption from the red side of your band. This can be very uncertain, of course. But if you want to compare a series of similar molecules, you may have similar uncertainties.
If you want to compare your HOMO-LUMO with computed values using DFT, the treatment of electron correlation can be very poor. What to do? Run a TDDFT calculation instead to include some more correlation. Compare frontier orbitals shapes and energies vs those obtained from static calculation. Anyway, since vibronic coupling is neglected, there will be discrepancies with experimental values most of the time.
Hope this was helpful!
Besitos
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VOTE