I recently computed an Absorption and Emission Spectra of AlexaFluor fluorescent Dyes (AF-488 & 594) using TD-DFT/B3LYP/6-31+g(d,p)/DMSO/IEFPCM-Solvent Model. I also tried with CAM-B3LYP but the results are same.
What I've got is that my computed spectra are much broader over a long range of wavelength in comparison to the experimental spectra. Can someone please give a explanation for this? Although I've reached a comparable lambda max, yet my spectra are much broader.
I've attached the Experimental and Theoretical Spectra of AF-488, Yellow line: Theoretical || Blue-Line: Experimental
Thank You :-)
Hi dear Ajay,
The reason may be because the theoretical study includes all the possibilities of transitions while the process is considering only the possible transition realistically and the theoretical study may not deal with sufficient accuracy with some factors such as hydrogen bonding and the effect of solvent as a practical study.
Hi dear Ajay,
The reason may be because the theoretical study includes all the possibilities of transitions while the process is considering only the possible transition realistically and the theoretical study may not deal with sufficient accuracy with some factors such as hydrogen bonding and the effect of solvent as a practical study.
You can try one more thing. TD-DFT calculates transition, not spectra, and latter on an appropriate FWHM is added around that transition (0.4 eV in GaussView). You can play with the width and see and what value your theoretical and experimental matches closely. In Chemissian software you easily vary this FWHM.
http://gaussian.com/uvvisplot/
www.chemissian.com
Thanks Everyone for your replies.. :-)
@Khulood Obaid..thanks for your answer..we were thinking the same at first but couldn't fit the same explanation for a number of systems.
@Tuhin Khan...thanks for the insight and for references..tweaking with the FWHM really helped I didn't knew about this..can you please give some references or information on gaussian ploted UV-Visible spectra? .......thanks again.
@Bojidarka B. Ivanova, I was thinking doing the same for last one month the problem is I don't know how setup vibronic calculations using gaussian, can you please extend your help in setting up the calculations with some simple example?
Regards
Ajay Khanna
Calculated spectra must be broadened to account for the vibrational sub structure and solvent effects. Change the bandwidth until it matches approximately the experimental spectrum. It would help to know what software you are using. Also it is strange that they would have the exact same peak intensity? is your software normalizing them in some way?
@Cody Covington, thanks for your answer, actually I',m using Gaussian09, and it didn't normalized the peak intensity, I did with respect to the maximum epsilon value and even the peak intensity were not same i added the resulting difference to the computed spectra to see how it would look like if superimposed on the experimental spectra..Theoretical values for Lambda Max is: 451nm while for Experimental it is: 495nm...changing the widht indeed forces the computed spectra to max experimental but it also changes the value of absorption/epsilon, how to resolve this issue?
changing the width will change the max epsilon, since the area under the curve is (more or less) the same. Why do you want it to match exactly? TDDFT is off for a number of reasons. Solvent effects and vibronic structure play a big part there.
@Cody Covington, actually I don't want to exactly match the spectra but computed spectra is more spread and I want to know why? is it possible to get some references where it states that solvent effects and vibronic structure have something to do with more spreading of the computed spectra?
From experimental data I know the exact epsilon maximum value so I can change the FWHM accodingly but what to do when you don't have experimental data available to tweak your spectra widths? What I mean is if you go for ab initio methods for entirely different system there is no way you going to know that your resulted spectra should look like something? What to do in those cases?
Thanks again for your time
One other thing you can try to do to evaluate if the problem of your computed spectra is more spread than the experimental one is due to solvent effects is simulate your spectra adding the first solvation shell explicitly. This Will give you more information about it.
The broadening of the peak in experiment accounts for losses and non-radiative processes that are not completely considered in theoretical modeling.
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