In order to calculate LUMO and HOMO levels for an organic substance, I need to determine the onset Potential (oxidation and reduction). How to do it with the attached picture?
Thanx in advance.
the onset potentials, Ea onset and Ec onset, where the oxidation and reduction peaks start.
In the picture, were are oxidation and reduction peaks that I'll consider for the onset potential determination?
The onset potential is an ill defined term. See the discussion at
https://www.researchgate.net/post/What_is_the_common_way_to_define_the_onset_potential_from_linear_sweep_voltammogram_LSV_of_PEC_measurement
In addition, your CV shows that an irreversible adsorption is involved. I would not try to interpret your curve.
@ Yurii V Geletii
Thank you for your response.
If this CV shows irreversibility, means I will not be able to determine HOMO and LUMO energy levels?
The electrochemistry is a pretty complicated field. You need to learn much how to collect the data and then how to interpret them. I feel that you are novice in the area.
Dear all,
I agree basically with Yurii that drawing a voluntary line into a cyclic voltammogram is not a series approach to determine HOMO and LUMO energies.
However, the reality is that people do this, including me, with some sucess. Attached you find our recent publication, where in Fig. 2 we defined an offset potential by (voluntarily) drawing a line at a position, where a half-wave potential would lie if the process was reversible and electrolysis would very probably lead to full conversion.
But it is not a precise and I can understand people that can this "airy".
Hope this helps
But specifically to your CV.
This must be measured again, starting at around 0 V and scanning both the cathodic (negative) and anodic (positive) range in separate scans.
You can also measure the cathodic and anodic range in one scan, but still you MUST start at around 0 V - where no material is reduced or oxidised.
You started in the negative range and reduced your analyte (the molecule or material you want to study) before scanning. This makes no sense if your analyte cannot be reduced reversibly. In this case you start your scan with changing = probably destroying your analyte.
The crossing lines at the end of the scan clearly show that you have electrolysed = destroyed the analyte in the beginning.
Best,
AXEL
Thank you Professor @Axel Klein for clear explanation that explain the trouble in this graph.
I'm not in field of electrochemistry, but I need de determine the LUMO Level for the substance.
Can I estimate it with the oxidation part in this graph or do I have to redo the experiment to have a reversible reaction?
Dear Mohammed (If I may),
Cyclic voltammetry cannot provide the energy of the LUMO and also not of the HOMO. The reason is the unclear zero energy. Electrochemistry is a relative method. We use different standards like the Normal Hydrogen Electrode, the Saturated Calomel Electrode, and in organic solvents we use Ferrocene as standard. But all of them are different and not of them represents something like a zero value for an energy.
The only thing you can do reliably is to measure the HOMO-LUMO gap. And also you can compare different potentials = energies for different compounds if you use the same standard and the same experimental conditions.
To your last question. You should repeat the experiment and make separate scans for a) the anodic and the b) cathodic part starting both at 0 V. Then c) you scan from 0 V to the anodic - return to the cathodic and then back to 0 V. Then d) you scan from 0 V to cathodic - return to the anodic part and then back to 0 V.
Comparison of scan a) with scan d) and of scan b) with scan c) will be very interesting.
And then you can draw these off-set potentials and say something about the HOMO-LUMO gap and about the relative position of the first reduction wave and the first oxidation wave relative to the applied standard potential.
You can compare these potentials with literature values for well-known redox couples like 2Cl-/Cl2 for the anodic part and Zn/Zn(II) for the cathodic part.
Hope this helps,
best wishes,
aXEL
Thank you Pr. Axel Klein for your assistance.
What I understood I can estimate the gap with the CV I would need other characterization techniques to locate LUMO and HOMO of my substance. what techniques do you think I need in addition with the CV?
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