Dear reader,
I've been having trouble with my cyclic voltammetry recently, but I then had a break through and managed to get apparently "clean" results using dry, deoxygenated THF and/or DMF, however each solvent (w/ the same electrolyte, conc. etc...) yields significantly different results.
In THF, my polycyclic aromatic compound returns a single reversible reduction at -1.44 V. In DMF, the same material returns two peaks at -1.0 and -1.6 V. Respectively, the peaks at -1.44 and -1.6 V yield close matches to LUMO energies calculated by DFT. The peak at -0.9 V, is 1 eV or so out.
Both solvents (with supporting electrolyte) return featureless blank scans. For one of my samples, the THF trace matches the literature (only available example). All scans are taken under an inert, dry atmosphere. Although I am discussing just one, I am experiencing similar issues with three other tested compounds.
Question: Why do I get an additional peak in DMF? This is essential if I am to estimate HOMO/LUMO energies.
Thanks in advance for any help.
Yes. Well, I gathered that much. I meant more in a physical sense. Obviously, the different results give wildly varying LUMO energies which is significant if I'm trying to understand their physical properties.
Do you have any idea, in the physical sense, how or why DMF yields different results to THF? I can't think of, nor have I have found, a suitable explanation.
I think it is important to stress how electrochemical reduction/oxidation proceeds in general. Firstly, an electron(s) is inserted/abstracted to/from a given compound's structure, giving rise to a charged species. This very often radical species can then irreversibly react with an electrophile/nucleophile present in the solution or be stabilized and then provide a counter peak in CV. Now, the solvent can be involved in this process, too, affecting the course of your CVs.
However, identifying the reaction mechanism (and therefore answering your question) is no trivial task. A rigorous approach to telling why you get the peaks you get would be: (i) performing quantitative electrolysis at ca -1.5 V in THF and at ca -1.1 V and -1.4 V in DMF, (ii) isolating the products, (iii) characterizing them by MS/NMR/IR etc., (iv) resolving their structures and only then (v) trying to resolve the reduction mechanism.
Moreover, I believe HOMO/LUMO energies are always only relevant to the given conditions (or calculated for vacuum), so the discrepancy you're seeing is understandable.
Hope this helps.
Best,
Jan
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