Hello,
I am doing a computational study with Gaussian software on certain compounds with antioxidant activity. I got the results of HOMO and LUMO energy. According to my results, the order of HOMO energy of my samples is T1>T2>T3>T4>T5
Does that mean the T1 sample has higher antioxidant activity than T5?
Moreover, In the articles that I have read, all mentioned that a compound with a low energy gap between HOMO and LUMO shows high antioxidant activity. But when I calculated the energy gap between HOMO and LUMO, the above order was not followed. The order for the energy gap is: T1
You have to tell me first, is that a transition metal? What spin state and method you used? Or is it singlet ground state? Singlet spin state are easier to understand but triplet states require different methods, Restricted or Unrestricted Open shell. The energy of the LUMO (empty orbital) is really a wild guess, depends a lot on the Basis Set you choose, a large one is better (ie cc-pTZV or larger). Check the orbital occupation orbital, last orbital with 2 electrons (homo) and an empty orbital just higher energy (lumo) above the last one. Substract the energies of these 2 orbitals and you get the Gap. Try to use Gauss-View.
Dont' expect any decent Energy of the LUMO, nor any property that depends on the LUMO energy (softness). An aproximate answer shall always deppend on a good quality basis-set (augmented with polarization) an in large molecules you may use CASPT to get just the Active Space, the Valence Orbitals plus the LUMO neighbors.
Dear victor,
Thank you very much for your explanation.
I used the DFT, B3LYP method with the basis set of 6.31G. And it is a singlet state.
No, none of them is transition metal. They are antioxidant molecules like Trolox and ascorbic acid.
B3LYP works very well for small molecules like Ascorbic Acid, to start 6-31G is OK but it is kind of small basis set. Have you OPTIMIZED the GEOMETRY before B3LYP/6-31G?
For SMALL ORGANIC molecules (singlets) B3LYP is enough. However if you are taking about O2 or TEMPO you have to know that doublet and triplet molecules (O2), require open-shell methods. Antioxidant molecules should have a SINGLET Ground state, so it should be OK. Once oxidized it is a different story (USE uB3LYP).
On the other hand, for very large molecules, say graphene, the HOMO-LUMO gap will be too small. Graphene (or coronene for example) are composed of too many degenerate orbitals, making it very questionable Eigen-states and wave functions.
I'm saying the B3LYP solution may be inaccurate for huge molecules (with minimum basis set), you could try using STO-4G (or STO-6G), if your computer doesn't allows for a very large molecule. In case of emergency (say you blame B3LYP) there are other dft methods, ie Perdew93A (doi 10.1103/PhysRevB.48.4978.2 ). The OPTIMIZED structure of Ascorbic acid is available in pubchem.ncbi.nlm.nih.gov/ , go to 1.2 3D Conformer coordinates, download'em as sdf. This conformer are optimized geometries. Here is also Trolox from ChemPub, in sdf coordinates (see CID 40624 trolox and 54670067 Ascorbic ac). Attached are pubchem coordinates in sdf format (text files, plain characters, use simplest editor software).
Another reason that T1, T2, T3, T4 order doeas not correlate with HOMO-LUMO gap, is that REDOX reactions are Acceptor-Donor couples, that is "intermolecular" electron transfer. The gap may not give a good clue, because you are calculating isolated molecules, not the redox-couple. The HOMO-LUMO gap IS going to change as Trolox approach its target oxidizer (to form the activated complex).
Some molecules such as Butylated-hydroxy-toluene (BHT), are antioxidant additives for oils. BUT the active group (phenol group) is sterically hindered, so it is quite different to 6-Hydroxy-chroman-carboxylic acid (TROLOX C). Don't go compare Ascorbic acid with protected phenol fragments (as in BHT). Trolox is rather a hydrophobic substance, while ascorbic ac is water soluble, consider that.
I recommend you to read:
TITLE "A DFT method for the study of the antioxidant action mechanism of resveratrol derivatives"
J Mol Model (2013) 19:2285–2298 DOI 10.1007/s00894-013-1770-7
Keywords AIP . Antioxidant activity . DFT method . HOMO and LUMO . Spin density
Mont Royale in French, nice place
Dear Victor,
Thanks a lot for the comments. They are really useful.
I will read the paper that you have recommended.
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