[tomek]

Hey,

I was wandering if anyone has ever tried to estimate the stability of a molecule by calculating energy difference between HOMO and LUMO. I am interested in meaning of absolute calculated values, not comparing values between different molecules. For example, would a difference of 4 eV indicate the molecule is unstable or should it be closer to 8 eV? I reckon it will depend on the level of theory applied to the calculations but I am curious whether someone has experience in this matter. A reference would be highly appreciated.

[Orcio_87]

No one tried it, as stability of molecule depends on its calculated energy (energy of electrons + energy of nuclei) not just two MOs.

[Corribus]

It depends on what you mean by stability. For instance, HOMO-LUMO gap, or just HOMO or LUMO, can definitely relate to photophysical stability of a material, or susceptibility of a material to oxidation or reduction processes. Your question is too general.

[tomek]

Sorry I wasn't precise enough. What I mean is compound susceptibility to oxidation, reduction, photosensitivity, etc. I would like to estimate a compound potential for degradation in various ways.

[Corribus]

The absolute HOMO and LUMO energies will certainly be related to susceptibility of a substance to oxidation and reduction processes because HOMOs and LUMOs tend to be correlated to redox potentials. E.g. a higher HOMO energy means that a substance is easier to oxidize. If the HOMO energy is high enough, a substance may not be air stable. The HOMO-LUMO energy gap is also related to certain photophysical or even thermal processes that could impact stability because photoexcited states tend to be more reactive than ground state analogs.

That said, these relationships would be complex and may not lend themselves well to a simplistic analysis of "HOMO-LUMO gap versus stability". You'd have to have a metric for stability for one thing. For another, stability will depend on a lot things such as the environment composition, temperature, and so forth. In the case of photo-stability, you also need to have the appropriate light wavelengths to bring about deleterious photochemistry.