I am trying to compare the electrostatic interaction between cation and anion of two salts, let's say AB and XY. So if I optimize the compounds AB and XY, separately, using Gaussian and compare the HOMO-LUMO energy gap for each of them, can I conclude either X (cation) and Y (anion) are having greater electrostatic interactions between them or A (cation) and B (anion) have higher one?
Optimize cation (A+) and anion (B-) separately. Cumulate their potential energy. Then optimize the AB salt and find the potential energy. Now
The cumulative potential energy of ions - salts' potential energy
= attraction energy
Do a similar job for XY
Now find which one is greater.
I am not sure to get the point. The HOMO-LUMO energy gap is not - by itself - a direct parameter pointing to the degree of the electrostatic interaction between A and B in a salt AB. This notwithstanding it is true that the largest the HOMO-LUMO gap, the hardest the compound AB: which in turn can be an indication of an mainly electrostatic interaction between A and B. This means that a hard-hard adduct in terms of the HSAB theory typically shows a large HOMO-LUMO gap. This said, obviously not all hard-hard adducts are necessarily salts.
On a different basis, you may want to analyse the nature of HOMO and LUMO of your salts in terms of contributions (%) of the component species A and B. If no or little covalent interaction holds, the HOMO will be basically made of the most electronegative component (say B), while the LUMO of the most electropositive (say A). The degree of participation of A to the HOMO or B to the LUMO will be an indirect measure of the covalence in A–B. If you repeat this procedure to a second salt XY under the same computational conditions (method, basis set) you could compare the nature of the interaction in A-B with respect to X-Y.
To qualitatively explore these concepts I recommend Chapters 5 (and in particular 5.3.2, "Ionic compounds and molecular orbitals") and 6 of Miessler, Inorganic Chemistry (ISBN-10: 0-321-81105-4).
I hope this helps.
Thank you Pablo Mtz , Sir. I also had the same doubt but as I read a paper where they talked about extent of electrostatic interaction from HOMO-LUMO energy annalysis, I asked the question here.I think better we can say about the charge transfer what Massimiliano Arca Sir mentioned. Large HOMO-LUMO energy gap indicated there is greater extent of charge transfer.
Thank you Massimiliano Arca sir, for your response
- Badges
- Science topic
- Similar topics
- Chemistry
- Sulfites
- Inorganic Chemistry
- Inorganic Chemicals
- Salts