I want to compare the stability of molecules that have the same number of atoms. I optimized these structures and calculated their vibrational frequencies. Gaussian09 gives a Gaussian Calculation Summary that shows the E(RB3LYP) value for each structure. I'm not sure if I can use this value to compare their stability. If the value can be used for that, how do I interpret it? The higher it is, the more stable the structure or the other way around? Thanks in advance for any help.
If you use exactly the same inputs for both molecules, it then makes sense to compare their total energies. In this case, a lower energy means higher stability. Also, as a double check you can compare their HOMO-LUMO gaps.A more stable system usually has a wider HOMO-LUMO gap.
Thank you Mohammad for your reply. They are not the same structure. They differ in the position of a proton. I'm trying to find which possible protonated form is more stable.
The structures do not need to be the same. As long as you have the same number of atoms (including H+ protons), you can compare their energies and related thermodynamic properties, as suggested by Ralf Steudel.
Hallo, Gabriela!
It is obvious to compare the relative stability of various isomers using their energies (see attachment). Nevertheless, the Gibbs free energy (with its temperature dependence) is a more relevant parameter (see textbooks of physical chemistry). . Unfortunately, the approximations used by its evaluation decrease its reliability (ideal gas, low temperature). For example, the equilibria between various TiO2 polymorps (rutile, anatase, brookite) depend on temperature.
Best wishes
mb
This is the right way, to compare stability of two molecules. If the molcules have the same atoms, and same number of atoms Gaussian gives quite accurate results!
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