please someone explain to me the importance of dipole moment in gaussian. I would like to know this as I want to explain my output of fractionation process using column chromatography.
Dear Mohd Amir Asyraf Mohd Hamzah ,
As far as I know, Gaussian gives you the total dipole moment in Debye at the end of the calculation. I don't know if this value could be useful in the case of column chromatography as more parameters have to be into account: the calculation is made in gas phase (if no solvent is asked in the calculation), even if the dipole moment depends on the nature of the molecule... It can be changed with solvent effects. Also, generally, the column is performed with a mixture of solvents, and it can be tricky to ask a mixture of solvents in calculation: some posts assume to give a dielectrical constant to Gaussian as a mean of the ensemble of used solvents. Also, this dielectrical constant is essential for a molecule, it is another parameter to take into account to understand the fractionation process. At last, but not least, appart dipole moment, solvent effects, dielectrical constant etc ... Interactions between silica, alumina etc... And your molecules are crucial and apparent very polar molecules... Are not so during your purification process! I don't know how it could be possible, via an easy method to quantify this parameter.
That's true Corentin Lefebvre
However, I would like to explain to certain extent why certain compounds e.g. phenols and derivatives was eluted when ratio of 9:1 (n-hexane : ethyl acetate) was used. But when ratio was increased to 8:2 ratio, catechol and derivatives were eluted as proven by using GC-MS. I found that there was positive correlation between Topological Polar Surface Area (referred to PubChem) and polarity of the solvent system. I somewhat interested in explaining the polarity of the compounds of interest using dipole moment of Gaussian as polarity of compound can be measured based on its dipole moment.
Dear Mohd Amir Asyraf Mohd Hamzah ,
yes, the dipole moment represents the polarity of the molecule, but this depends on the charge distribution formalism. The standard dipole moment in Gaussian is based on Mullinen charges. You could alternatively calculate this at NBO level (keyword 'dipole' in the nob section). Of course dipole moments strongly depends on the level of theory adopted in the calculation.
This said, in all cases you are getting the polarity of your model compound in the gas phase. Even introducing a solvation model, this is by no means representative of column chromatography. In fact, the capability of the column to separate the compounds of a mixture depends on their different affinity with the eluent and with the stationary phase. It is this balance that determines the effectivity of a chromatography. In other words, the order of solubility is not necessarily the order of elution of a specific column.
Therefore, in my opinion even if the order of (Mulliken) polarity may be in agreement with the separation capability of a column, this result should not be overemphasised.
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