Computing alpha and beta parameters for ILs

Predicting α by Computing EP_nuc

Calculating the electrostatic potential at the position of an atom is very straightforward. We need only run a single point calculation on our optimised cation with the additional property keyword. (Single point means we don't need to move the atoms as in an optimisation, we are only completing a single set of SCF cycles to obtain the correct electron density)

Open your completed log file from your frequency calculation, choose calculation set-up (all of the options should be set because you are opening an already completed job). Unselect Freq (as this is expensive and you don't want to repeat it) In the Additional Keywords line add the keyword prop, then submit this job. Don't forget to save it to a new name or you will overwrite your frequency analysis file!

Example input and output files for the property calculation:

• C4C1im_prop.com
• C4C1im_prop.log

Once the job has completed, open it in gaussview. We obtain the calculated EP_nuc values by looking at the relevant section of the log file. The log file is the actual text based output, so from the menu at the top of the screen choose "Results" and then "view file" and the text file will be displayed. You need to scroll down until you find the following heading:

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            Electrostatic Properties Using The SCF Density

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All the calculated electrostatic properties are found after this heading. The calculated EP_nuc values for each atom in the system are found in a table called "Electrostatic Proerties" and are under the column heading Electric Potential. An example for the [C₄C₁im]⁺ cation is shown below.

The EP_nuc values of smallest magnitude will correspond to the most H-bond acidic proton in the system (these numbers are negative, so the smallest number corresponds to the "most positive" of the negative values).

In the example above, atom 8 has the EP_nuc of smallest magnitude, with a value of -0.873206. To identify the atomic numbering you need to choose from the menu at the top of the screen "View" and then "labels", and the atom numbers will appear on the molecule.

Unsurprisingly, atom 8 corresponds to the NCHN proton.

Just because the output presents the value to 6 decimal places does not mean this number is meaningful to that accuracy! We consider differences in EP_nuc up to the second decimal place (2dp) meaningful for comparison, and so recording this number to 3dp is normally sufficient.

ΔEP_nuc for this example is therefore is -0.873.

Predicting Alpha α

The computed value of EP_nuc for [C₄C₁im]⁺ is -0.873, putting this value in the equation α=5.153*EP_nuc +5.136, yields a value of α=0.64.