Nonempirical Prediction of the Relative Electrospray Ionization Efficiencies of Nitroanilines by Combined CBS-QB3 and SCC-DFTB Calculations.

Computational study for three isomeric nitroanilines was carried out to find a nonempirical model to predict their relative ionization efficiencies compared to the corresponding experimental logIE values. The CBS-QB3 method was used for the gas-phase calculations of protonated nitroanilines, and the SCC-DFTB method for the calculations of spherical droplets, containing one protonated nitroaniline molecule, one hydronium cation, and 48 acetonitrile molecules, randomly generated by the PACKMOL program. The obtained results show that neither the gas-phase energy of protonated nitroaniline (or gas-phase basicity) nor the average energy of droplets can be used to predict the ionization efficiency ordering of those isomers. However, the difference of gas-phase and droplet energies gives a good correlation with logIE values and, thus, can be used for the prediction of relative ionization efficiencies.