An assessment of theoretical protocols for calculation of the pK(a) values of the prototype imidazolium cation

The highly accurate complete basis set method CBS-QB3 has been used in conjunction with the conductor-like polarized continuum (CPCM) method to predict the aqueous pK(a) values for the three different hydrogen atoms in the imidazolium cation. Excellent agreement was obtained with the available experimental values. The pK(a) for the deprotonation of imidazole was also calculated and found to be quite different from the experimental estimate. The protocol for the pK(a) calculation was carefully analyzed and some recommendations made about the choice of levels of theory.