The 6-311++G(d,p) basis sets and B3LYP functional, as implemented in Q-Chem 6.0, were used for all optimized geometries and calculated energies. 3.0 × 10 -3 Hartree/Bohr and 1 × 10 - 8 Hartree, respectively, were the thresholds for the maximum gradient and the self-consistent field (SCF) energy shift between successive optimization cycles. Using the optimized gasphase geometries as a basis, the conductor-like polarization continuum model (C-PCM) was used to determine single point energies. To incorporate the effect of the surrounding water medium, a relative dielectric constant of 78.39 with respect to the bulk water was chosen. The pKa value was computed under identical experimental circumstances with T = 80 °C and 1 atm.