Simplification of the CBS-QB3 Method for Predicting Gas-Phase Deprotonation Free Energies

Abstract

[eng] Simplified versions of CBS-QB3 model chemistry were used to calculate the free energies of 36 deprotonation reactions in the gas phase. The best such version, S9, excluded coupled cluster calculation [CCSD(T)], and empirical ( Eemp) and spinorbit ( Eint) correction terms. The mean absolute deviation and root mean square thus obtained (viz. 1.24 and 1.56 kcal/mol, respectively) were very-close to those provided by the original CBS-QB3 method (1.19 and 1.52 kcal/mol, respectively). The highaccuracy of the proposed simplification and its computational expeditiousness make it an excellent choice for energy calculations on gas-phase deprotonation reactions in complex systems.