The best theory-experiment agreement for Δ ‡ H is herein found for the B3LYP/6-311++G(2df, 2p)/SDD level, with Δ ‡ H= +254 kJ/mol at T= 298 K, thus overestimating the experimental value given by 235 kJ/mol in 8 %. Similar results were previously reported by Cardelino et al. at the level of theory given by B3LYP/6-311G(d,p)/3-21G(d,p) augmented with AKR4 extended basis set. For matters of comparison, the PBE functional, in combination with a plane-wave basis set of cutoff energy equals to 800 eV, has a calculated Δ ‡ H= 231 kJ/mol at 300 K. Hereafter, all calculations are assumed to have been performed within the B3LYP/6-311++G(2df, 2p)/SDD level of theory.