Theoretical calculations of proton affinities in phenol

It is shown that a relatively simple MP2(fc)/6–31G∗∗//HF/6–31G∗ model is capable of providing quantitative description of protonation in phenol. The use of the 6–31G∗∗ basis set in the single-point MP2 calculation is crucial in this respect. The zero-point energy (ZPE) contribution to the proton affinity (PA) is estimated at the HF/6–31G∗ level of approximation. It appears that the contribution of the ZPE energy to relative ΔPA proton affinities is negligible. The simple additivity rule for calculating empirical ZP energies works relatively well for the protonated species too. The energetically most favourable site of the proton attack is para to the OH substitution in accordance with the experimental finding. Performance of the MP2(fc)6–31G∗∗+ZPE(HF/6–31G∗) model in reproducing protonation at the oxygen atom is tested in some medium size alcohols and ethers. The calculated PA values are in good agreement with the measured data.