Protonation of gaseous halogenated phenols and anisoles and its interpretation using DFT-based local reactivity indices

The local proton affinities of phenol and its halogenated derivatives, X-C₆H₄-OH (X = H, F, Cl, Br, and I) in the C₂ (ortho), C₃ (meta), and C₄ (para) ring carbon positions are determined using DFT and MO methods. Similar to the process in the parent phenol, the C₄-protonation is the most preferable following a X-substitution at either the C₂ or C₃ position. Except for X = I, in para-X-phenols, a C₂-protonation provides the most stable protonated forms; for para-I-phenol, a C₄-protonation remains more favorable. At the modest B3LYP/6-31+G(d,p) + ZPE level, the proton affinities (PA's) are reasonably reproduced with a quasi systematic overestimation of about 10 kJ/mol with respect to available experimental data. The calculated PA's for X-phenols are as follows (values in kJ/mol, 2, 3, and 4 stand for the substitution positions and experimental values are given in parentheses): 2-F, 797 (788); 3-F, 813 (802); 4-F, 787 (776); 2-Cl, 801; 3-Cl, 815; 4-Cl, 789; 2-Br, 806; 3-Br, 818; 4-Br, 792; 2-I, 813; 3-I, 823; and 4-I, 816; with a probable error of ± 12 kJ/mol. A portion of the potential energy surface describing the excess proton migration over the phenol ring is elaborated. A correlation between the local PA's and the shifts of the ν_OH and τ_OH vibrational modes under protonation suggests that a resonance mechanism is likely responsible for the trend of changes in PA. Attempts to rationalize the regioselectivity of protonation are made using local reactivity indices derived from density functional theory, such as the condensed Fukui function (f) and local softness (s). While these indices could predict the preferential protonation site among C(H) atoms at various positions, which are the sites of a similar nature, they are unable to differentiate either a C- or an O-protonation or even the processes at C(H) and C(X) atoms. Proton affinities of anisole (C₆H₅-O-CH₃) and fluoroanisoles are also evaluated (kJ/mol): anisole, 845; 2-F, 820 (exptl, 807); 3-F, 835 (826); and 4-F, 809 (796).