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-C6 H4-OH (X = H, F, Cl, Br, and 1) in the C-2 (ortho), C-3 (meta), and C-4 (pa ra) ring carbon positions are determined using DFT and MO methods. Similar to the process in the parent phenol, the C-4-protonation is the most prefer able following a X-substitution at either the C2 or C3 position. Except for X = I, in para-X-phenols, a C-2-protonation provides the most stable proto nated 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 k J/moI 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 subst itution positions and experimental values are given in parentheses): 2-F, 7 97 (788); 3-F, 813 (802); 4-17, 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 desc ribing the excess proton migration over the phenol ring is elaborated. A co rrelation between the local PA's and the shifts of the nu (OH) and tau (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 indice s derived from density functional theory, such as the condensed Fukui funct ion (f) and local softness (s). While these indices could predict the prefe rential protonation site among C(H) atoms at various positions, which are t he sites of a similar nature, they are unable to differentiate either a C- or an O-protonation or even the processes at QH) and C(X) atoms. Proton aff inities of anisole (C6H5-O-CH3) and fluoroanisole, 845; 2-F, 820 (exptl, 80 7); 3-F, 835 (826); and 4-F, 809 (796).