O. Tishchenko et al., Protonation of gaseous halogenated phenols and anisoles and its interpretation using DFT-based local reactivity indices, J PHYS CH A, 105(38), 2001, pp. 8709-8717
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).