Protonation of fluorophenols and fluoroanisoles in the gas phase: experiment and theory

The proton affinities (PA) of the 2-, 3- and 4-fluorophenols and related anisoles have been determined with the use of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Based on proton equilibria with suitable reference bases the following proton affinities have been obtained: 788 kJ mol(-1) (2-fluorophenol), 802 kJ mol(-1) (3-fluorophenol), 775 kJ mol(-1) (4-fluorophenol), 808 kJ mol 1 (2-fluoroanisole), 825 kJ mol(-1) (3-fluoroanisole) and 795 kJ mol(-1) (4-fluoroanisole). The experimental proton affinities have been evaluated on the basis of ab initio calculations performed with the G3, G3(MP2) and MP2(fc)/6-11G(2d,p)//HF/6-31G(d,p) procedures. The main aspects of the calculations are: (i) the G3( MP2) proton affinity for the most basic site in each molecule is in good agreement with the experimental value, (ii) the values calculated at the MP2 level are systematically lower than the experimental values and (iii) the relative order of the proton affinities is essentially the same at the different levels of theory. The calculations indicate that the 4- position of the aromatic ring is the most basic site in 2- and 3-fluorophenol, whereas the 2-positon of 4-fluorophenol is associated with the higher proton affinity. For the fluoroanisoles, only MP2-calculations were performed and exclusively for the ring position expected to be associated with the higher PA value.