The proton affinities of furan, 2-, 3-, and 4-methylphenol, and the related anisoles have been determined with Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The proton affinity of furan is redetermined to be 812 kJ mol-1 on the basis of the experimental equilibrium constant for the proton transfer to acetone. The present value is significantly higher than that recommended in the literature (803 kJ mol-1), but in agreement with ab initio G3(MP2) calculations, which suggest a proton affinity of 814 kJ mol-1 for the 2-position in furan. The determination of the equilibrium constant for the reaction between a protonated methylphenol or methylanisole and a suitable reference base results in the following proton affinities:  2-methylphenol, 832 kJ mol-1; 3-methylphenol, 841 kJ mol-1; 4-methylphenol, 814 kJ mol-1; 2-methylanisole, 850 kJ mol-1; 3-methylanisole, 860 kJ mol-1; and 4-methylanisole, 841 kJ mol-1. Calculations at the G3(MP2) level indicate that the 4-position is the most basic site in the 2- and 3-methyl-substituted phenols, whereas almost the same proton affinity is obtained for the 2- and 4-position in 4-methylphenol. The G3(MP2) proton affinity for the most basic site in a given methylphenol is in agreement with the present experimental values.

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