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

Abstract

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 À1 (2-fluorophenol), 802 kJ mol À1 (3-fluorophenol), 775 kJ mol À1 (4-fluorophenol), 808 kJ mol À1 (2-fluoroanisole), 825 kJ mol À1 (3fluoroanisole) and 795 kJ mol À1 (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.

The proton affinities of saturated and unsaturated heterocyclic molecules

“…A case in point is the protonation of unsaturated molecules, which often can take place at more than one position. Computational studies can provide a reliable estimate of the proton affinity of the molecule [1][2][3][4][5][6][7] as well as a measure of the thermodynamic difference between the various possible points of attachment of the proton [8][9][10][11][12][13]. An example is the protonation of pyridine and aniline, which in both cases could take place on nitrogen or on any of the carbon atoms.…”

Experimental and computational study on the molecular energetics of the three monofluoroanisole isomers

“…For 3-fluoroanisole and 3,4-difluoroanisole, have been recently published the gas-phase enthalpies of formation obtained by theoretical calculations, as well as the standard entropies, S (T), heat capacities, C p ðTÞ and enthalpies, [H (T) À H (0)] [26]. The proton affinities (PA) of the three monofluoroanisole isomers are known having been determined with the use of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry [27]. Hence, this paper reports the standard (p = 0.1 MPa) molar energies of combustion of the three liquid isomers of monofluoroanisole, measured by rotating bomb combustion calorimetry, as well as their standard molar enthalpies of vaporization, at the T = 298.15 K, measured by Calvet microcalorimetry, and the derived values for the standard molar enthalpies of formation, in the gas phase, at T = 298.15 K. In addition to the experimental work, we have calculated the gas-phase enthalpies of formation for the three monofluoroanisoles by applying the empirical method suggested by Cox [28] and by computational thermochemistry using the density functional theory at the B3LYP/6-311++G(d,p) level of theory.…”

10.1007/s11178-008-1006-y

“…In the recent years, considerable progress was achieved in the calculations of acidity parameters (such as proton affinity ΔH H and basicity ΔG) of molecules of many organic compounds in the gas phase [21,[25][26][27][28][29]. In the recent years, considerable progress was achieved in the calculations of acidity parameters (such as proton affinity ΔH H and basicity ΔG) of molecules of many organic compounds in the gas phase [21,[25][26][27][28][29].…”

“…In the recent years, considerable progress was achieved in the calculations of acidity parameters (such as proton affinity ΔH H and basicity ΔG) of molecules of many organic compounds in the gas phase [21,[25][26][27][28][29]. Therefore, it became possible to draw reliable relations between ΔH H and structural parameters [31][32][33]36] or other properties [27,28,[30][31][32][33][34][35][36][37] and describe competing protonation processes involving multicenter molecules [27,28,34,35] and gas-phase tautomeric protontransfer equilibria [34,37]. In the past 5 years, the error in the calculation of the above acidity parameters for the gas phase was smaller than 1% [25,30,41,44].…”