The gas phase acidity of trifluoromethanesulphonic acid. An ab initio MO and density functional theory study

Trifluoromethanesulphonic acid (CF3SO2OH) and its conjugated anion (CF3SO2O-) have been investigated by applying ab initio MO and density functional theory (DFT) methods. All geometries were fully optimised at the 3-21 G((*)) level of calculation and then, energies were improved by using correlated wavefunctions up to MP2(fc)/D95 + + (d, p). Six stationary points of CF3SO2OH have been studied. Assuming two principle arrangements, eclipsed and staggered, several directions of the proton towards the remainder of the molecule have been regarded: cis (C-s), trans (C-s) and a C-1 geometry between them. The absolute minimum corresponds to the staggered C-1 structure while all other stationary points, with the exception of the eclipsed trans geometry (hilltop) were found to be transition states. For CF3SO2O- two stationary points have been considered: the staggered minimum geometry (C-3v) and the eclipsed (C-3v) transition state structure which is higher in energy by 4.2 kcal mol(-1) {MP2(fc)/6-31 + (d, p)//3-21 G((*)) + [ZPVE (3-21 G((*)))]}. The deprotonation energy at 0 K (Delta E-acid) calculated for CF3SO2OH at the same level of investigation amounts to 296.7 kcal mol(-1) being in agreement with DFT (B88-LYP/DNP) calculations (297.6 kcal mol(-1)). Furthermore, thermodynamic corrections have been taken into consideration and the DFT calculations provide a free energy Delta G(acid) (enthalpy Delta H-acid) Of deprotonation (kcal mol(-1)) of 291.0 (298.2) being clearly smaller than the corresponding experimental values of 298.8 +/- 2.4 (305.9 +/- 2.4). The more sophisticated and reliable MP2(fc)/D95 + + (d, p) method, however, agrees quite well (Delta E-acid = 301.9 kcal mol(-1)) giving evidence that in the gas phase CF3SO2OH has a strength comparable to HClO4 and FSO3H but it was predicted to be weaker than ClSO3H. (C) 1998 Elsevier Science B.V. All rights reserved.