Structures, thermochemistry, and infrared spectra of chloride ion-fluorinated acetone complexes and neutral fluorinated acetones in the gas phase: experiment and theory

The thermochemistry of chloride ion clustering onto acetone and four fluorinated acetones (CH3C(O)CH2F, CF3C(O)CH3, CF3C(O)CF2H, and CF3C(O)CF3) under thermal equilibrium conditions has been determined by pulsed-ionization high pressure mass spectrometry (PHPMS). The standard enthalpy (DeltaHdegrees) and entropy (DeltaSdegrees) changes obtained indicate a variety of different types of bonding in these complexes. Ab initio computational methods have been used to obtain more insight into the structures and energetics. Surprisingly, in the Cl-(CF3C(O)CF2H) complex the chloride ion is not linearly hydrogen bonded, despite the presence of a very acidic CF2-H bond. Instead, coordination with the carbonyl group carbon atom seems to be more pronounced, as found for the Cl-(CF3C(O)CF3) complex. For the Cl-(CF3C(O)CF3) clustering equilibrium a DeltaSdegrees value of -37.6 cal mol(-1) K-1 has been measured, indicating that one or both CF3 group rotations will be hindered upon complex formation. Excellent agreement between DeltaH(298)degrees values calculated at the MP2/[6-311 ++G(3df,3pd)/6-311+G(2df,p)]//MP2/[6-31+G(d)/6-31G(d)] level of theory and experimental DeltaHdegrees values has been obtained. In addition, Delta(acid)H(298)degrees values for a set of small to medium sized organic and inorganic acids, and the fluorinated acetones have been determined at the G3 and G(3)(MP2) levels of theory. Good to excellent agreement was obtained compared to experimental data, indicating that these composite methods perform well for the determination of reliable Delta(acid) H(298)degrees results for medium sized molecules containing up to eight second row atoms. No linear correlation between clustering DeltaH(298)degrees and Delta(acid)H(298)degrees values was found, indicating that most likely ion-dipole and ion-induced dipole interactions are determining the observed thermochemical trends. Finally, gas phase FT-IR spectra of CH3C(O)CH2F, CF3C(O)CH3, CF3C(O)CF2H, and CF3C(O)CF3 have been obtained, and the normal mode vibrational frequencies compared to results from calculations at the HF/6-31G(d) level of theory, scaled by 0.8953, and the B3LYP/6-311++G(3d,3p) level of theory. For the Cl-(CF3C(O)CF3) complex some large shifts in frequencies and IR absorption intensities are observed relative to CF3C(O)CF3, especially for the CO stretch.