Direct measurement of the stability of the supramolecular synthon C6H6•C6F6

The 1:1 complex between benzene and hexafluorobenzene has been studied in liquid krypton. Molecular dynamics simulations on the solutions predict that the lifetime of the complex varies between 33(20) ps at 126 K and 17(5) ps at 158 K, long enough to allow detection of the complex by infrared spectroscopy. Infrared spectroscopy in the 125-165 K temperature range of solutions in liquid krypton containing hexafluorobenzene and benzene, benzene-d(1), sym-benzene-d(3), or benzene-d(6) reveal the presence of bands due to the complex. From an analysis of the spectra as a function of temperature, the complexation enthalpy in solution was determined to be -6.0(2) kJ mol(-1). The liquid-phase complexation enthalpy was transformed into a gas-phase value of - 12.3 kJ mol(-1) by correcting for the solvent influences using free energy perturbation theory Monte Carlo simulations of the solutions. Ab initio calculations, at the MP2/6-31 G(d) level, predict a complexation energy of - 16.8 kJ mol(-1) and show that the complex has C-s symmetry with the two aromatic rings slightly shifted and tilted away from 6-fold symmetry. A critical comparison of the ab initio frequencies with experimentally observed complex bands confirms literature data on the failure of the out-of-plane force field for benzene at the ab initio level employed.