Intermolecular interactions between tropolone and fluoromethanes

Van der Waals complexes of tropolone (TRN) with CF4, CFH3, CF2H2, and CF3H have been synthesized by expanding mixtures of TRN and the fluorinated methane (FM) in a supersonic free-jet and have been examined using laser induced fluorescence excitation spectroscopy. The sign and magnitude of the microscopic solvent shifts and the magnitude of the tunneling doublet splittings of the origin bands of each distinct complex have been determined from the LIFE spectra. These data, together with both empirical and ab initio calculations of the potential energy surfaces of the 1:1 complexes, have been used to assign the structures of the complexes and determine their approximate binding energies. Expansion of TRN with CF4 produces one identifiable 1:1 complex in which the solvent is primarily dispersively bound and lies above the TRN ring in a symmetric three-legged stool conformation. Expansion of TRN with CFH3 produces two 1:1 complexes, both primarily dispersively bound, in which the solvent molecule lies above the seven-membered ring of TRN in a three-legged stool conformation but which differ in the conformational orientation of the CFH3 species on the TRN surface. Expansion of TRN with CF2H2 produces one 1:1 complex in which the solvent molecule lies above the plane of the TRN ring, but is considerably displaced from its centre of mass and in which binding is primarily electrostatic rather than dispersive. All three partially fluorinated methane molecules produce 1:1 complexes with TRN in which the solvent is bound in the TRN plane by intermolecular hygrogen-bonding. Such structures partially disrupt the intramolecular hydrogen bond of the chromophore and consequently exhibit LIFE spectra characterized by intense, strongly blue-shifted origin bands in which the proton tunneling doublets are unresolvable because of a large decrease in the intramolecular proton tunneling rate. The existence of good correlations between the solute-solvent binding energy and the microscopic solvent shift and between the binding energy and the proton affinities of the solvent for the entire group of hydrogen-bonding solvents, including the partially fluorinated methanes, suggests that C-F ... H-O and F-C-H ... O = C interactions result in weak hydrogen bonds which are not qualitatively different from those of more traditional hydrogen-bonding species.