Dispersion interaction between fluoranthene and benzene in cyclohexane

The electronic absorption spectra of fluoranthene (abbreviated as FA) were measured in various solvents together with the polarization spectrum with the aid of the stretched polymer film technique. The electronic structure of FA is discussed in comparison with MO calculated results. The polarization directions of the electronic transitions of FA were determined. For instance, the 404 (very weak) and 360.3 nm bands are polarized along the shorter and the longer (C-2-) molecular axes, respectively. A weak van der Waals type FA-benzene complex formation was recognized in cyclohexane. The equilibrium constants for this complex formation were determined at various temperatures, e.g. K = 0.147 +/- 0.03 dm(3).mol(-1) at 298 K. The enthalpy, entropy, and free-energy changes are estimated to be DeltaHdegrees = -5.9 +/- 1.2 kJ.mol(-1) (-1.4 +/- 0.3 kcal.mol(-1)), DeltaSdegrees(298) = -36 +/- 1.0 J.K-1 mol(-1) (-8.7 +/- 0.24 cal.K-1 mol(-1)), and DeltaGdegrees(298) = 4.7 +/- 0.4 kJ.mol(-1) (1.1 +/- 0.1 kcal.mol(-1)). This DeltaHdegrees value is considerably small compared with the familiar hydrogen bonding energy, ca. - 12 kJ.mol(-1)(-3 kcal.mol(-1)).