Raman Study of Intermolecular Interactions in Supercritical Solutions of Naphthalene in CO 2

The high-pressure Raman spectra of the v₁ and 2v₂ Fermi doublet of CO₂ and the C-H stretching, C-H bending and C-C-C breathing modes of naphthalene have been studied at pressures varying up to 2000 bar and temperatures between 60 and 90°C. The naphthalene bands show a blue frequency shift with increasing density, whereas a red shift for the Fermi resonance free stretching mode of CO₂ is observed with increasing density. The blue shift is explained in terms of repulsive interactions probed by the naphthalene vibrations, while the red shift is related to the attractive forces dominating in the intermolecular potential as seen by the CO₂ stretching mode. The experimental results support the validity of the site-to-site model of intermolecular potential. The intermolecular potential between naphthalene and CO₂ is assumed to be anisotropic, and the proposed electrostatic quadrupole-quadrupole model of these interactions effectively explains the anisotropy in the intermolecular potential, the energy of association, and the frequency shifts.