Intermolecular Vibrational Spectra of C_3v CXY₃ Molecular Liquids, CHCl₃, CHBr₃, CFBr₃, and CBrCl₃

We report the quality anisotropic intermolecular vibrational spectra within the frequency range 0.5–800 cm^–1 of four C_3v CXY₃ molecular liquids, CHCl₃, CHBr₃, CFBr₃, and CBrCl₃, by means of femtosecond optical-heterodyne-detected Raman-induced Kerr effect spectroscopy. The results show that the first moment of the intermolecular vibrational spectrum is proportional to the square root of the value of the surface tension divided by the liquid density. This implies that the intermolecular vibrational spectrum reflects the bulk properties of the liquids. To understand the molecular-level aspects of the intermolecular vibrational spectra of the liquids, the spectra are compared with the molecular properties such as molecular weight, rotational constants, and bimolecular interaction energy. Overall, the first moment of the spectrum moderately correlates to the inverse square roots of both the molecular weight and the fast rotational constant. Therefore, the molecular properties are responsible for the intermolecular vibrational spectrum. Plots of the first moment of the intermolecular vibrational spectrum vs the square root of the value of the simple bimolecular interaction energy divided by the molecular surface area and the molecular weight show a linear correlation in the case of the oblate symmetric top molecular liquids, CHCl₃, CHBr₃, and CFBr₃. However, CBrCl₃, which is a prolate symmetric top molecular liquid, does not show the same correlation for the oblate molecular liquids.