Atom-atom interaction model and internal vibrational exciton splittings in organic charge-transfer complexes: Factor group splittings of naphthalene vibrations in several crystalline complexes

Factor group splittings of naphthalene vibrations are experimentally investigated for naphthalene: 2SbCl₃ (C₁₀H₈ - 2SbCl₃), naphthalene: octafluoronaphthalene (C₁₀H₈ : C₁₀F₈), naphthalene: TCNB, and naphthalene: TNB crystalline complexes by Raman spectroscopy and using the isotopically mixed crystal technique. It is found that only 386 cm^-1 mode of C₁₀H₈ shows factor group splittings in the first two complexes. The splitting increases from 5 cm^-1 in pure C₁₀H₈ crystal to 5.5cm^-1 in C₁₀H₈ : 2SbCl₃ but decreases to 1cm^-1 in the C₁₀H₈ : C₁₀F₈ complex. Also as SbCl₃ is successively replaced by SbBr₃ in the complex C₁₀H₈ : 2SbCl₃, the factor group splitting of 386 cm^-1 C₁₀H₈ mode decreases and the mean of the factor group frequencies goes through a minimum near 0.5 mole fraction of SbBr₃. A theoretical calculation using atom-atom potential model and considering only naphthalene-naphthalene interactions predicts that the factor group splitting on 386 cm^-1 band should increase from pure C₁₀H₈ crystal to the C₁₀H₈ :2SbCl₃ crystalline complex and decrease in C₁₀H₈ : C₁₀F₈. However, the calculation also predicts a similar trend for 943 cm^-1 band of naphthalene which shows a factor group splitting of 5 cm^-1 in pure C₁₀H₈ but none in the C₁₀H₈ : 2SbCl₃ complex. Furthermore, the atom-atom interaction model does not explain the effect of SbBr₃ substitution on the factor group splitting. The importance of electrostatic multipole interactions in explaining the behavior of factor group splitting is discussed.