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A new approach to the analysis of the internal phonons of tris(quinolin-8-olato) aluminum(III) is presented, which enlightens the role played by the ligands in determining the vibrational properties of the organometallic compound and evidences the importance of the contributions arising from the coupling terms among the three quinolinato fragments. An accurate exam of the normal modes of the meridianal isomer evidences the role of the interactions among the fragments in the vibrational dynamics of the ground state. Due to the special attention paid to the quinolinato fragments, a preliminary investigation on the vibrational properties of 8-hydroxyquinoline, taken as a model fragment, was also performed. The vibrational properties of the polymorph species β of the organometallic molecule were obtained refining the calculated frequencies, the dipole moment derivative matrix, and the polarizability derivative tensor derived by the hybrid density functional B3LYP/6-31G* comparing with the frequencies and intensities recorded by the infrared and the Raman spectroscopies performed on a polycrystalline sample. One thus obtains the most accurate intramolecular force constants up to date for the meridianal isomer in a crystalline phase. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The polarized infrared absorption and Raman spectra on the bc plane of a single crystal of α-sexithienyl (α-T6) have been recorded in the range from 50 up to 3500 cm-1. The intermolecular interactions give rise to the Davydov splitting of the ungerade phonon energy levels originated by the molecular pyramidalization along with the CCH bending modes. A μ-Raman investigation on the crystal surface allows identification of the perturbed local packing of the molecules, which are spectroscopically crystal defects. The selective resonant enhancement of the scattering intensity of some Raman modes, obtained by exciting at λ[sub ex]=632.8 nm, indicates that these defects are associated with the lowering of the electronic excited states due to the increased π-orbital overlapping. Moreover, the resonant enhancement allows detection of some of the gerade intramolecular phonons not observed in the bulk. The comparison between the far-infrared absorption spectra recorded at room temperature and at 80 K evidences three lattice phonons. The spectral analysis, based on a simulation, which uses a scaled ab initio molecular force field, well interprets the experimental data assigning all the internal phonons and elucidates some of the effects of the intermolecular interactions. The calculations are performed at the HF/6-31 G* level of the theory to obtain the harmonic frequencies and the dipole moment and polarizability tensor derivatives of the α-T6 molecule. Both polarized infrared and Raman spectra simulations are carried out calculating the optical response of a unit cell of the α-T6 single crystal belonging to the C[sub 2h] factor group in the monoclinic P2[sub 1]/n space group. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]