Molecular vibrations and lattice dynamics of ortho-terphenyl

Infrared and Raman spectra of crystalline, melted and solvated ortho-terphenyl and its perdeuterated isotopomer, D₁₄-ortho-tephenyl, have been recorded. Optimized geometries and vibrational frequencies were calculated by the semiempirical RHF/AM1 method and by DFT using the B3LYP functional and 6–31G(d) basis set. In both cases the lowest energy conformation is of C₂ symmetry. With the scaled AM1 and B3LYP/6-31G(d) force fields the average error in reproducing the experimental molecular vibrational frequencies is 13cm^?1 and 5cm^?1, respectively. The AM1 potential energy surface for phenyl torsions was mapped on a 15° grid. The barrier to concerted internal rotation is estimated to lie between 3 kJ mol^?1 and 6kJ mol^?1. The calculations of the lattice dynamics at k = 0 in the low temperature fully ordered crystal phase of parent and deuterated ortho-terphenyl were performed with inclusion of six low lying intramolecular vibrations. The conformational change of the ortho-terphenyl molecule induced by crystal packing forces was taken into account by re-defining the unperturbed molecular vibrational state. Although an accurate assignment of lattice vibrations was not possible, the calculated spectra give quite a reasonable picture of the low frequency dynamics in crystalline ortho-terphenyl. The relevance of the results obtained to the glass forming property of ortho-terphenyl is discussed.