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Molecular vibrations and lattice dynamics of ortho-terphenyl

Infrared and Raman spectra of crystalline, melted and solvated ortho-terphenyl and its perdeuterated isotopomer, D_14-ortho-terphenyl, have been recorded. Optimized geometries and vibrational frequencies were calculated by the semiempirical RHF/AM1 method and by means of DFT method using B3LYP functional and 6-31G(d) basis set. In both cases the lowest energy conformation is of C_2 symmetry. With the scaled AM1 and B3LYP/6-31G(d) force fields the average error in reproducing the experimental molecular vibrational frequencies is 13 and 5 cm^-1, respectively. The AM1 potential energy surface for phenyl torsions was mapped on a 15 deg grid. The barrier to concerted internal rotation is estimated to lye between 3 and 6 kJmol^-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 redefining the unperturbed molecular vibrational states. Although an accurate assignment of lattice vibrations was not possible, the calculated spectra give quite reasonable picture of the low-frequency dynamics in crystalline ortho-terphenyl. The relevance of the obtained results to the glass forming property of ortho-terphenyl is discussed.

ortho-terphenyl ; vibrational spectrum ; lattice dynamics

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