Molecular vibrations and lattice dynamics of ortho-terphenyl

Abstract

Infrared and Raman spectra of crystalline, melted and solvated orfho-terphenyl and its perdeuterated isotopomer, D14-ortho-tephenyl, have been recorded. Optimized geometries and vibrational frequencies were calculated by the semiempirical RHF/AM 1 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 13 cm-' and 5cm-*, 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-' and 6 kJmol-'. 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.The present study is an attempt to complete the assignment of the vibrational spectrum of OTP using both Raman and IR spectroscopy and to incorporate scaled semiempirical and DFT quantum mechanical calculations with subsequent use of these results in crystal dynamics calculations. Particularly interesting in the vibrational dynamics of OTP is the assignment of the low frequency intramolecular modes, and the evaluation of their possible role in the formation of the glassy state. There has been speculation in the literature about whether torsional motion of the side phenyl rings is responsible for the secondary relaxation in OTP. A secondary or ,&relaxation in the OTP glass was observed by dielectric relaxation [5] and by neutron diffraction 121. An important contribution in this respect is the molecular dynamics simulation study [lo], in which an empirical intramolecular torsional potential was used. A qualitative picture of the short-term dynamics in OTP has been achieved without, however, stating explicitly the extent to which side ring flips are responsible for the secondary, either fast or slow, relaxation. ExperimentalThe OTP sample was purchased from Aldrich and used without further purification. The sample of per-Molecular Physics ISSN 0026-8976 print/ISSN 1362-3028 online 0 2001 Taylor

Structural and Solvent Control of Nonadiabatic Photochemical Bond Formation: Photocyclization of o-Terphenyl in Solution

“…The spectrum of the photochemical product formed after UV excitation of OTP in our work exhibits striking similarities with the absorption bands of DHP: For each species a broad, featureless absorption band appears in the visible region and corresponds with the lowest energy transition. Furthermore, both species exhibit a near-UV transition with discernible vibronic progressions with spacings of ∼1340 cm –1 (DHP) and ∼1000 cm –1 (OTP photoproduct), closely matching the ∼1300 cm –1 carbon–carbon inter-ring and the ∼1000 cm –1 symmetric in-plane ring stretching frequencies of unsaturated cyclic hydrocarbons . Based on these similarities, we assign the photoproduct observed in our time-resolved measurements to the analogous cyclized structure, 4 a ,4 b -dihydrotriphenylene (DHT).…”

“…Furthermore, both species exhibit a near-UV transition with discernible vibronic progressions with spacings of ∼1340 cm −1 (DHP) and ∼1000 cm −1 (OTP photoproduct), closely matching the ∼1300 cm −1 carbon−carbon inter-ring and the ∼1000 cm −1 symmetric in-plane ring stretching frequencies of unsaturated cyclic hydrocarbons. 48 Based on these similarities, we assign the photoproduct observed in our time-resolved measurements to the analogous cyclized structure, 4a,4bdihydrotriphenylene (DHT). This correspondence is supported by the fact that both bands observed from the OTP photoproduct appear at energies ∼0.3−0.5 eV lower than those measured from DHP.…”

Molecular Dynamics Study of Translational and Rotational Diffusion in Liquid Ortho-terphenyl

“…There are no accurate microwave or electron diffraction studies of OTP due to its low vapor pressure. However, there are a number of experimental or computational estimates: 48° (from gas-adsorption chromatography), 55° (from molar Kerr constants; liquid), 63° (from HeI photoelectron spectrum), 54° (from intramolecular potential derived from crystal data + MM; isolated molecule), 52° (B3LYP Calculation), 54° (AM1 Calculation), and 48° (INDO calculation) . One can see, therefore, that the twist angles computed by minimization and dynamics using COMPASS are consistent with the broad range of experimental estimates.…”

Interfacial effects on vitrification of confined glass-forming liquids

“…33,34 First we focus on the hydroxyl vibration region from 4000 to 2900 cm −1 of the spectra shown in Figs. These spectral features are superimposed at 1700-400 cm −1 by vi- brations associated with confined liquids which frequencies are in good agreement with the literature data.…”