Conformational Analysis of Stereoisomers of Triphenylheptane by Depolarized Rayleigh Scattering

Abstract

S:Average molecular optical anisotropies <r 2 ) of the three stereoisomers of 2,4,6-triphenylheptane were measured by depolarized Rayleigh scattering. Analysis of these results led to the determination of the relative stability among the various conformational structures of these isomers.KEY WORDS Optical Anisotropy / Rayleigh Scattering / Local Conformation / Triphenylheptane / In a previous work 1 • 2 we studied by depolarized Rayleigh scattering (DRS)3 the stereoregularity of polystyrene (PS) chains in solution. The quantitative interpretation 2 of our results led to the study of short range interactions in model molecules of PS, the meso and racemic 2,4-diphenylpentane (DPP). This paper is devoted to the study by depolarized Rayleigh scattering of conformational structure of three stereoisomers of 2,4,6-triphenylheptane (TPH) which are the higher members of the series. These results will be compared with those already published 4 • 8 elsewhere. The experimental and theoretical basis for the method used will not be developed here but may be found in ref 1-6. EXPERIMENT ALThe synthesis of 2,4,6-triphenylheptane was carried out7 by condensing cumyl magnesium bromide with 1,3-diphenyl-l-butanone. The different configurational isomers were at the first step separated by preparative gas chromatography using a Thomson SRTI model THN 102 with a stationary phase of the silicon SE-30 type. The purity of the heterotactic isomer was estimated at about 95% and that of isotactic and syndiotactic isomers at 99,96. On completion of this work we noted that the heterotactic isomer was colorless but, that other isomers had a slightly yellow color. This coloration which may be introduced by eventual fluorescent molecules was very inconvenient for subsequent measurement of Rayleigh scattering. Also in the second step, we carried out a repurification of the compounds in question. Thus, isotactic and syndiotactic TPH were dissolved in cyclohexane and the solutions obtained were filtered on active coal. The stereoisomers were at last recovered by evaporation under vacuum of the solvent. Each compound so obtained was a transparent colorless liquid. MOLECULAR OPTICAL ANISOTROPY AND CONFORMATIONAL ANALYSISThe average molecular optical anisotropy <r2>3 of TPH in cyclohexane solution at 25°C was determined* by measuring the scattered depolarized intensity from the solutions at a wavelengh of 5460 A. The technique used has been previously described. 3 ' 1 The accuracy of <r2> is * In practice the intensity measurements are carried out by comparison with a standard liquid, cyclohexane, for which these has been determined at a wavelength of 5460 A a scattered depolarized intensity per unit volume and unit of incident photometric illumination of 23, 15-10-8 cm-1• 537

Conformations of 2,4-Diphenylpentane: A Quantum Chemistry and Gas-Phase Molecular Dynamics Simulation Study

“…These compounds can be considered polystyrene dimers and trimers, respectively. Far-infrared and depolarized Rayleigh scattering methods have been applied in the study of phenyl ring motion in these compounds, while conformational populations have been investigated by NMR vicinal coupling measurements, − depolarized Rayleigh scattering (optical anisotropy), , far-infrared and Raman spectroscopy, ultrasonic relaxation measurements, , and epimerization reactions. , These compounds have also been the subject of both molecular mechanics − and limited ab initio electronic structure calculations . These studies seem to establish racemic tt (r- tt ) as the lowest energy conformer of DPP.…”