A study of rotational isomerism in 1,2-diphenylethane, its pp′-disubstituted derivatives (Cl, Br, and CN), 2,3-diphenylbutane, and its pp′-dibromo-substituted derivatives, by infrared and Raman spectroscopy and molecular polarisability measurements

Abstract

An examination of the Raman spectra of 1,2-diphenylethane, 1,2-di-(p-chlorophenyl)ethane, meso-2,3-di-phenylbutane, and its pp′-dibromo-derivative reveals the general absence of corresponding frequencies from the i.r. spectra of the solids. This fact, taken together with the general appearance of certain extra bands in the i.r. and Raman spectra of various solutions, strongly suggests the existence of only the trans-rotamer in the solid phase but of a dynamic equilibrium mixture of both trans- and gauche-rotamers in solution. This conclusion is also considered to be essentially true in the case of 1,2-diphenylethane, notwithstanding the lack of extra bands in the i.r. spectra of its solutions. The relatively weak intensities of the bands attributed to the gauche-rotamers suggest that in these compounds the proportion of the gauche-rotamer is appreciably less than that of the trans in solution. The greater number of absorption bands observed in the i.r. spectra of the racemic (±) than in that of the corresponding solid meso-compound, is in accord with predictions based on simple selection rules. The dihedral angles of some of the gauche-rotamers are determined by molecular polarisability measurements.