Rotational isomerism in 1,2-dinitro-1,2-diphenylethane and 2,3-dinitro-2,3-diphenylbutane

Abstract

I.r. and Raman spectra of the meso and (±) isomers of 1,2-dinitro-1,2-diphenylethane and 2,3-dinitro-2,3-diphenylbutane are reported and assignment of frequencies made. Comparison of the Raman and i.r. spectra of both the solid and solution states of these four isomers provides information on the conformations they adopt in the various states. Thus, meso- 1,2-dinitro-1,2-diphenylethane is shown to exist as a mixture of gauche and trans rotamers in the solid and solution states with the trans predominating in a series of solvents including benzene. There is evidence for the existence of the (±) isomer as three different rotamers in the solid but in only two different rotamers in solution. These are believed to be the non-polar and one of the polar rotamers. Similarly, meso-2,3-dinitro-2,3-diphenylbutane exists as a mixture of gauche and trans rotamers. However, the population of the trans rotamer here is considerably less than that of the gauche rotamer in the solid and possibly the solution state as well. The (±) isomer of 2,3-dinitro-2,3-diphenylbutane, like its ethane analogue, exists as three rotamers in the solid state and only two in solution. Dipole-moment (in C 6H6, CCl4, and C6H12) and Kerr-effect (C6H6 and CCl4) measurements of these isomers are consistent with the spectral evidence.