LOW RESOLUTION MICROWAVE SPECTROSCOPIC DETERMINATION OF THE CONFORMATION OF ETHYL BENZENE

Author Institution: Department of Chemistry, University of ConnecticutIn order to use B + C values obtained from low resolution microwave band spectra to determine molecular conformation, it is necessary to establish confidence limits concerning their geometrical significance. We have compared low resolution B + C values with rigid rotor constants for seventeen species studied by both low and high resolution techniques spanning the asymmetry range, $- 0.56 0.85$. Three compounds deviate significantly: methyl $chloroformate^{1}$, $phenylborondifluoride^{2}$, and especially methyl $trifluoroacetate.^{3}$ The deviations are clearly associated with vibrationally excited species which contribute to the low resolution spectra. Low resolution B + C values have been determined for p-ethyl-benzaldehyde and m-iodo-, m-bromo-, and m-chloroethylbenzene. In each case only one conformational species is observed. If one assumes that the meta and para substituents do not seriously affect the relative configuration of the ethyl and phenyl groups, the data require that the ethyl carbon atoms lie in a plane orthogonal to the phenyl $plane.^{1}$D. G. Lister and N. L. Owen, JCS Faraday 11, 69, 1036 (1973). $^{2}$D. Christen and N. L. Owen, unpublished work. $^{3}$G.I.L. Jones, T. D. Summers, and N. L. Owen, Ibid., 70, 100 (1974)