Abstract
Dipolar couplings obtained previously by analyzing the proton NMR spectra of samples dissolved in nematic solvents of thioanisole and 4-chloroseleno anisole were interpreted in terms of jumps between symmetry-related forms for the motion about the ring-X and X-CH3 bonds. Two models were found to fit the data: jumps between four positions of minimum energy having the X-CH3 bond rotated through ϕ about 40° out of the ring plane, or jumps between planar and orthogonal structures with unequal weightings. The proton and fluorine dipolar couplings for 2,3,5,6-tetrafluoro anisole were interpreted previously in terms of free rotation about the ring—O bond, but with the assumption that the orientational order parameters are independent of the rotational angle ϕ. The data for all three molecules is re-interpreted here, plus new data for thioanisole dissolved in a second nematic solvent, using a theoretical model which allows for the dependence of the orientational ordering on ϕ, thus permitting continuous potentials to be tested, and which also involves fewer variable parameters. This constraining of the model leads to a rejection of the non-planar positions for the minimum in the rotational potential, but leaves two other possible solutions: a two-fold continuous potential for rotation about the ring-X bond with a minimum when the C-X bond is in the ring plane and a maximum when it is at 90°, or a four fold continuous potential with a secondary minimum at 90°. In the case of the tetrafluoro anisole the two-fold potential corresponds to virtually free rotation.
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Celebre, G., Longeri, M. & Emsley, J.W. An NMR investigation using liquid crystalline solvents of the conformations of hindered anisoles, thioanisole and selenoanisole. Appl Magn Reson 2, 611–625 (1991). https://doi.org/10.1007/BF03166070
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DOI: https://doi.org/10.1007/BF03166070