Dipolar couplings obtained previously by analyzing the proton NMR spectra of sampies dissolved in nematic solvents of thioanisole and 4-chloroseleno anisole were interpreted in temas of jumps between symmetry-related forros for the motion about the ring-X and X-CH(3) bonds. Two models were found to fit the data: jumps between four positions of minimum energy having the X-CH(3) bond rotated through phi about 40 degrees out of the ring plane, or jumps between planar and orthogonal struct. ures with unequal weightings. The proton and fluorine dipolar couplings for 2,3,5,6-tetrafluoro anisole were interpreted previously in temas of free rotation about the ring-O bond, but with the assumption that the crrientational order parameters are independent of the rotational angle phi. The data for aU 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 phi, thus permitting continuous potentials to be tested, and which also involves fewer variable parameters. This constraining of the model leads to a reject. ion of the non-planar positions for the mY in the rotational potential, but leaves two other possible solutions: a two-fold continuous potential for rotafion 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 degrees, or a four fold continuous potential with a secondary mY at 90 degrees. In the case of the tetrafluoro anisole the two-fold potential corresponds to virtually free rotation.
An NMR Investigation Using Liquid Crystalline Solvents of the Conformations of Hindered Anisoles, Thioanisole and Selenoanisole
CELEBRE, Giorgio;
1991-01-01
Abstract
Dipolar couplings obtained previously by analyzing the proton NMR spectra of sampies dissolved in nematic solvents of thioanisole and 4-chloroseleno anisole were interpreted in temas of jumps between symmetry-related forros for the motion about the ring-X and X-CH(3) bonds. Two models were found to fit the data: jumps between four positions of minimum energy having the X-CH(3) bond rotated through phi about 40 degrees out of the ring plane, or jumps between planar and orthogonal struct. ures with unequal weightings. The proton and fluorine dipolar couplings for 2,3,5,6-tetrafluoro anisole were interpreted previously in temas of free rotation about the ring-O bond, but with the assumption that the crrientational order parameters are independent of the rotational angle phi. The data for aU 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 phi, thus permitting continuous potentials to be tested, and which also involves fewer variable parameters. This constraining of the model leads to a reject. ion of the non-planar positions for the mY in the rotational potential, but leaves two other possible solutions: a two-fold continuous potential for rotafion 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 degrees, or a four fold continuous potential with a secondary mY at 90 degrees. In the case of the tetrafluoro anisole the two-fold potential corresponds to virtually free rotation.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
