INTERNAL-ROTATION POTENTIAL FUNCTION FOR ANISOLE IN SOLUTION - A LIQUID-CRYSTAL NMR-STUDY

The H-1 and C-13 NMR spectra of anisole-methyl-C-13 dissolved in a nematic solvent ZLI 1115 have been analyzed. The set of dipolar couplings obtained is used to test various models for the internal rotations about the ring-O and O-CH3 bonds. It is shown that the data are not consistent with the simplest models for this motion: a 2-fold jump about the ring-O bond between planar structures or a four-site jump between nonplanar structures, in both cases with a three-site jump model for motion about the O-CH3 bond. Two models are consistent with the data: a continuous potential for rotation through an angle phi about the ring-O bond, V(phi) = 1/2V2(1 - cos 2-phi) + 1/2V4(1 - cos 4-phi), while retaining the three-site jump motion about the O-CH3 bond or a continuous rotation about both bonds such that the barrier for rotation through theta about the O-CH3 bond is a maximum when phi = 0-degree and decreases to zero for phi = 90-degrees. This concerted motion is described by the potential V(phi,theta) = 1/2V2(1 - cos 2-phi) + 1/2V3,4 cos2-phi(1 - cos (3-theta + 2-phi)), and the values obtained for the coefficients V2 and V3,4 are in excellent agreement with previously reported molecular orbital calculations. It is concluded that V(phi,theta) is closer to reality.