Coupled internal rotations in substituted ethoxybenzenes: maximum entropy analysis of nuclear magnetic resonance of nematic solution data and ab initio rotameric distributions

The dipolar couplings observed by means of nuclear magnetic resonance spectra of solutes in nematic solvents (LX-NMR) are used to build the maximum entropy probability distribution for the orientational and the internal variables that describe the conformational state of the solute molecule. This approach is applied to the cases of ethoxy (beta,beta,beta-d 3) benzene, 4-fluoro ethoxy (beta,beta,beta-d 3) benzene, and 4-chloro ethoxy (beta,beta,beta-d 3) benzene that contain two internal degrees of freedom, and yields a bidimensional probability map for the two relevant internal rotations. A detailed map of the ab initio potential for ethoxybenzene is also obtained with full optimization of internal coordinates. The two results are compared, showing that they are in good agreement, which allows one to say that the two rotations are strongly correlated especially in the region of the maximum probability (absolute potential minimum).