Maximum entropy analysis of NMR data of flexible multirotor molecules partially oriented in nematic solution: 2,2':5',2''-tertiophene, 2,2'- and 3,3'-ditiophene

The dipolar couplings observed by NMR spectroscopy of solutes in nematic solvents (LX-NMR) are used to build up the maximum entropy (ME) probability distribution function of the variables describing the orientational and internal motion of the molecule. The ME conformational distributions of 2.2'- and 3,3'-dithiophene and 2,2':5',2''-terthiophene (alpha-terthienyl) thus obtained are compared with the results of previous studies. The 2,2'- and 3,3'-dithiophene molecules exhibit equilibria among cisoid and transoid forms; the probability maxima correspond to planar and twisted conformers for 2,2'- or 3,3'-dithiophene, respectively. 2,2':5',2''-Terthiophene has two internal degrees of freedom; the ME approach indicates that the trans,trans and cis,trans planar conformations are the most probable. The correlation between the two intramolecular rotations is also discussed.