Conformational analysis of 2,2'-bithiophene revisited: the maximum entropy method applied to large sets of H-H and 13C-H partially averaged dipolar couplings.
Two recently determined sets of partially averaged H-H and (13)C-H NMR dipolar couplings of 2,2'-bithiophene as a solute in nematic liquid-crystalline solvents have been analyzed by means of the maximum entropy method. This investigation originated to ascertain the real information content of the new (13)C-H data. To this end, a procedure has been employed which consists in adding the experimental data one after the other and monitoring, during each of these steps, the behavior of the minimum of the associated work function as well as that of the root-mean-square deviation. It turns out that nearly all (13)C-H data do not provide new pieces of information with respect to a subset of seven H-H dipolar couplings. Inclusion of the very few which do, however, alters significantly the conformational distribution function calculated taking into account the sole H-H dipolar couplings. The final form of this function is characterized by a prominent peak corresponding to the trans conformation and two smaller peaks corresponding to the cis conformation and of the conformation in which the two rings are mutually orthogonal. The form of the conformational distribution function is the result of a considerable orientational-conformational coupling. The implications of the present results on the form of the torsional potential-energy function of the isolated molecule are discussed.
Cinacchi
Giorgio
G
School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, England. giorgio.cinacchi@bristol.ac.uk