MOLECULAR-DYNAMICS SIMULATION OF BIPHENYL DISSOLVED IN A LIQUID-CRYSTALLINE SOLVENT - A TEST OF THEORETICAL METHODS OF DERIVING ROTATIONAL POTENTIALS FROM PARTIALLY AVERAGED NUCLEAR-SPIN DIPOLAR COUPLINGS

A molecular dynamics simulation has been carried out of biphenyl disso lved in a solvent comprised of rigid particles interacting with the Ga y-Berne potential, The solution is investigated in isotropic, nematic, and smectic phases, and the probability distribution, P-LC(phi) obtai ned, where phi is the angle between the two ring normals. This is comp ared with P-mol(phi), the distribution calculated for an isolated mole cule, and it is found that the positions, phi(max) of the maxima of th e two distributions differ by about 2 degrees, The molecular dynamics trajectory is used to calculate averaged nuclear spin dipolar coupling s, D-ij, and these are used to test the maximum entropy (ME) and addit ive potential (AP) theoretical models which have been used previously to obtain the distribution P-LC(phi) from dipolar couplings obtained o n real solutions of biphenyl in liquid crystalline solvents. It is con cluded that the AP method is able to recover the true distribution PLC (St) from the simulated D-ij with good precision at all the temperatur es studied, whereas the ME method achieves good precision only when th e orientational order is high. The AP method also succeeds in obtainin g the correct difference between P-LC(phi) and P-iso(phi), the distrib ution for an isotropic phase at the same temperature. (C) 1996 America n Institute of Physics.