Structure and molecular ordering extracted from residual dipolar couplings: A molecular dynamics simulation study

A molecular dynamics (MD) simulation, based on a realistic atom-atom intera ction potential, was performed on 4-n-pentyl-4(')-cyanobiphenyl (5CB) in th e nematic phase. The analysis of the trajectory was focused on the determin ation of molecular structure and orientational ordering using nuclear dipol e-dipole couplings. Three sets of couplings were calculated: C-13-C-13, C-1 3-H-1, and H-1-H-1. These dipolar couplings were used for investigation of the biphenyl and the ring-chain fragments in 5CB. The models employed in th e analysis were based on the rotational isomeric state (RIS) approximation and the maximum entropy (ME) approach. The main questions addressed in this article are: (i) How sensitive are the various sets of dipolar couplings t o the long-range orientational order and molecular conformation? (ii) Which model predicts a molecular structure that is in best agreement with the tr ue conformation? Computer simulation is an attractive method to address the se questions since the answer is provided: we know the true orientational o rder and the molecular structure. We found that all sets of dipolar couplin gs analyzed using the two models predict correct orientational order for th e biphenyl fragment. The structure of this moiety was unambiguously determi ned in all analyses except for the ME method applied on the C-13-C-13 coupl ings. The RIS approximation failed to discriminate between a large range of possible structures of the ring-chain fragment. (C) 2001 American Institut e of Physics.