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 interaction potential, was performed on 4-n-pentyl-4(')-cyanobiphenyl (5CB) in the nematic phase. The analysis of the trajectory was focused on the determination of molecular structure and orientational ordering using nuclear dipole-dipole couplings. Three sets of couplings were calculated: C-13-C-13, C-13-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 the 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 to the long-range orientational order and molecular conformation? (ii) Which model predicts a molecular structure that is in best agreement with the true conformation? Computer simulation is an attractive method to address these questions since the answer is provided: we know the true orientational order and the molecular structure. We found that all sets of dipolar couplings analyzed using the two models predict correct orientational order for the biphenyl fragment. The structure of this moiety was unambiguously determined in all analyses except for the ME method applied on the C-13-C-13 couplings. The RIS approximation failed to discriminate between a large range of possible structures of the ring-chain fragment. (C) 2001 American Institute of Physics.