Conformational Distribution of n-Hexane in a Nematic Liquid Crystal Obtained from Nuclear Spin Dipolar Couplings by Monte Carlo Sampling

Abstract

Motionally averaged proton-proton dipolar couplings measured by nuclear magnetic resonance (NMR) spectroscopy can provide information about the conformations and orientations sampled by partially oriented molecules. In this study, the measured dipolar couplings between pairs of protons on n-hexane dissolved in a nematic liquid crystal solvent are used as constraints in a Monte Carlo sampling of the conformations and orientations of n-hexane. Rotation about each carbon-carbon bond in the molecule is modeled by the complete sinusoidal torsional potential of Ryckaert and Bellemans rather than by the three-state rotational isomeric states (RIS) model that has been used in previous studies. Comparison of the results of the simulations using the Ryckaert-Bellemans potential and the RIS model indicates little difference in the values of the adjustable parameters and the quality of the fits to the experimental data. The primary difference between the models appears in the calculated conformer probability distributions for n-hexane, highlighting the importance of the exact shape of the torsional potential used to model carbon-carbon bond rotation in organic molecules.

Structure−Property Correlations in Model Compounds of Oligomer Liquid Crystals

“…The maximum entropy (MaxEnt) method is employed in the fitting procedure to derive the most probable solution from a limited amount of experimental data. Using the modified method, we have investigated orientational and conformational characteristics of n -alkanes, , 1,6-dimethoxyhexane, and alcohols , dissolved in nematic LCs and successfully obtained results consistent with those obtained by other methods. − The simulation scheme has been further extended to lyotropic LCs of sodium octanoate/1-decanol/water , and sodium octanoate/1-butanol/water …”

“…Using the modified method, we have investigated orientational and conformational characteristics of n-alkanes, 15, 16 1,6dimethoxyhexane, 16 and alcohols 17,18 dissolved in nematic LCs and successfully obtained results consistent with those obtained by other methods. [19][20][21][22][23] The simulation scheme has been further extended to lyotropic LCs of sodium octanoate/1-decanol/ water 24,17 and sodium octanoate/1-butanol/water. 18 In this study, we have treated model compounds of oligomer LCs (i.e., oligomeric models for polymer LCs with different spacer lengths; see Figure 3): monomeric models, 1-phenoxypentane (monomer-5) and 1-phenoxyhexane (monomer-6); dimeric models, 1,3-diphenoxypropane (dimer-3), 1,4-diphenoxybutane (dimer-4), 1,5-diphenoxypentane (dimer-5), and 1,6diphenoxyhexane (dimer-6); tetrameric models, 1,5-bis{p- [5-(phenoxy)pentyloxy]phenoxy}pentane (tetramer-5) and 1,6-bis{p- [5-(phenoxy)hexyloxy]phenoxy}hexane (tetramer-6).…”

Prediction from Molecular Shape of Solute Orientational Order in Liquid Crystals

“…It is interesting that most work has concentrated on the liquid crystal itself, − and that only a few simulations account for the presence of a solute. ,,− The solutes that have been studied with these techniques until now are hydrogen, benzene, ,, hexane, ,, biphenyl, and a series of solutes modeled by hard ellipsoids of various lengths and widths. , The simulations have been used to explore to what extent conformational probabilities of the solute were affected by the liquid-crystal environment.…”

Spatial Configuration and Thermodynamic Characteristics of Main‐Chain Liquid Crystals

“…Whereas simpler models predict some slight enhancement of elongated conformers [Samulski, 1980;Gochin et al, 1987Gochin et al, , 1990Janik et al, 1987;Sasanuma and Abe, 1991;Sasanuma, 2000a,b], the more elaborated chord model proposed by Photinos et al [1990aPhotinos et al [ ,b, 1991Photinos et al [ , 1992 has led to the conclusion that conformer probabilities are similar to those computed for the isotropic arrangements. The chord model was tested favorably by various groups, including two-dimensional 1 H-NMR analysis [Rosen et al, 1993], MD simulation [Alejandre et al, 1994], Monte Carlo sampling [Luzar et al, 1996], and the maximum entropy approach [Berardi et al, 1996]. In connection with the aforementioned arguments, it may be noteworthy that the biphasic boundary lines of the phase diagram exhibit a weak odd-even oscillation when plotted against the number of skeletal atoms of n-alkane solute [Martire, 1979].…”