title: A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals.
abstract: The proton NMR spectra of n-pentane orientationally ordered in two nematic liquid-crystal solvents are studied over a wide temperature range and analysed using covariance matrix adaptation evolutionary strategy. Since alkanes possess small electrostatic moments, their anisotropic intermolecular interactions are dominated by short-range size-and-shape effects. As we assumed for n-butane, the anisotropic energy parameters of each n-pentane conformer are taken to be proportional to those of ethane and propane, independent of temperature. The observed temperature dependence of the n-pentane dipolar couplings allows a model-free separation between conformer degrees of order and conformer probabilities, which cannot be achieved at a single temperature. In this way for n-pentane 13 anisotropic energy parameters (two for trans trans, tt, five for trans gauche, tg, and three for each of gauche+ gauche+, pp, and gauche+ gauche-, pm), the isotropic trans-gauche energy difference Etg and its temperature coefficient Etg(') are obtained. The value obtained for the extra energy associated with the proximity of the two methyl groups in the gauche+ gauche- conformers (the pentane effect) is sensitive to minute details of other assumptions and is thus fixed in the calculations. Conformer populations are affected by the environment. In particular, anisotropic interactions increase the trans probability in the ordered phase.
authors.affiliation: Chemistry Department, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada.
authors.forename: E Elliott
authors.initials: EE
authors.lastname: Burnell
authors.affiliation: Chemistry Department, Brandon University, 270-18th Street, Brandon, Manitoba R7A 6A9, Canada.
authors.forename: Adrian C J
authors.initials: AC
authors.lastname: Weber
authors.affiliation: Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia V6T 1Z1, Canada.
authors.forename: Ronald Y
authors.initials: RY
authors.lastname: Dong
authors.affiliation: Radboud University, Institute for Molecules and Materials, Heyendaalseweg 135, NL-6525 AJ Nijmegen, The Netherlands.
authors.forename: W Leo
authors.initials: WL
authors.lastname: Meerts
authors.affiliation: Laser Centre, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.
authors.forename: Cornelis A
authors.initials: CA
authors.lastname: de Lange
mesh_terms: D050866:Liquid Crystals / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D010420:Pentanes / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013696:Temperature
chemical_list: D010420:Pentanes; D012997:Solvents; C033353:pentane