NMR of orientationally ordered short-chain hydrocarbons

The proton NMR spectra of the straight-chain hydrocarbons from methane to n-hexane orientationally ordered in nematic liquid-crystal solvents are reviewed. The theory behind reorientation-vibration coupling (that explains the anisotropic splittings observed in the spectrum of methane) is discussed. This coupling is also important for ethane where the torsional motion about the C–C bond becomes an issue. The ratios of propane to ethane dipolar couplings are found to be independent of liquid crystal and of temperature: this demonstrates that the hydrocarbons are ‘magic solutes’, i.e. their orientational order results from a single anisotropic interaction that involves short-range size-and-shape effects. The longer chains n-butane, n-pentane and n-hexane (which give extremely complicated NMR spectra) exist in several symmetry-unrelated conformers. The spectra are analysed with the aid of evolution strategies. The dipolar couplings obtained are used in a model-free analysis (that is based on the constant ratio between ethane and propane dipolar couplings) to explore important information about the intermolecular potential. For example, the orientational order of longer conformers is favoured by the nematic phase.