Dynamic NMR in liquid crystalline solvents—ring inversion of s -trioxane

The proton magnetic resonance spectrum of s-trioxane in nematic liquid crystalline solvents was studied over a wide range of temperatures. The spectra exhibit striking line shape variations with temperature which are attributed to the ring inversion of s-trioxane. At high temperatures (≳100 °C) the spectra correspond to an average Hamiltonian due to the two fast interconverting chair forms of the molecule. As the temperature is lowered the resonance lines undergo selective broadening due to incomplete averaging of the two conformers. In order to obtain quantitative kinetic results the line shape theory for dynamic NMR in anisotropic liquids is formulated and is used to calculate theoretical spectra for s-trioxane undergoing ring inversion. Comparison of the experimental and theoretical spectra yields kinetic parameters for the reaction in the temperature range at which the liquid crystalline solutions were studied, i.e., −20 to +110 °C. The proton NMR of s-trioxane was also studied in the isotropic solvent freon in the temperature range −80 to −40 °C. Analysis of these spectra yield essentially the same kinetic parameters as in the anisotropic solvents. The final overall results are ΔH≠ = 11.2 kcal/mole, ΔS≠ = +3.1 e.u., and 1/τ (300 °K) = 2.0×105 sec−1. The advantages and limitations of dynamic NMR in liquid crystalline solvent are discussed.