Dynamic NMR Study of 1,3,5,7-Tetraoxacyclooctane in Liquid and Liquid Crystalline Solutions and in the Solid State

Article abstract of DOI:10.1021/j100041a007

Deuterium, proton, and carbon-13 NMR spectra of 1,3,5,7-tetraoxacyclooctane (tetroxocane) were recorded in liquid crystalline and isotropic solutions as well as in the solid state.In solution, tetroxocane exists as a mixture of the boat-chair (BC) and crown (Cr) conformers.Pseudorotation and inversion is extremely fast in the BC form, while inversion in the crown conformer is relatively slow (on the NMR time scale).Analysis of the NMR line shape and 2D exchange experiments shows that the ring inversion in the Cr form occurs via the BC form.Interconversion rate constants and equilibrium constants between the two forms over a wide temperature range were determined in several normal and liquid crystalline solvents.In the solid state tetroxocane crystallizes in a (slightly distorted) crown form.Deuterium NMR in a powder sample of solid tetroxocane-d8 exhibits dynamic spectra consistent with molecular 4-fold jumps about their pseudo-C₄ axes.Quantitative analysis of the dynamic line shape is consistent with a small C₂ deformation of the molecule, as found previously by X-ray analysis, and yields the following kinetic equation for the jump rate k_J(s^-1)=5.9E16exp(-15.9/RT), where R is in kcal mol^-1 K^-1.