L. Calucci et al., DYNAMIC NMR-STUDY OF 1,3,5,7-TETRAOXACYCLOOCTANE IN LIQUID AND LIQUID-CRYSTALLINE SOLUTIONS AND IN THE SOLID-STATE, Journal of physical chemistry, 99(41), 1995, pp. 14942-14948
Deuterium, proton, and carbon-13 NMR spectra of 1,3,5,7-tetraoxacycloo
ctane (tetroxocane) were recorded in liquid crystalline and isotropic
solutions. as well as in the solid state. In solution, tetroxocane exi
sts as a mixture of the boat-chair (BC) and crown (Cr) conformers. Pse
udorotation and inversion is extremely fast in the BC form, while inve
rsion in the crown conformer is relatively slow (on the NMR time scale
). Analysis of the NMR line shape and 2D exchange experiments shows th
at the ring inversion in the Cr form occurs via the BC form. Interconv
ersion rate constants and equilibrium constants between the two forms
over a wide temperature range were determined in several normal and li
quid crystalline solvents. In the solid state tetroxocane crystallizes
in a (slightly distorted) crown form. Deuterium NMR in a powder sampl
e of solid tetroxocane-d(g) exhibits dynamic spectra consistent with m
olecular 4-fold jumps about their pseudo-C-4 axes. Quantitative analys
is of the dynamic line shape is consistent with a small Ct deformation
of the molecule, as found previously by X-ray analysis, and yields th
e following kinetic equation for the jump rate, k(J) (s(-1)) = 5.9 x 1
0(16) exp (-15.9/RT), where R is in kcal mol(-1) K-1.