13C-dipole dipole relaxation time and internal motions of oligo cyclicethers and ether-esters. Part IV. The mechanism and activation energies of the ring Pseudorotation of 15.Crown.5, 18.Crown.6 and 2,6-dioxo-18.crown.6.

Abstract

The 13C dipole-dipole relaxation time values of 1,4,7,10,13- pentaoxacyclopentadecane,(15.Crown.5) and 1,4,7,10,13,16-hexaoxa cyclooctadecane,(18.crown.6) in CDCl 3 and 2,6-dioxo-1,4,7,10,13,16 -hexaoxacycloctadecane,(2,6-do-18.crown.6) were determined in CD 2Cl 2 at 15 MHz of various temperatures by inversion recovery method. The results observed in frequency range of ( W O+ W H 2T 2«1 extreme narrowing conditions were explained in terms of internal motions of macrocyclic backbone and the ring isomerization. The free energies obtained from the 13C dipole-dipole relaxation time measurments were assigned properly to the pseudorotation rate and the conformational barrier of the oxyethylene groups of the macrocyclic ethers and ether-ester. The segmental motion CC and CO bonds of dioxo-hexaoxa ether, observed as the specific T DD 1 and energy barrier of different 13C nuclei was also discussed.