Reorientational dynamics of two isomers of thiacalix[4]arene

Abstract

Molecular reorientation and local mobility of two isomers of 25,26,27,28-tetrapropoxy-2,8,14,20-tetrathiacalix[4]arene, cone and 1,3-alternate, were studied by means of NMR relaxation. The chemical shift anisotropy of 13C nuclei was determined in the solid state. Nuclear relaxation due to dipole-dipole interaction and chemical shift anisotropy was analysed in the framework of the Lipari-Szabo approach. The aromatic core of both isomers was found rigid on the subnanosecond time scale. The amplitude of the local motion of the lower rim propoxy substituents increases towards the methyl ends. The methylene groups of the propoxy residues of the cone isomer possess more motional freedom than in the 1,3-alternate, where they are sterically hindered by the adjacent aromatic rings. Global reorientation of the cone is slower than the motion of the 1,3-alternate, which rotates partly anisotropically. Copyright © 2003 John Wiley & Sons, Ltd.