Molecular motion in solid p-nitrophenol inclusion compounds of cyclomalto-hexaose and -heptaose: n.m.r. and dielectric studies

Abstract

13C-C.p.-m.a.s. n.m.r. spectra are reported for two polymorphs of p-nitrophenol and its hydrated inclusion compounds with cyclomalto-hexaose (αCD) and -heptaose (βCD). From temperature-dependent 13C-n.m.r. line-widths an activation energy of 50 kJ/mol was obtained for the motion of p-nitrophenol in the cavity of αCD. The motion was confirmed to be a two-fold flip of the p-nitrophenol molecule about the C-1-C-4 axis by 2H-n.m.r. spectroscopy. Dielectric measurements were used to define a dipole reorientation process with an activation energy of 56.8 kJ/mol for p-nitrophenol-αCD, which was attributed to reorientation of the water molecules. The motion of p-nitrophenol is faster and less restricted in the βCD complex than in that of αCD and depends strongly on the degree of hydration of the complex.