Molecular Motion and Phase Changes in Long Chain Solid Normal Alkanes as Studied by 1H and 13C NMR

Abstract

Abstract Proton spin-lattice and dipolar relaxation times and 13C CP/MAS NMR spectra were measured on crystalline samples of n-C50H102 and long chain normal alkanes (n-alkanes) with average molecular weight of 595, 1214, and 2305 in order to characterize the static and dynamic molecular structures in the solid state. The 13C CP/MAS spectra indicate that the molecules take all-trans conformation in the orthorhombic unit cell and do not undergo any discernible change on annealing and/or quenching of specimens. On the other hand, the T1d in each material assumes a deep and clear minimum value in the orthorhombic phase; this relaxation process was unambiguously assigned to the molecular uniaxial reorientation about the long chain axis. There is a linear relation between the activation energy and the reciprocal of the chain length. In the case of n-C50H102 a solid state phase transition was found at 351 K. The T1d in the high temperature phase led to extremely high activation energy, 150 kJ mol−1, which may be attributable to molecular self-diffusion or other motion accompanying drastic change in the molecular structure.