STRUCTURAL STUDIES OF N-ALKANES BY VARIABLE-TEMPERATURE SOLID-STATE HIGH-RESOLUTION C-13 NMR-SPECTROSCOPY

High-resolution C-13 NMR spectra of some n-alkanes (n-C19H40, n-C24H50, n-C32H66 and n-C44H90) in the solid state have been measured by means of variable-temperature, solid-state, high-resolution C-13 NMR spectroscopy as a function of temperature, in order to clarify the structural change over a wide range of temperatures. From these results, it was found that the C-13 chemical shift value of the internal CH2 carbons in n-CH24H50 moves from 34.2 to 33.3 ppm in going from the triclinic form to the rotator phase, and the C-13 chemical shift of the internal CH2 carbons of n-C19H40 and n-C32H66 moves from 32.8(32.9) to 33.3 ppm in going from the orthorhombic form to the rotator phase. This shows that the C-13 chemical shift values of n-alkanes in the rotator phase are the same irrespective of their chain length. Further, it was found that the structure of the chain-end parts in the intermediate phase and the rotator phase of longer n-alkanes is similar to that in the noncrystalline phase in which fast transition between trans and gauche isomers occurs.