Characterization of the methylene 13C chemical shift tensor in the normal alkane n-C20H42

Abstract

13C NMRchemical shifts in a single crystal of n‐eicosane (n‐C20H42) have been measured using the method of high power proton decoupling and the chemical shifttensors were characterized. Principal values in ppm relative to CS2 are: (a) For CH3: σ11=166.7±2, <22=171.2±2.0, σ33=189.8±2; (b) For the α‐methylene: σ11=156.0±2.5, σ22=163.1±2.5, σ33=178.0±2.5; and (c) For the interior methylenes: σ11=142.6±2.0, σ22= 154.6±2.0, σ33=175.6±2.0. From x‐ray studies, only the unit cell parameters are known. On the basis of isolated molecule symmetry considerations, cross‐polarization rate studies, and oriented polyethylene spectra, the interior methylene chemical shifttensor is assigned with respect to molecular orientation. The crystallographic axes are also related to the interior methylene chemical shifttensor and a prediction is made for the C–CH3bond direction relative to these crystallographic axes. The principal axes of the α‐CH2 and the CH3chemical shifttensors do not coincide with any bond directions. The difference (5.6 ppm) between the interior methylene average chemical shielding in solid n‐eicosane and the isotropic liquid value is also discussed.