D. Vanderhart
Jan 15, 1976
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Influential Citations
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Journal
Journal of Chemical Physics
Abstract
13C NMR chemical shifts in a single crystal of n‐eicosane (n‐C20H42) have been measured using the method of high power proton decoupling and the chemical shift tensors 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 shift tensor is assigned with respect to molecular orientation. The crystallographic axes are also related to the interior methylene chemical shift tensor and a prediction is made for the C–CH3 bond direction relative to these crystallographic axes. The principal axes of the α‐CH2 and the CH3 chemical shift tensors do not coincide with any bond directions. T...