Discrimination of Rotational Isomeric States in Cycloalkanes by Solid-State CP-MAS 13C NMR Spectroscopy

Article abstract of DOI:10.1021/ja00330a010

The solid-state behavior of three cycloalkanes, cyclododecane, cyclotetraeicosane, and cyclohexatriacontane, was investigated by means of temperature dependent magic angle cross-polarization 13C NMR experiments.For the two smaller ring molecules a state of high internal mobility like the "rotator phase" in n-alkanes was detected.It could be correlated with a phase transition in the solid state visible by means of DSC.In the case of (CH2)12 this is 151 K below the melting point, and in the case of (CH2)24 it is 25 K below the melting transition.The CP-MAS 13C NMR spectra show a transition from the fast exchange to the slow exchange regime of magnetically nonequivalent states.By comparison with X-ray diffraction data the well-resolved resonance signals for the low-temperature phases were assigned to molecular segments distinguished by the rotational isomeric states of the carbon-carbon bonds.Chemical shift differences due to conformational isomerism were as large as 12 ppm; thus, they exceed "packing effects" by far.