Temperature Dependence of 13C NMR Spectra

Cyclononane prefers a twist boat-chair (TBC) conformation (see references cited in [110]), with two classes of CH2 groups differing in symmetry ( and ). At room temperature, a rapid inversion between two enantiomeric TBC forms probably occurs, involving an intermediate boat-chair form An averaged spectrum having one signal as in Fig. 3.10(e) is observed (Fig. 3.12) [110]. At very low temperature, this inversion is frozen, 

Intramolecular rotations in carboxylic and carbonic acid amides have been thoroughly investigated by 1H NMR [108]. The thermodynamic data for rotation in N,N-di-methyltrichloroacetamide as obtained by 1H and 13C 1H NMR agree quite well CH AG294 = 76.5 kJ/mol 13C AG BB = 73.6 kJ/mol) [109]. 

Any interconversion in a sample may give rise to temperature-dependent NMR spectra. For example, the 13C NMR spectrum of dimeric cyclopentadienyl iron dicarbonyl is temperature dependent [114]. This was attributed to intermolecular exchanges of carbonyls and interconversion between cis and trans complexes. 

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