Signal splitting bond rotation

H-MA-MMA at 90°C, along with its simulation, is shown in Fig. 23. A 12-Une spectrum with a very small quartet in each spectroscopic line was detected. The 12-line spectrum is caused by a quartet of triplets (4 X 3) from three equivalent methyl protons and two methylene protons. An additional very small splitting of a doublet (0.80 G) due to the y-proton in the MA moiety can also be observed. The signal intensity of each spectroscopic line displays a temperature dependent change due to hindered rotation around the Ca- Cp bond. At 90°C, the bond rotates freely and the simulated spectrum is shown in Fig. 23. The ESR spectra of such model dimeric radicals, with well-defined structures, have not been observed before. Analyses of the values obtained from the hyperfme coupling constants clearly show the dimeric radicals have the stractures indicated in Fig. 22. 

For 20- [11] and 40-membered [12] macrocyclic di- and tetra-Schiff bases being derivatives of (R)-BTNOL, the chemical shift differences (AS) measured for diastereotopic methylene protons of BINOL unit in CDCI3 have suggested the presence of the partial rotation around the Ar—O—CH2 bond.27 The splitting of some 1H signals in acetone-d6 was explained as a result of a dynamic interconversion process on the NMR time-scale. 

Chiral lanthanide chelates have been used to study compounds that are chiral but do not have an asymmetric carbon atom. 5-Oxidovinylphenanthridinium species (40) have two pairs of diastereomeric rotamers . The number of methyl singlets of 40 doubled in the presence of Eu(tfc)3, confirming the chirality under conditions of slow rotation. 1,3-Dienes such as 41 can exist in EE, EZ and ZZ forms and are chiral by virtue of slow rotation . The methoxy signal was conveniently monitored in the presence of Eu(hfc)3 and could be used to determine the barrier to rotation. Slow rotation about the disulfide bonds of substrates of structure 42 gives rise to enantiomers, as was demonstrated by a splitting of the signals in the presence of Eu(hfc)3 . 

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