Coalescence of NMR signals

Below 100 °C tri-t-butyldiaziridinimine (176) only undergoes inversion at the exocyclic nitrogen, as evidenced by coalescence of NMR signals at about 50 °C. Heating for 1 h to 150 °C, however, results in clean formation of ( )-azoisobutane and t-butyl isocyanide. 

The exchange activation energy calculated at the temperature of coalescence of NMR signals is 59 kJ mol (14 kcal mol ). It was also pointed out that, because of the strong Tr-accepting ability of the NO unit, the electron density at the central atom is reduced, thereby aiding the ni -type coordination of the hydrazido(l -) ligands. 

No coalescence of NMR signals was observed for the pseudotetrahedral alkylidene and vinylidene complexes [Re(C5H5)(NO)(PPh3)(=CHPh] and the related CH2 and C=CH2 complexes which sets lower limits for rotation barriers about the Re = C bonds of 18-19 kcal mol". " However, slow isomerizations of kinetically formed isomers of the CHPh and C=CMePh complexes to thermodynamic mixtures could be followed by NMR methods and this has allowed kinetic activation parameters for the ligand rotation to be determined in these cases. 

The H nmr spectrum of Zr(Hf)(acac)4 in CHCIF2 shows two Me proton resonances of equal intensity in the slow exchange region only at lowered temperature ( —170°Ci). This observation is consistent with a square antiprism geometry 11. At — 145°C coalescence of the signals occurs arising from complicated intramolecular rearrangements. 

Metalated vinyl ethers are configurational stable up to —20°C in tetrahydrofuran. H-NMR measurements of 1-ethoxy-1-lithioethene TMEDA did not show any coalescence of the signals for the vinyl protons until the onset of decomposition. Thus, there is no evidence of inversion in this case . Similar configurational stability is displayed by a-lithiated thioethers in tetrahydrofuran no inversion occurs up to 0°C. On the contrary, deprotonated vinyl sulfoxides and sulfones are configurationally less stable . ... 

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