Investigation Of Rotational Barrier In Nicotinamide And Picolinamide Using Temperature-dependent 1h Nmrr Spectra

Rotational barrier in nicotinamide and picolinamide have been investigatedrnemploying temperature-dependent proton nuclear magnetic resonance (1H NMR)rnspectrometry. In nicotinamide and picolinamide the two protons on the nitrogenrnappeared at different chemical shift positions revealing that they are magneticallyrnnon-equivalent.rnIn the experiments, nicotinamide showed two distinct peaks corresponding to thernamine group of the amide molecule, and the peak separation decreased as a functionrnof temperature indicating a rotational barrier about the amide bond. The coalescencernof the two peaks was observed at 328 K. Using through line shape analysis of therntemperature dependent spectra; a rotational barrier of 17.9 kcal molrn−1was calculated.rnInterestingly the opposite temperature dependence of the peak separation wasrnobserved in picolinamide in which the separation increased as the temperature wasrnincreased and no convergence was observed in the temperature ranges thernexperiments were conducted. The observed behaviour clearly demonstrates that thernsplitting of the peaks cannot be attributed to the rotational barrier about the C−N bondrnrather it suggests that the two peaks originate from two different chromophores (O−Hrnand =N−H chromophores) resulting from a tautomeric equilibrium.

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