Investigation of Rotational Barrier in Nicotinamide and Picolinamide Using Temperature-Dependent 1H NMrR Spectra
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Date
2018-05-05
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Addis Ababa University
Abstract
Rotational barrier in nicotinamide and picolinamide have been investigated
employing temperature-dependent proton nuclear magnetic resonance (1H NMR)
spectrometry. In nicotinamide and picolinamide the two protons on the nitrogen
appeared at different chemical shift positions revealing that they are magnetically
non-equivalent.
In the experiments, nicotinamide showed two distinct peaks corresponding to the
amine group of the amide molecule, and the peak separation decreased as a function
of temperature indicating a rotational barrier about the amide bond. The coalescence
of the two peaks was observed at 328 K. Using through line shape analysis of the
temperature dependent spectra; a rotational barrier of 17.9 kcal mol
−1was calculated.
Interestingly the opposite temperature dependence of the peak separation was
observed in picolinamide in which the separation increased as the temperature was
increased and no convergence was observed in the temperature ranges the
experiments were conducted. The observed behaviour clearly demonstrates that the
splitting of the peaks cannot be attributed to the rotational barrier about the C−N bond
rather it suggests that the two peaks originate from two different chromophores (O−H
and =N−H chromophores) resulting from a tautomeric equilibrium.
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Keywords
1H NMR, Nicotinamide, Picolinamide, Rotational Barrier, Rate Constant