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Isotope Effects in NMR Spectra as Indication of Conformational Change upon Protonation of Imino and Azo Group in Model Compounds

The protonation of both imino and azo group plays a very important role in many chemical and biochemical processes and transformations. In this work trans-N-benzylideneaniline, trans-N-salicylideneaniline and trans-azobenzene were chosen as model molecules for aromatic compounds containing imino and azo group, respectively. In order to study deuterium isotope effects in 13C NMR spectra a number of deuterium labeled isotopomers were synthesized. The protonation of studied compounds was performed using trifluoroacetic acid (CF3COOH), where no decomposition of samples due to hydrolysis was observed. The deuterated species were also prepared using CF3COOD. There is an indication that the long range deuterium isotope effect over six bonds (6D) in para deuterium labeled isotopomers can be well correlated with the N-Ph torsional angle in the molecules studied. On the other hand, the range of effects could be used as an indication of conformational change upon protonation (deuteration). The general idea is that longer range of effects indicates a better overlapping of p orbitals, which is more effective at smaller torsional angle. Taking this into account, one can predict trends of conformational changes upon protonation in binuclear aromatic compounds containing imino or azo group.

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