13C CP/MAS NMR studies of tetraazaannulenes: fast proton transfer in the solid state

Authors:
  • AMAlan C. McGregor
    Alan C. McGregor
  • PLPeter J. Lukes
    Peter J. Lukes
  • JOJulian R. Osman
    Julian R. Osman
  • JCJoe A. Crayston
    Joe A. Crayston
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Abstract
Full assignments of the high resolution 13C CP/MAS solid state NMR spectra of a series of substituted dibenzotetraazaannulenes based on the parent macrocycle tetraazaannulene (TAA) are reported. The expected number of resonances observed is compatible with the crystal structures in all cases, except that in the case of TAA only one resonance is observed in the region expected for C-8 and -15. This observation is interpreted as being a consequence of fast proton transfer in the solid-state coupled with rapid molecular rotation. Variable temperature studies of the maximum line broadening of the peaks indicates an activation energy of 8.4 kJ mol–1. Methyl substituents on the ring lead to two peaks in the CN/C–NH region; this is attributed to unequal populations of tautomers with no rotation of the molecule. The results are compared to literature 15N data. Variable temperature data on the methyl substituted systems are also consistent with the reported 15N data, confirming that the tautomers are unequally populated.