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The NMR spectra of porphyrins 20 —proton and 13 C characterization of porphyrin atropisomers
Author(s) -
Abraham Raymond J.,
Plant James,
Bedford Geoffrey R.
Publication year - 1982
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270190409
Subject(s) - porphyrin , atropisomer , chemistry , ring (chemistry) , dihedral angle , chemical shift , proton nmr , substituent , crystallography , free base , ring current , stereochemistry , photochemistry , molecule , organic chemistry , hydrogen bond , physics , earth's magnetic field , salt (chemistry) , quantum mechanics , magnetic field
The preparation, isolation and characterization by proton and 13 C NMR of the four possible atropisomers of meso ‐tetra(2‐methoxy‐1‐naphthyl)porphyrin is described. Chemical shift differences due to atropisomerization effects are observed in the porphyrin and naphthyl rings. Comparison of the naphthyl chemical shifts with those of the model compound 1‐isopropenyl‐2‐methoxynaphthalene allows the shifts due to the porphyrin ring current to be isolated. The observed Δδ values of the naphthyl protons agree well with those calculated from the previously described porphyrin ring current model, and allow both the angle of tilt of the naphthyl ring and the dihedral angle of the 2‐methoxy substituent to be estimated. In contrast, the Δδ values for the naphthyl carbons bear no relationship to the calculated ring current shifts. Calculations of the total ring current contribution (porphyrin plus naphthyl rings) at the different naphthyl rings of the unsymmetric type III isomer show that at least part of the observed atropisomerism effects are due to the long‐range current shifts of the naphthyl rings. The results also provide a clear demonstration of the identity of the porphyrin ring current in the free base and porphyrin dication.