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NMR spectra of porphyrins. 26 —conjugation versus steric repulsions in a planar chiral meso ‐ Aminoporphyrin
Author(s) -
Abraham Raymond J.,
Smith Kevin M.,
Bobe Frank W.,
Minnetian Ohannes M.
Publication year - 1984
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.1270221208
Subject(s) - chemistry , steric effects , spectral line , nmr spectra database , nuclear overhauser effect , crystallography , proton nmr , proton , nuclear magnetic resonance spectroscopy , stereochemistry , physics , quantum mechanics , astronomy
The proton NMR spectra of the free base porphyrins γ‐ meso ‐dimethylaminopyrroetioporphyrin‐XV (1) and the corresponding γ‐diethylamino compound 2 show no temperature dependence, and are interpreted as being due to an essentially orthogonal conformation of the γ‐NR 2 substituents. However, both of the corresponding dications 3 and 4, respectively, in CDCl 3 –TFA solution, give temperature‐dependent proton NMR spectra. Nuclear Overhauser enhancement difference spectra allow complete assignment of the spectrum of 3, and hence the interpretation of the temperature dependence. The spectra arise from a skew conformation of the NR 2 groups in which the barrier to rotation through the orthogonal conformation is ca 13.6 kcal mol (56.8 kJ mol −1 ), and the barrier to rotation through the planar conformation is greater than this.