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| - Modulation of the Porphyrin Inner Proton Exchange Rates by theSteric Effects of Bridge Substitution
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| - The tautomeric equilibria and H−N proton transfer taking place in the nonsymmetricallysubstituted water-soluble 2-sulfonato-5,15-bis(4-sulfonatophenyl)porphyrin (1) and in its 10-bromo-substituted derivative (2) were analyzed by NMR methods: 1H and 13C spectroscopies andheteronuclear multiple bond correlation (HMBC) and heteronuclear single quantum coherence(HSQC) 1H−13C and 1H−15N techniques. The existence of preferred pathways of H−N transferwas detected. The conclusions are rationalized by taking into account the effect partial meso-substitution exerts on the relative energies of the different cis-tautomer intermediates involved inthe tautomerism. These results underline the experimental consequences stemming from thenonequivalence in porphyrins between the ‘tautomeric interconversion' and ‘proton transfer' terms,when observed by NMR techniques, as a consequence of the existence of two pairs of degeneratetautomers and transformation pathways.
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