Fine structure of 5,10,15,20-tetrakis(m-hydroxyphenyl)chlorin (m-THPC): a 1H, 13C and 15N NMR study

Abstract

The 1H, 13C and natural abundance 15N NMR spectra of tetrakis(m-hydroxyphenyl)chlorin (m-THPC) are assigned through the complementary analysis of the data from one-dimensional, and both homo- and hetero-nuclear two-dimensional NMR experiments. The analysis of the 1H and 13C spectra showed that the symmetry of the structure includes an effective mirror plane, σv, and the 1H–15N coupling constant of 99 Hz (measured from the ‘inverse-mode’1H–15N 2D experiment) showed that the central imino protons undergo slow intramolecular tautomeric exchange. The single 15N chemical shift (δ 111.8 relative to 15NH4+) of the proton-bearing nitrogens is characteristic of a ‘pyrrole’ type, and the long range coupling 1H chemical shift correlation (2D) spectrum showed that, on the NMR time scale, the imino hydrogens are localised on five-membered rings that are not in the σv plane. Taken together these data prove that the fine structure of m-THPC is correctly represented with the imino hydrogens placed on opposite rings which are not reduced, as shown in structure 4.