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, σ_v, 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 σ_v 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.