Pros And Cons To The Formation Of Porphyrin Ring

The chemical shifts of peaks detected at δ –2.74 and –2.93 were attributed to the highly shielded inner N-H groups of porphyrin rings of TPP and TSPP, respectively. The pyrrolic N-H was strongly shielded by the induced ring current (delocalization of π electron ring), known as a diamagnetic anisotropic effect. However, the diamagnetic anisotropic effect caused adverse effect on β-H atoms at the periphery of the porphyrin ring, leading to a substantial shift of their signals in the 1H NMR spectra. As depicted in Figure 4.2, β-H showed a single peak resonance at ~δ 8.8 ascribed to the H atom strongly deshielded by the diamagnetic anisotropic effect. Furthermore, the eight β-H resonances gave rise to a singlet due to the symmetry and the planarity of the porphyrin macrocycle ring which rendered a magnetically and chemically equivalent environment to the β-H atoms of the porphyrin ring.

Consequently, the Ho’ and Hm’ of TPP showed multiple peak resonances at δ 8.20 and δ 7.79. The sulphonation of TPP was observed in the 1H-NMR spectrum of TSPP as shown in Figure 4.2, which displayed changes in the multiplet peaks to a doublet at Ho’ (8.20 ppm) and Hm’ (8.06 ppm). The substitution of sulphonate (SO3-) in the phenyl ring caused the coupling of neighbouring protons which improved the symmetrical environment of Ho’ and Hm’ giving rise to the doublet peak resonances. The 1H-NMR data of TPP and TSPP are tabulated in Table 4.1. Changes in the peak resonances demonstrated that sulphonation of TPP had occurred at meso-position of the TPP. Then, TPP and TSPP were characterized with 13C-NMR to further examine the sulphonation process of