Evidence for unusually strong intramolecular hydrogen bonding in highly nonplanar porphyrins

The 1H NMR spectra of the 5,10,15,20-tetraalkylporphyrins 2–4 reveal a large downfield shift of the signal for the NH protons and a reduction in the activation energy for NH tautomerism as the alkyl substituents become larger; these unusual changes can be rationalized in terms of greatly enhanced intramolecular hydrogen bonding of the NH protons as the bulkier substituents distort the macrocycle from planarity and contract the porphyrin core.