Frequencies of the NH vibrations and macrocycle structure of sterically distorted porphyrins

Abstract

The behavior of the bands of the NH stretching vibrations of sterically distorted porphyrins whose macrocycles are undergoing different types of deformation (saddling, elongation, waving, etc.) is studied in CHCl3 and CCl4 solutions. The geometry of sterically distorted porphyrins and the frequencies of their normal vibrations are calculated in terms of the density functional theory. The relative strength of intramolecular hydrogen bonds is estimated in terms of the theory of natural (localized) orbitals. The frequency of the stretching NH vibrations of porphyrins with saddling, elongation, or waving deformation of their macrocycles is shown to be inversely proportional to the energy of stabilization E (2), arising due to the charge transfer from the orbitals of unshared electron pairs of the nitrogen atoms of the pyrrolenyne rings to the antibonding σ*NH orbitals. It is shown that this frequency can serve as a measure of the strength of intramolecular hydrogen bonds. A specific feature of the ruffling deformation of the porphyrin macrocycle is that the strengthening of the hydrogen bonds (due to a decrease in the size of the internal porphyrin window) is accompanied by a shortening of the NH bonds, whereas, depending on the magnitude of the ruffling, the frequency of the stretching NH vibrations decreases or increases. This phenomenon is associated with the fact that the shortening of the NH bond facilitates a decrease in its effective van der Waals radius and weakening of destabilizing interactions of the σNH orbitals.