Theoretical study of subporphyrins

We have characterized the ground-state geometries, electronic structures and IR spectra of a series of contracted porphyrinoids termed subporphyrins with the aid of density functional theory calculations. The calculations indicate that all molecules are not flat with curvatures that depend upon the type of substituent atoms present. The metal-free (M{double bond, long}2H) subporphyrin molecule (1) is characterized by a low-energy intramolecular hydrogen bond (N-H⋯N) and its HOMO-LUMO energy gap is ∼0.55 eV larger in magnitude than that of the metal-free porphyrin homologue. A minimal model of the frontier orbitals of 1 comprises two pair of nearly-degenerate HOMOs and LUMOs.