DFT and HF Studies of the Geometry, Electronic Structure, and Vibrational Spectra of 2-Nitrotetraphenylporphyrin and Zinc 2-Nitrotetraphenylporphyrin

The ground-state geometries and electronic structures of 2-nitrotetraphenylporphyrin (H 2 -2-NO 2 -TPP) and zinc 2-nitrotetraphenylporphyrin (Zn-2-NO 2 -TPP) with C s and C 1 symmetry have been determined from density functional theory, using the Becke-Lee-Yang-Parr composite exchange-correlation functional (B3-LYP) and ab initio RHF method and 6-31G(d) basis set. The optimized geometries are then compared with the crystallographic data of related compounds. The energy and electronic structures of different conformers are analyzed and compared with each other. The conformers with C 1 symmetry are found to be more stable than that of C s symmetry. The relative order of the highest occupied a 2 u and a 1 u orbitals determined by B3LYP (a 2 u > a 1 u ) is reversed by RHF (a 1 u > a 2 u ). The vibrational wavenumber, IR, and Raman intensities are also calculated at B3LYP/6-31G(d) optimized geometries. The calculated wavenumbers are scaled by a uniform scaling factor and compared with the experimental one. Most of the scaled modes are found to be in good agreement with the observed fundamentals. A single β-NO 2 substitution slightly changes the geometries, the vibrational wavenumbers, and the frontier orbitals energy level.