Structural and Vibrational Properties of Tetraoxaporphyrin Dication, the Oxygen Analogue of Porphyrin, and of Isoelectronic Diprotonated Porphyrin

Abstract

Structural calculations by means of the density functional method have been performed on tetraoxaporphyrin dication and on isoelectronic diprotonated porphyrin as well as on the sulfur and carbon analogues of porphyrin. A detailed study of the stable conformations of these compounds is reported starting with the most symmetrical conformations and lowering the symmetry along the vibrational coordinates with imaginary frequency. The calculated geometries are related to experimental structures available from X-ray diffraction studies. The Raman spectra of tetraoxaporphyrin dication exciting with micro-Raman instrumentation at 785 nm and of diprotonated porphyrin in near-resonance conditions with the Soret band have been measured. The correlation between frequencies calculated with the DF/B3-LYP/cc-pVDZ procedure for porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication has allowed for making a vibrational assignment for the latter two systems in excellent agreement with experiment using a single frequency scale factor.

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Other two stable conformers with approximate C 2h and C s symmetry, the so-called quasi-wave and quasi-ruffled isomers [285], have been found by calculations [275,286] ≈ 5 kcal/mol above the D 2d conformer. In qualitative discussions on porphyrins the concept of main conjugation path embedded in the macrocycle and responsible of the electronic absorption has been advanced [287,288].…”

“…Tetraoxaporphyrin dication, the oxygen analogue of porphyrin, has the highly symmetrical D 4h structure [272,273] which may be modulated through five redox stages [274]. The structural parameters of the three molecular species have been the object of a large number of computational studies all of which indicate that the stable forms of porphyrin and tetraoxaporphyrin have D 2h and D 4h geometries, respectively, when electron correlation is taken into account [275][276][277][278][279][280][281][282]. The experimental [272,283,284] and calculated [275] bondlengths of the three macrocycles are reported in Figure 45.…”

“…In the inset the experimentally observed spectrum of PH A large amount of vibrational data have been acquired on porphyrin from normal and resonance Raman [290,291], infrared [292] and fluorescence/phosphorescence [293] spectroscopies and the assignment of the fundamental modes has been established in great detail [290,294]. The infrared and Raman spectra of the tetraoxa-and diprotonated porphyrin have been equally obtained and completely assigned making use of ab initio calculated frequencies and intensities and correlation with porphyrin [275]. The near resonance Raman spectra of porphyrin and diprotonated porphyrin excited at 406.7 nm and the normal Raman spectrum of tetraoxaporphyrin exciting at 785 nm are reported in Figure 46 and Figure 47, respectively.…”

Theoretical Study on Resonance Raman Spectra of Tetraoxaporphyrin Dication by TDDFT Calculation

“…The discovery of TOPs also opens the door to synthesize other porphyrin-like tetrafuran macrocycles [2−4]. For these reasons, various spectroscopic experiments have been conducted to study the ground and excited state properties of TOPs [1][2][3][4][5][6]11]. The dicationic TOP 2+ is the isoelectronic analogue of normal closed-shell metalloporphyrins (MPs), and it shares many similarities in UV-visible absorption spectra with MPs [1,6,11].…”

“…DFT calculations have been extensively used to study the structures as well as the electronic and vibrational spectra of porphyrin-related compounds. Previously, Jelovica et al have studied IR absorption and offresonance Raman (RR) spectra of TOP 2+ and proposed the assignments of the observed vibrational bands with the assistance of DFT calculations [5]. On the other hand, the resonance Raman scattering (RRS) of TOP 2+ has been measured [11] but not been theoretically studied yet.…”