Quantum Chemistry Calculation And Spectroscopic Studies Of Tetraazaporphyrins

Porphyrins and their derivatives have been extensively studied due to their participation in many biological processes. Tetraazaporphyrin is formed when the four meso-methine groups of normal porphyrins are replaced by aza nitrogens. Tetraazaporphyrins have a variety of technical applications. The spectroscopic and quantum chemistry computational studies of tetraazaporphyrins are expected to lead to a better understanding of their structure-property relationship and thus provide theoretical basis for their various applications. In this thesis, by using density functional theory calculations and spectral measurements, we investigated the ground-state structures and properties of protonated tetrabenzotetraazaporphyrin (phthalocyanine), the vibrational spectra of tert-butyl substituted metallo-tetraazaporphyrin (MT(tBu)TAP), as well as the molecular structures and the electronic spectra of hydrogenated tetraazaporphyrins. Another part of this thesis is the theoretical study about the nuclear-spin-induced magneto-optical effects. An expression is derived for nuclear-spin-induced optical Cotton-Mouton (NSIOC-M) effect for molecules with the prepolarized nuclear magnetic moments and correlated it with the molecular structures. The thesis is divided into five chapters.In Chapter I, we introduced the basic principle of quantum chemistry calculation, Raman scattering, and molecular polarizability. The research developments of tetraazaporphyrins are reviewed in this chapter, and the main contents of the thesis is also introduced briefly.Despite of numerous experimental efforts on the protonation reaction of phthalocyanines in acidic media, the site and the degree of protonation of the free base phthalocyanine have not been well understood. In Chapter II, we studied the structures and relative energies of twenty-two N-protonated species of the free base phthalocyanine (H2Pc) with the density functional theory at the B3LYP/6-31G(d) level of theory. It was revealed that the inner protonation at the isoindole-nitrogen atoms causes significant out-of-plane deformation of the macrocycle, mainly due to the steric hindrance of the central cavity and mutual repulsion with other H atoms. The outer protonation at the meso-nitrogen atoms remains planar configuration and is energetically more favorable than the inner protonation at the isoindole-nitrogen atoms. Among the studied twenty-two protonated species, the most stable one is H6Pc4+(IS1), for which all the outer meso-nitrogen atoms are protonated. TDDFT calculations have been performed for selected species (H2Pc, H4Pc4+(IS4) and H6Pc4+(IS1)), and the results are consistent with the optical spectroscopic and the hole-burning experiments and may be helpful in clarifying the long-term dispute about the sites and degrees for H2Pc protonation.In Chapter III, we studied the infrared absorption and 514.5nm excited Raman spectra for the metallo-tetra-(tert-butyl)-tetraazaporphyrin (MT(tBu)TAP, M=Cu, Co, Ni, Zn). The ground-state structures and vibrational spectra of MT(tBu)TAPs have been calculated at the B3LYP level of theory. The observed Raman and IR bands have been assigned based on the calculation results and by comparing with the normal metalloporphyrins. The relationship between the Raman/IR frequencies and the structures of TAP ring was investigated. The results show that the frequencies of CβCβ'stretch (Ag, v2), asymmetric CaNm stretch (Ag, v19), and symmetric CaNm stretch (Bg, v28) modes increase linearly with the decrease of the core-sizes of TAP ring. The v28 and v19 modes are more sensitive with respect to the ring expansion in comparison with the v2 mode. Our results correlate the vibrational spectra of tetraazaporphyrins with their molecular structures and will be useful for their further applications.In Chapter IV, we investigated the ground state structures and electronic spectra for the cobalt, nickel, copper, and zinc complexes of tetraazachlorin and tetraazabacteriochlorin with the density functional theory. The corresponding metal complexes of chlorin and bacteriochlorin were also studied for comparison. Seven of the total sixteen studied species were found to display non-planar ruffling deformation. It was revealed that meso-tetraaza substitution tends to reduce the degree of the deformation while the hydrogenation of the Cβ=Cβbond tends to increase it. The cause of ruffling deformation is considered to be the mismatching of the size of the porphyrin coordinating hole with the metal ions. The electronic spectra of nickel and zinc complexes were calculated with TDDFT. The results indicate that the meso-tetraaza substitution and the hydrogenation of the Cβ=Cβbond make the Q band blue-shift while the B band red-shift and enhance the absorption intensity of the Q band. Hydrogenated porphyrins play an important role in the second generation PDT. The theoretical investigation on the hydrogenated TAPs will be helpful in understanding the structure-function relationships for these novel compounds.Savukov et al. reported the detection of nuclear magnetic resonance (NMR) signals using nuclear-spin optical rotation (NSOR) method. However, this brand new method can't give information on molecular structures. In Chapter V, we theoretically studied the relations between NMR signals detected by nuclear-spin-induced Faraday effect and Cotton-Mouton effect for molecules in fluids and nuclear spin-spin coupling tensors. An expression is derived for nuclear-spin-induced optical Cotton-Mouton (NSIOC-M) effect for molecules with the prepolarized nuclear magnetic moments. Nuclear-spin-induced optical Cotton-Mouton effect in fluid of molecules with the prepolarized nuclear magnetic moments consists of two parts. The temperature dependent part is direct proportional to the product between the nuclear spin-spin coupling tensors and anisotropy of molecular polarizabilities. The magnitude of NSIOC-M effect is estimated and for some molecules with a large polarizability anisotropy, such as conjugation molecules, new effect might be detectable. Different from NSOR, NSIOC-M effect can give information on molecular structure parameters in fluids, such as bond lengths and angles and thus provide a new spectroscopic tool for the investigation of molecular structures.