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Benzo-fused metalloporphyrins and metallotetraaza-tetrabenzoporphyrins : vibrational characterization by a local-to-global strategy

Authors Kong, Hippocrates H. K
Issue Date 1993
Summary The objective of this M.Phil. project is to study a very special class of metalloporphyrins, tetrabenzoporphyrins, which are formed by the fusion of benzo rings into the pyrrole-rings of normal porphyrin macrocycle, by means of Raman Scattering and Infrared absorption spectroscopies, and by vibrational analysis. The systems being studied are (a) Zn(II)-tetrabenzoporphyrins (ZnTBP) and its pyrrole-15N4, benzo-dl6 isotopomers, (b) Zn(II)-tetraphenyltetrabenzoporphyrins (ZnTpTBP) and its pyrrole-15N4, benzo-dl6 isotopomers, (c) metalloazatetrabenzoporphyrins or metallophthalocyanines (MPc: M=2H+, Mn++, Fe++, Co++, Ni++, Cu++, Zn[sperscript ++]), and (d) a series of indene, indole, benzofuran, thianaphthene, phthalic anhydride and its benzo-deuterated isotopomer. Chapter 2 summarizes briefly the principles of the techniques used in the study, and the methods by which the compounds were prepared. Chapter 3 reports a detailed Raman and IR study of ZnTBP and its pyrrole-15N4 and benzo-dl6 isotopomers. The main vibrational features in the spectra are assigned to the vibrations of isoindole unit of TBP and the meso-bridges. In particular, it was found that the in-phase CalphaCm bridge stretching mode of ZnTBP is located at ~1620 cm-1 and is much higher than that of normal porphyrins (~ at 1500 cm-l). This result is consistent with the observation from X-ray single crystal diffraction that CalphaCm bond in TBP (~ 1.34 ÅA) is shorter than that of normal porphyrins (~ 1.38 ÅA). Chapter 4 presents studies on ZnTpTBP. To identify the vibrational features contributed by phenyl substituents, a comparison study is made to that of Zn(II)-tetraphenylporphyrin (ZnTPP), the normal modes of which have been well established by other projects in this group. It is found that phenyl modes are quite localized and are decoupled with that of porphyrin skeletal modes. The unique feature of ZnTpTBP is that its macrocycle is significantly ruffled. As a consequence, π-conjugation macrocycle is affected and the stretching frequencies assignable to the CalphaCm bridge vibrations shift down quite a lot. The accurate determination of isotope shifts facilitate greatly to the main characters of porphyrin skeletal modes localized on the isoindole subunits. Chapter 5 summarizes a systematic vibrational study on MPc (M=2H+, Mn++, Fe++, Co++, Ni++, Cu++, Zn[sperscript ++]). Two questions are addressed in this study. The first question is the attribution of the normal modes to the vibrations of aza-bridges and isoindole rings, based on 15N isotope shift in free base Pc established in this group (Li et al, unpublished results). The second question is the dependence of Pc skeletal modes on the properties of transition metal ions such as ionic radii. The effect of aza substitution at the bridge on the electronic structure of the macrocycle is discussed. In the last chapter, Chapter 6, Raman and IR spectra of several small compounds are recorded. The vibrational characters of the main normal modes are assigned on the basis of AM1 quantum mechanical calculation. The study of all these small compounds is based on the observation that vibrations of all tetrabenzoporphyrins can be divided into those localized on isoindole subunits, and those of aza-bridges. We, thus, have adopted a divide-and-conquer approach to complementing the analysis made for TBPs and Pcs. This strategy of local-to-global analysis is based on the idea that the vibrations of "local" subunits have to be adapted to the "global" symmetry of the whole molecule. Indeed, a detailed analysis of calculation results reveals that the main features of fused Six-Five ring system are all similar to each other both in terms of frequency and cartesian displacements.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 1993
Language English
Format Thesis
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