||The goal of this M.Phil. project is to study the vibrational and electronic properties of three types of metalloporphyrins (MP) in their ground electronic state by means of Fourier Transform (FT) Raman scattering and infrared absorption sprctroscopies and by vibrational analysis. The metalloporphyrins being studied are (a) free-base and metallo-meso-tetraphenylporphyrins (MTPP : M=2H+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Pd2+, and Pt2+) and the four isotopomers of NiTPP, (b) free-base and metallooctaethyl-porphyrins (MOEP : M=2H+, Fe2+, Fe3+, Co2+, Ni2+(tetragonal), Ni2+(triclinic), Cu2+ and Zn2+), and (c) free-base and metallo-etioporphyrin- Is (MEtioP-I : M=2H+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+). In addition, free-base and Ni2+ complex of porphine, the simplest porphyrin ligand without any peripheral substituents, are studied to explore the similarity and contrast between the ground electronic states of the two apparently symmetrically different compounds. In chapter 2, the principles of the methodologies used in this study are briefly summarized to facilitate the understanding of the advantages and limitations of the approaches adopted in this project. Chapter 3 reports a systematic study of NiTPP and its pyrrole-15N4, meso-13C4, pyrrole-d8 and phenyl-d20 isotopomers. A complete record of the isotope shifts make it possible to carry out a detailed analysis of the normal mode characters of the observed peaks. Several unresolved problems are discussed on the basis of the vibrational analysis. The problems discussed include : (1) the locations of the phenyl internal vibrational modes in both IR and Raman spectra; (2) how phenyl motions are coupled with the porphyrin skeletal motions; and (3) the identification of the candidate peaks for the porphyrin skeletal out-of-plane modes. Chapter 4 summarizes the vibrational study of the first- and second-row transition metal complexes and free-base TPP (MTPP : M=2H+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Pd2+, and Pt2+). The purpose of this study is to explore the nature of metal to porphyrin bonding, and the dependence of porphyrin skeletal modes on the properties of metal ions such as ion radii, charge, etc. The perturbation to the effective symmetry of the ground electronic state by the binding of metal ions is discussed by comparing Raman spectra of free-base versus metalloporphyrins. In chapter 5, two important questions of OEP system are addressed by studying their Raman and IR spectra. The first question is the structural polymorphism and its manifestation in vibrational spectra. This question is carefully studied by identifying the crystal dependent vibrational modes of NiOEP in two different crystalline forms, tetragonal and triclinic. The second studies the isotope (meso-d4) shifts of the structural sensitive modes. Chapter 6 presents the dependence of OEP skeletal modes on the transition metals, and the influence of metals binding on the electronic ground state of free-base OEP. The systems include free-base as well as the complexes of Fe2+, Fe3+, Co2+, Ni2+(tetragonal), Ni2+(triclinic), Cu2+ and Zn2+. Last but not the least, chapter 7 includes a summary of Raman and IR spectra of MEtioP-I (M=2H+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+). This study is aimed at complementing the discoveries and conclusion discussed in chapter 6. In particular, we found that (a) MEtioP-Is bear the most similarity to their OEP counterpart in terms of vibrational normal mode characters properties, (b) the effective symmetry of the ground electronic state of MEtioP-I is similar to that in free-base form. To confirm the conclusions drawn from the comparative study of free-base versus metalloporphyrins reported in chapter 6 and 7, I also conducted a study on free-base porphine and its Ni(II) complex. The results are summarized in the Appendix.