This dissertation is concentrated on the theoretical studies of pyrrole-based macrocycles by using quantum mechanics and molecular mechanics methods. Several systems, that are, neutral metal complexes of porphyrin isomers, heteroporphyrins, calixarenes, and calixpyrroles are investigated. 44...[ Read more ]
This dissertation is concentrated on the theoretical studies of pyrrole-based macrocycles by using quantum mechanics and molecular mechanics methods. Several systems, that are, neutral metal complexes of porphyrin isomers, heteroporphyrins, calixarenes, and calixpyrroles are investigated.
In chapter 2, systematic theoretical studies of the geometrical features, metal binding properties and electronic features of a variety of neutral metals (Ni, Cu, Zn, Cd, Mg, Ca) complexes of porphyrin isomers are presented. For various metals, the parent metal-porphyrin-( l,l,l,l) is the most stable. It has been found that the N4-core size of the porphyrin isomers is an important factor for the metal-binding stabilization. Generally, large-sized metals prefer to bind large N4-core ligands; while small-sized metals favor to bind small N4-core macrocycles. Natural bond orbital analysis shows that the electron configuration of Ni, Cu, Zn, Cd are d[superscipt 8], d9, d10, and d10, respectively. Moreover, the redox behaviours of these metal porphyrins can be qualitatively predicted. In chapter 3, the β-octasubstituent effect on the Z/E preference of metalloisoporphycenes is studied. Our study showed that the introduction of eight β-ethyl groups would destabilize the (Z)-isomers more than the (E)-isomers in each case of metal complexes. The effect is qualitatively correlated to the size of the metal cation: Ni > Cu > Zn > Pt. In chapter 4, our study is focused on the structural properties, aromaticity, and two electron oxidation potentials of heteroporphycenes (N,O,S). According to our calculated results, heteroatoms prefer to occupy the trans-position in the N4-core of porphycene either in 20 π-electron or 18 π-electron structures. Calculated NICS values demonstrate that all 18 π-electron structures are aromatic that obeys the Huckel 4n+2 rule. Meanwhile, most 20 p-electron structures are predicted to be anti-aromatic. Some exceptional cases are also found owing to the non-planarity of the structures. Frontier orbital energy analysis shows that Pyc-S4 is difficult to be oxidized. However, structures 7a, 10a and 13a are easier to be oxidized to their 18 π-electron structures. The prediction of UV-Vis spectra indicates that 4b, 7b, l0b, llb, 12b and 13b have considerable absorption near IR region ([is more than]700nm).
Another objective of this thesis is to investigate the conformational features and binding properties of calixpyrrole and its related structures. In chapter 6, a comparison of conformational features of calixaromatics including parent calix arene, O-tetramethyl calixarene, calixbenzene, calixfuran, calixthiophene, and calixpyrrole is carried out. Results show that calixarene favors the cone conformation due to four hydrogen bonds. On the contrary, calixfuran, calixthiophene, and calixpyrrole all prefer a 1,3-alternate conformation, while the cone conformation is least stable. In chapter 7, the conformational feature and anion-binding properties of calixpyrrole in the gas phase and dichloromethane solution are studied. The stability sequence is predicted to be 1,3-altemate [is more than] partial cone [is more than] 1,2-alternate [is more than] cone either in the gas phase or in CH2C12 solution. Anion binding analysis reveals that 1:1 calixpyrrole-F- binding pattern with the cone structure is more favourable than 1:2-pattern with the 1,2-alternate structure. N-H---F- hydrogen bond strength and anion binding energies are also established in the work. Finally, the β-substituent effects on the conformational preference and anion-binding abilities of three series of substituted calixpyrroles are investigated. It reveals that the introduction of electron-withdrawing groups at β-positions enhances the anion binding of calixpyrroles.