A computational and theoretical study of the molecular, vibrational, and electronic structures of metalloporphyrins

This dissertation explores the molecular, vibrational, and electronic structures of metalloporphyrins (MPs) by using computational and theoretical methods. Nonplanar distorted porphyrin macrocycles have been observed in many proteins, including the cytochrome c and the peroxidases. Interestingly, the nonplanar distortions were found to be conserved for proteins of the same type, but from different species. In order to effectively study the nonplanar distortion of MPs, a reliable classical force field is desired.; A new porphyrin force field (HBCV FF) is presented to study the structures and vibrational spectra of substituted Ni porphyrins. The HBCV FF is a hybrid force field, which treats the porphyrin macrocycles with an accurate Hessian-biased (HB) force field derived from DFT and experimental IR and Raman spectra using NiP as a model. The substituents are treated by the commercial consistent valence (CV) force field. The HBFF includes extra 1,3 interactions to account for longer range interactions in the porphyrin macrocycle and thereby improve the vibrational frequency calculations (Chapter 3). To further improve the vibrational accuracy, the stretching force constants are allowed to respond to bond length changes. The HBCV FF accurately reproduced the geometries and vibrational frequency shifts of three different conformers of Ni(II) octaethyl porphyrin (Chapter 5). It also calculated the structures of four Ni(II) tetra-alkyl substituted porphyrins (NiTAPs) with high fidelity, but could not accurately address vibrational shifts (Chapter 6). Careful examination of the results suggests that the problem is in merging of the macrocycle and substituent force fields.; Additionally, high level DFT was employed to study ruffling potential energy surfaces of four model metalloporphines (NiP, BeP, MgP, and ZnP) and their structure-sensitive modes (Chapter 4). The results show that the ruffled porphyrins were stabilized by metal-porphyrinato core interactions for all of the metal studies, but particularly strongly for Ni and Be. Extensive DFT studies of the NiTAPs were performed as well. The calculated geometries and vibrational shifts agreed well with the experimental results, validating DFT as an appropriate method for these types of studies.; In addition to the study of the molecular and vibrational structures of MPs, theoretical approaches are developed to examine the electronically excited wavefunctions. These studies lead to understanding of the resonance Raman excitation profile of NiP (Chapter 7) and electronic communication in porphyrin dimers used as molecular electronics models (Chapter 8).