Characterization of Polynuclear Mixed-Valence Manganese Complexes as Models of the Water Oxidation Site in Photosystem II

Chang, Hsiu-Rong

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Description

Title

Characterization of Polynuclear Mixed-Valence Manganese Complexes as Models of the Water Oxidation Site in Photosystem II

Author(s)

Chang, Hsiu-Rong

Issue Date

1988

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Chemistry, Inorganic

Abstract

• In order to understand the electronic structure of the manganese water oxidation site of the photosystem II, a number of polynuclear mixed-valence manganese complexes are characterized by means of various physical methods such as electrochemistry, EPR spectroscopy and magnetic susceptibility measurements. Distributions in the weak antiferromagnetic interactions in two mixed-valence Mn$\sp{\rm II}$Mn$\sp{\rm III}$ complexes (LMn$\sb2$Cl$\sb2$Br) $\cdot$H$\sb2$O ($\sp\sim -2$cm$\sp{-1}$) and (LMn$\sb2$Br$\sb3$) $\cdot$1/2CH$\sb2$Cl$\sb2$ ($\sp\sim -1$cm$\sp{-1}$) together with distributions in single-ion zero-field interactions for these two Mn$\sp{\rm II}$Mn$\sp{\rm III}$ complexes lead to g- and A-strains which give broad signal in the EPR spectra. The antiferromagnetic interactions in (Mn$\sb2$(bpmp) ($\mu$-OAc)$\sb2$) (ClO$\sb4)\sb2$ ($-6$cm$\sp{-1}$) and (Mn$\sb2$(bcmp)($\mu$-OAc)$\sb2$) (ClO$\sb4)\sb2\cdot$1/2CH$\sb2$Cl$\sb2$ ($-7.7$cm$\sp{-1}$) result in well-resolved 29-line Mn hyperfine patterns in the g = 2 region at liquid-helium temperatures. Experimental and simulated X- and Q-band EPR spectra for these 29-line patterns indicate that they are due to only the z-component of the g tensor. The x- and y-components of the g-tensor are broadened to the point of not contributing to the observed EPR spectra. The differential broadening could be a reflection of g- and A-strain effects and increased nitrogen superhyperfine interactions in the g$\sb{\rm x}$ and g$\sb{\rm y}$ directions.
• The magnetochemistry for twelve more polynuclear manganese complexes are investigated. The data are all fit to the suitable theoretical models (equations) assuming pairwise isotropic magnetic exchange interactions. The antiferromagnetic exchange interactions for Mn$\sp{\rm II}$Mn$\sp{\rm III}$ pairs are quite weak ($\vert$J$\vert$ $<$ $\sp\sim$10cm$\sp{-1}$) relative to those reported for Mn$\sp{\rm III}$Mn$\sp{\rm IV}$ complexes (40 cm$\sp{-1}$ $<$ $\vert$J$\vert$ $<$ 220cm$\sp{-1}$). These results are applied to explain why the S$\sb2$ state of PSII gives rise to an EPR signal, whereas, the S$\sb0$ state does not. The relatively strong antiferromagnetic interaction in Mn$\sb3\sp{\rm III}$Mn$\sp{\rm IV}$ unit of the S$\sb2$ state could give a ground state fairly far removed in energy from excited states. On the other hand, the relative weak antiferromagnetic interaction in Mn$\sp{\rm II}$Mn$\sb3\sp{\rm III}$ unit of the S$\sb0$ state would lead to appreciable thermal populations in the ground state and excited states even at liquid-helium temperature.

Graduation Semester

1988

Type of Resource

text

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