High-resolution probes of electronic states: Magnetic circular dichroism spectroscopy and electron density distributions by x-ray diffraction.
AuthorCarducci, Michael David.
Committee ChairEnemark, John
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractMagnetic Circular Dichroism spectroscopy (MCD) has been used to assign the optical spectra in high and low symmetry molybdenyl complexes. High resolution single crystal polarized absorption spectra of (PPh₄) (MoOCl₄) and (AsPh₄) (MoOCl₄(H₂O)) show two ligand field transitions whose polarization, MCD and vibronic fine structure are compatible with previously accepted assignments. In the case of (AsPh₄) (MoOCl₄(H₂O)), the splitting of the ²E by spin-orbit coupling is 810(2) cm⁻¹. The absorption and MCD spectra of LMoOX₂ complexes (where L is hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X = O, Cl or S) can be interpreted within the same framework of ligand field and ligand π to metal transitions adopted for (AsPh₄) (MoOCl₄(H₂O)). For LMoO²⁺ complexes of ⁻SCH₂CH₂S⁻ and toluene-3,4-dithiolate, the features between ∼9000 and 16000 cm⁻¹ are assigned to transitions from sulfur out-of-plane π orbitals to d(xy) and overlapping ligand field bands. The pattern of intense MCD peaks between 17,000 and 35,000 cm⁻¹ is assigned to transitions from out-of-plane sulfur π orbitals to molybdenum d(xz,yz} orbitals. On the basis of these molybdenyl model complexes, an interpretation is proposed for the recently reported MCD spectra of DMSO reductase. The suitability of variable temperature MCD for identifying contributions due to paramagnetic molybdenum(V) in the presence of diamagnetic porphyrin centers was investigated. For a molybdenum(V):free-base porphyrin ratio of 5:1, the Mo(V) MCD could be clearly identified. In a sample of sulfite oxidase which contained about 25% Mo(V), the temperature dependent MCD signal was not significantly above background noise and not assignable as arising from Mo(V). The electron density distributions of two compounds were measured by high resolution, low temperature X-ray diffraction. As a working example of the technique, the deformation density of oxalic acid dihydrate was redetermined and compared to multiple reports. Positional and thermal parameters are in excellent agreement while deformation density and multipole populations differ, but are found to be within the ranges reported. Using the same techniques, the electron density of the macrocyclic thioether 1,4,8,11-tetrathiacyclote adecane (TTCD) was determined at 100 K.