Doctoral Dissertations

Spring 1996

Dissertation

Chemistry

Degree Name

Doctor of Philosophy

A robust yet sensitive Q-band (35 GHz) cavity has been designed for routine variable temperature EPR (electron paramagnetic resonance) and ENDOR (electron nuclear double resonance) measurements down to 2 K. It consists of an aluminum or brass (plain, silver or gold plated) ribbon imbedded in a cylindrical epoxy or epoxy/quartz composite with a tunable piston at the bottom. The cavity has all the advantages of the traditional silver wire-wound cavity often used for Q-band measurements but is much more robust and easier to construct. The cavity suppresses degenerate resonant modes and minimizes wall eddy currents induced by field modulation. With standard Varian Q-band modulation coils, a 100-kHz modulation field of 27 G peak-to-peak is obtained at the sample. The cavity Q$\rm\sb{L}$ ($\approx$2500) and weak pitch signal/noise ($\approx$1000:1) with a Varian/E-110 microwave bridge are comparable to those obtained with either a traditional wire-wound cavity or a cavity constructed with a thin silver wall on an epoxy/quartz substrate. A set of parallel posts along the axis of the cavity (bidirection) form the ENDOR coil.
Dinitrosyl iron model complexes with ligands of cysteine, mercaptoethanol, thioglycolic acid, ethanethiol, penicillamine and imidazole were studied by EPR and ENDOR spectroscopy. X-band and Q-band ENDOR measurements have been made at various fields across the EPR envelope at temperatures from 2 K to 100 K. We have assigned most of the $\sp1$H resonances with reasonable certainty. The data confirm the thiol groups binding in the complexes with mercaptan ligands at room temperatures, and suggest that a structural rearrangement occurs with dinitrosyl iron cysteine and penicillamine upon freezing the sample, in which case coordination of both the thiol and amino group takes place. The assignment of axial EPR spectra to thiol coordination in every instance has also been cast in doubt by our data.