Experimental and Theoretical Studies of Paramagnetic Transient Reaction Intermediates

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Our research program involves the study of the formation mechanisms, structure-bonding relationships, properties and chemical reactivity of selected paramagnetic reaction intermediates. They are sulfur-nitrogen heterocycles and organometallic transition metal half-sandwich complexes. These intermediates are fundamental building blocks that react to form important new magnetic, conducting and spin-electronic materials. We would like to understand how their structure and bonding influences their magnetic, electronic, chemical and physical properties. This will be done by generating the intermediates and determining their experimental parameters, such as hyperfine (A) and g tensors by electron paramagnetic resonance (EPR), vibrational frequencies by Raman and infrared (IR) spectroscopy and their electronic excitation energies by ultraviolet-visible (UV-VIS) spectroscopy. These parameters will also be calculated by hybrid density functional, CAS-SCF or configuration interaction (CI) methods. The agreement between the computed and experimental parameters results in a comprehensive clear bonding picture from which concrete conclusions are made about the role of the intermediate in the mechanism of the overall formation reaction. We have successfully used this technique in the past. Organic semiquinone free radical intermediates that are involved in single electron transfer reactions of biological importance will also be studied by the same methods. When the reactive intermediates are short-lived, they will be trapped in rare-gas matrices at low temperatures (8-12 K). By matrix-isolating them, their experimental parameters are determined, via EPR, IR and UV-VIS spectroscopy, without the necessity of expensive ultra-fast techniques. EPR spectroscopy is our most important tool and we will continue to develop new methods of improving our home-built pulsed and conventional multi-frequency (4.5 - 35.0 GHz) EPR spectrometers. These include new pulsed electron-spin-echo techniques, phase-modulation detection and dielectric resonator probes that increase the spectrometers' sensitivities by a factor of 30. This will enhance their spectral resolution and the detection of smaller amounts of intermediates.