David M. Bartels
Hope College, Holland, Michigan, B.A. (1977)
Northwestern University, Evanston, Illinois, Ph.D. (1982)
Hope College, Holland, Michigan, B.A. (1977)
Phone: (574) 631-5561
Office: 203D Radiation Research Building
Fast Kinetics of Radiation-Initiated Chemistry
Fast Kinetics of Free Radical Reactions
Free radicals are generated in virtually all radiation-initiated processes, and are responsible for most of the permanent chemical changes. The recombination reactions are often diffusion limited or nearly so, but also depend on pairing of spin to produce stable singlet products. This gives rise to the fascinating Chemical Induced Dynamic Electron Polarization (CIDEP) phenomenon in their time-resolved EPR spectra, and Chemically Induced Dynamic Nuclear Polarization (CIDNP) in NMR spectra of the recombination products, where some lines appear with negative phase due to population inversions.
Reaction Rates of the Hydrated Electron
When electrons are injected into water by ionizing radiation, a large fraction of them become "hydrated", as the water dipoles arrange themselves to stabilize the negative charge. The resulting hydrated electron is easily measured by its strong optical absorption in the red, and many of its reaction rates have been measured. However, these reactions are not well described by any established theory of reaction rates (e.g. Marcus Theory for electron transfer). The challenge is to work out why and find a modified theory with predictive power.
Solvent Effects on Reaction Rates in Supercritical Water
Supercritical water is proposed as the coolant for efficient Generation-IV nuclear reactors, and is the medium for an important advanced oxidation technology for hazardous waste destruction. The properties of water change dramatically in the supercritical region as the water density changes continuously between zero and 1 g/cc. The primary free radicals in water - hydrated electrons, H atoms, and OH radicals - are respectively ionic, hydrophobic, and dipolar, providing opportunity to investigate nearly all possible solvent effects using radiolysis excitation. Many strange effects are being found, such as rate constants that decrease as the temperature is raised.
Three mechanisms can be postulated for the larger corrosion rates observed in nuclear power plants relative to non-irradiated systems. (a) The water is irradiated to produce products like H2 and H2O2 which will change the electrochemical corrosion potential. (b) The surface metal-oxide layer is directly excited to cause "photochemical" corrosion. And (c) the neutron field produces displacement damage to accelerate movement of oxygen ions through the surface oxide to the neat metal below. Present research funded by the nuclear industry is aimed at sorting out the relative importance of these various effects in hopes of finding new mitigation strategies.
Hydrated Electron Electron Structure
Solvation "structure" of the hydrated electron has been a subject of hot debate since its discovery in 1962. The most recent controversy has raged over whether the electron primarily exists in a "cavity" between water molecules, or is solvated by a densified patch of solvent. In collaboration with Kumar and Sevilla of Oakland University, we recently found that virtually all of the hydrated electron properties could be reproduced using a minimal four-water ab initio model in dielectric continuum, where one bond of each water molecule points toward a central void. Given the large spin density on the water molecules, the ab initio model is much better described as a "multimer solvent anion" than as an "electron in a solvent cavity".
Hyperfine Coupling of the Hydrogen Atom in Water
High precision measurement of hydrogen atom EPR splitting in water shows its hyperfine coupling is about one ppt below the vacuum hfc, and moves further away from the vacuum value at higher temperature. Eventually at ca. 250oC, the hfc turns around and heads back toward the vacuum value. A simple model explains this in terms of the frequency of collisions between H and the water molecules. However, very high level ab initio calculations predict the opposite behavior, that the hfc in water should be higher than the vacuum value. Experiment and theory seem to be at an impasse on this supposedly simple problem.
Small Free Radical Recombinations in High Temperature Water
Near room temperature, recombination of small free radicals like H and OH are nearly diffusion limited in aqueous solution, i.e. once they meet their reaction is certain. We have been surprised to learn that above about 200C, "barrierless" reactions involving H and OH are no longer limited by diffusion. Diffusion becomes so fast that the solvent "caging effect" fails to average over all possible angles of approach, and the reaction rate is limited by a "steric effect." We showed that for recombination of hydroxymethyl radicals, the diffusion limit is not even reached at room temperature. The great surprise has been that the rates measured in water, where hydrogen bonding was assumed to be important, are identical to the "high pressure limit" rate in the gas phase. Water is "merely" a very effective third body for energy transfer.
Modeling of Nuclear Reactor Chemistry
Surprisingly, the radiation chemistry occurring in nuclear power reactors has not been successfully modeled until recently. A review of all reaction rates and radiolysis product yields was prepared in collaboration with John Elliot of Atomic Energy of Canada in 2008, which included all of the new high temperature information generated in our laboratories. Simulation of the "Critical Hydrogen Concentration" or excess added hydrogen needed to suppress radiolysis in the reactor cores was still not successful. Additional experiment and modeling shows that radiolysis yields due to neutron radiation has not been correctly measured in laboratory experiments. This key missing information is now a primary target of research.
Walker, J.A., S.P. Mezyk, E. Roduner, and D.M. Bartels. "Isotope Dependence and Quantum Effects on Atomic Hydrogen Diffusion in Liquid Water." Journal of Physical Chemistry B 120 (2016): 1771-1779. link
Sterniczuk, M., and D.M. Bartels. "Source of Molecular Hydrogen in High-Tempurature Water Radiolysis." Journal of Physical Chemistry A 120 (2016): 200-209. link
Kumar, A., J.A. Walker, D.M. Bartels, and M.D. Sevilla. "A Simple ab Initio Model for the Hydrated Electron that Matches Experiment." Journal of Physical Chemistry A 119 (2015): 9148-9159. link
Kanjana, K., B. Courtin, A. MacConnell, and D.M. Bartels. "Reactions of Hexa-aquo Transition Metal Ions with the Hydrated Electron up to 300 degrees C." Journal of Physical Chemistry A 119 (2015): 11094-11104. link
Kanjana, K., J.A. Walker, D.M. Bartels. "Hydroxymethyl Radical Self-Recombination in High-Temperature Water." Journal of Physical Chemistry A 119 (2015): 1830-1837. link
Nuzhdin, K., D.M. Bartels. "Hyperfine Coupling of the Hydrogen Atom in High Temperature Water." Journal of Chemical Physics 138 (2013): 124503. link
Bartels, D.M., J. Henshaw, and H.E. Sims. "Modeling the critical hydrogen concentration in the AECL Test Reactor." Radiation Physics and Chemistry 82 (2013): 16-24. link