Professors David Anderson of the University of Wyoming and Robert Hinde of the University of Tennessee are supported by the Experimental Physical Chemistry Division for a collaborative investigation of the infrared (IR) spectroscopy and chemistry of cryogenic molecular hydrogen solids containing open-shell atoms as substitutional impurities. Through a combination of experiment (Anderson) and first principles simulations (Hinde) the project goal is to use the IR spectroscopy to quantify how the presence of the hydrogen ?solvent? perturbs the electronic structure of the open-shell atom. Most chemistry occurs in the presence of a solvent and these fundamental studies are aimed at understanding how solvent effects are manifested in simple atomic radical reactions with molecular hydrogen. This will be accomplished by first developing a theoretical framework for understanding the solvent-induced perturbation of the electronic structure of chlorine atoms (Cl) trapped in solid hydrogen. The knowledge gained in these halogen atom studies will then be applied to oxygen atom (O) doped solid hydrogen to study solvent perturbations of the prototypical combustion reaction of O-atoms with molecular hydrogen. The goal of this research is to develop the spectroscopic analysis such that solid hydrogen doped with atomic radicals can be used as a new reaction medium to study the details of low temperature chemistry in a condensed phase.
Frozen molecular hydrogen is a unique crystalline solid that is distinguished as a quantum solid due to pronounced quantum mechanical effects of the hydrogen molecule at the low temperatures at which hydrogen freezes (T<13.8 K). These studies of chemical reactions in a quantum solid will have direct impact on our understanding of quantum mechanical effects in low temperature chemical reactions, the ability to use light to control chemical reaction pathways, and potentially provide transformative insight into both high gravimetric storage and clean production of molecular hydrogen. This research will also provide a rich vehicle for undergraduate and graduate research experience and training.
One of the most fundamental concepts in chemistry is the rate of a chemical reaction typically increases at higher temperature because the reactants can use the greater thermal energies to surmount the chemical barrier between reactants and products. This is why refrigeration is used to keep food fresh; at low temperature the chemical reactions that spoil food are much slower. However, at extreme low temperatures chemistry can still occur but now by a qualitatively new pathway, the reactants quantum mechanically tunnel through the barrier separating reactants from products, and this "under the barrier" chemistry has not been studied as extensively as high temperature chemistry. With support from the National Science Foundation, our group in the Chemistry Department at the University of Wyoming explores the low temperature limits of chemistry by conducting chemical reactions in the 1.7 to 4.3 K temperature range (4.3 K = -268.85°C). To accomplish this goal we form frozen molecular hydrogen ices that contain small concentrations of potential chemical reactants and use lasers to irradiate the ice sample to create highly reactive species such as atomic hydrogen. In this low temperature limit all reactions occur exclusively through tunneling, very different from traditional chemical reactions at elevated temperatures. We observe some reactions only start to occur below a sharply defined threshold temperature, which is opposite the behavior found at high temperature. In this work we develop the experimental methodology to measure these tunneling-mediated low temperature reactions and provide preliminary data on a number of reactions of atomic hydrogen with formic acid, nitrous oxide and methanol. This work tests our fundamental understanding of chemistry under these low temperature extremes and is relevant to the chemistry that occurs on small dust particles in the low temperature conditions of interstellar space. The large variety of molecules now detected in interstellar space, not thought to exist under the harsh conditions of space even just 30 years ago, likely have their origins from low temperature tunneling reactions on icy dust grains. This work helped support the research activities of four PhD students and two undergraduates. We have a long-standing collaboration with the theoretical chemistry group of Robert J. Hinde at the University of Tennessee, Knoxville who carry out quantum Monte Carlo simulations that can be used to model our experimental results, but also to make predictions on the properties of still unexplored low temperature chemical reactions.
