The Center for Simulation and Modeling (SaM) at the University of Pittsburgh is dedicated to supporting and facilitating computational-based research across campus. SaM serves as a catalyst for multidisciplinary collaborations among professors, sponsors modeling-focused seminars, teaches graduate-level modeling courses and provides individual consultation in modeling to all researchers at the University. Our areas of research include: energy and sustainability, nanoscience and materials engineering, medicine and biology, and economics and the social sciences.
SaM Researchers in the News
We review the role that gas-phase, size-selected protonated water clusters, H+(H2O)n, have played in unraveling the microscopic mechanics responsible for the spectroscopic behavior of the excess proton in bulk water. Because the larger (n ≥ 10) assemblies are formed with three-dimensional cage morphologies that more closely mimic the bulk environment, we report the spectra of cryogenically cooled (10 K) clusters over the size range 2 ≤ n ≤ 28, over which the structures evolve from two-dimensional arrangements to cages at around n = 10. The clusters that feature a complete second solvation shell around a surface-embedded hydronium ion yield spectral signatures of the proton defect similar to those observed in dilute acids. The origins of the large observed shifts in the proton vibrational signature upon cluster growth were explored with two types of theoretical analyses. First, we calculate the cubic and semidiagonal quartic force constants and use these in vibrational perturbation theory calculations to establish the couplings responsible for the large anharmonic red shifts. We then investigate how the extended electronic wave functions that are responsible for the shapes of the potential surfaces depend on the nature of the H-bonded networks surrounding the charge defect. These considerations indicate that, in addition to the sizable anharmonic couplings, the position of the OH stretch most associated with the excess proton can be traced to large increases in the electric fields exerted on the embedded hydronium ion upon formation of the first and second solvation shells. The correlation between the underlying local structure and the observed spectral features is quantified using a model based on Badger’s rule as well as via the examination of the electric fields obtained from electronic structure calculations.
Dehydration reactions play an important role to convert biomass-derived alcohols (e.g. ethanol) to value-added chemicals (e.g. ethylene, an important building block for the production of polymers). Dehydration chemistry on metal-oxide catalysts has been an area of research for more than half a century now, albeit, with contradictory results. Prof. Mpourmpakis’ group at Pitt developed a theoretical model based on quantum chemical calculations that relates the dehydration activity with key physicochemical properties of the metal oxides (catalysts) and the alcohols (reactants). These descriptors are the catalyst’s surface Lewis acidity (alcohol binding energy on the metals) and basicity (proton affinity of the surface oxygens or hydroxyl-groups) and the carbenium ion stability of the alcohols. The model’s predictions were further verified by dehydration experiments in Prof. Raymond Gorte’s lab at the University of Pennsylvania. The ramification of this simple, but yet very powerful model is that we can apply it to screen a variety of different alcohols and metal-oxide catalysts according to their dehydration activity, avoiding trial-and-error experiments in the lab.
Publication source: http://pubs.rsc.org/en/content/articlelanding/2014/cy/c4cy00632a