Bond Dissociation Energies and Electronic Structure of Small Transition Metal and Lanthanide Molecules

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In this project funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Chemistry Division, Professor Michael Morse of the University of Utah is using sophisticated laser techniques to measure the amount of energy that is required to break chemical bonds. The measured bond dissociation energies are key to determining whether a reaction can proceed spontaneously or not. Although some of these bond dissociation energies have been measured previously, the associated measurement errors are quite large. The PI has developed new methods that reduce the measurement errors by factors of 10 to 100, thereby establishing bond dissociation energies to unprecedented accuracy. The species that are amenable to these studies are small molecules containing transition or lanthanide metals, which are particularly difficult to treat by computational methods. The new data obtained by the PI will provide a useful reality check, allowing computational chemists to determine what methods are suitable for modeling of these difficult but important species. The broader impacts of this work include developing a systematic database of bond dissociation energies, providing the data necessary to benchmark computational methods for the transition and lanthanide metals, and providing opportunities for the training of students in the design and construction of advanced experimental instrumentation and spectroscopic data analysis. Students working on these projects develop strong problem-solving skills, which are transferable to many different arenas of scientific endeavor. In an outreach effort to the local community, Dr. Morse continues to give twice-yearly workshops for high school Advanced Placement (AP) chemistry teachers.The project focuses on diatomic and triatomic molecules that possess a high density of electronic states at the ground separated fragment limit. Specific molecules include MX species, where M is an open d-subshell transition metal or lanthanide, and X is a p-block element or ligand, such as B, C, N, O, Si, P, S, Se, F, Cl, Br, CN, or CH. The molecules are formed by laser ablation of the metal in a carrier gas stream containing a source of the ligand. The molecules are cooled by supersonic expansion into vacuum and probed by resonant two- or three-photon ionization schemes. Due to the exceptionally high density of states in these metallic systems, there is a plethora of intersecting potential energy curves at the energy of the ground separated atom limit. When the molecule is excited below this limit, a dense quasicontinuous vibronic spectrum is recorded. When the molecule is excited above the ground separated atom limit, however, spin-orbit and nonadiabatic interactions enable it to hop from curve to curve and find its way to separated atoms on a subnanosecond to few-nanosecond time scale. The abrupt drop in ion signal is readily identified as the bond dissociation energy. In an extension of the method, a cryocooled ion trap photodissociation instrument will be employed, using similar methods to precisely measure bond dissociation energies of metal-containing cations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.