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

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Professor Michael Morse of the University of Utah is using laser spectroscopy to study the electronic structure and chemical bonding in small molecules that contain d- and f-block metals, with emphasis on providing highly precise values of bond dissociation and ionization energies. These measurements will be useful for developing accurate and efficient computational methods for larger systems relevant to the catalytic and technological applications of the d- and f-block elements. Dr. Morse will examine the broad trends in chemical bonding as one moves across the periodic table in this work, and will investigate whether the dissociation energies of metals bonded to chemically related p-block elements, such as silicon and germanium, are strongly correlated. Professor Morse and his students will use resonant two-photon ionization spectroscopy to study neutral d- and f-block molecules, and cryo-cooled ion photodissociation experiments to study metal-containing cations. In addition to training the graduate students who are directly involved in this project, Dr. Morse also organizes a biannual workshop for high school chemistry teachers, where he presents on physical chemistry topics that they have selected. Other speakers also present on chemical education research and educational psychology. In this project, the Morse group will employ laser ablation of a metal sample in a pulsed supersonic expansion of helium with small amounts of a reactant gas to generate a molecular beam of the species of interest. They will then use a one laser pulse, which is scanned, to excite the molecule electronically and a second laser pulse to ionize the molecule. Ions will then be detected using time-of-flight mass spectrometry. Molecules with a high density of electronic states, such as the open d- and f-subshell molecules, fall apart on a subnanosecond time scale when they are excited above their dissociation energy, causing a sharp drop in ion signal. This provides a highly precise measurement of the bond energy. The Morse team is also able to use the high density of electronic states in these species to measure ionization energies to high precision. Employing a different instrument, the research team is using similar methods to measure the threshold for dissociation in a range of d- and f-block cryo-cooled ions. The bond dissociation energies of the neutral molecules and their cations can then be combined with the ionization energies of the neutral molecules and of the metal atoms using a thermochemical cycle to confirm the accuracy of all four measured values. These studies are likely to be particularly useful for the development of density functional theory methods for accurate computations of molecules containing the d- and f-block elements. 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.