A new way to capture and structurally characterize highly reactive intermediates
We specialize in creating experimental methods that can isolate the encounter between two reactive species and map out the key structures that occur in a chemical transformation. Our strategy is to extract one of the reactive species (R1) from native solution (with or without surrounding solvent) and encapsulate it in the gas phase with a layer of weakly bound, inert cryogenic material such as rare gas atoms (RG) to form R1∙RGm clusters. This is accomplished using ultracold ion sources relying on either supersonic free jet or radio frequency ion traps held at 10 K. The second reactant (R2) is then condensed onto this cold cluster assembly and the reaction exothermicity is released by rapid evaporation of RG atoms. We then determine the structures of the cold ions that are frozen into well-defined local minima corresponding to reaction intermediates. Structural information is recovered from high resolution spectra that are obtained with great sensitivity by monitoring the photoinduced evaporation of the weakly bound cryogen using a second stage of mass spectrometry. Many types of spectroscopic diagnostics are being employed including UV/VIS (electronic), IR (vibrational) and, for negative ions, photoelectron spectroscopy. Chemical systems most interesting presently are the mechanisms of catalytic CO2 activation and water splitting in addition to proton-mediated covalent bond formation through the cooperative action of water networks.
The methods we utilize to study gas phase ion chemistry
We design and build the hybrid mass and optical spectrometers required to both structurally characterize chemical intermediates and photoinduce early reaction complexes along the reaction coordinate leading to products. These spectrometers are largely custom built in-house in order to integrate the cryogenic ion source technology and the laser photodissociation aspects in a coherent fashion. One versatile platform that we have refined over the past decade involves double (or higher order) focusing, tandem time-of-flight photofragmentation. In that approach, ions with a single mass (m/z) are brought to a transient focus for nearly 100% efficient interaction with broadly tunable, high power pulsed lasers. By combining sequential stages of laser interaction and mass analysis, we have developed a powerful array of multidimensional tools that routinely yield isomer-selective spectra of complex mixtures at a given mass. Most interesting are the distinct chemical species that correspond to different bonding arrangements between the same set of atoms. An exciting project is now underway in which we are interfacing our cryogenic spectroscopy methods with the ultra-high resolution available in an ion-cyclotron resonance (ICR) mass spectrometer equipped with a superconducting (7 T) magnet.
Both the TOF and ICR instruments provide routine structural characterization of mass-analyzed compounds presently under study by our synthetic colleagues. Our techniques offer profound opportunities for further expansion as the tools we build yield ever clearer pictures of the mechanisms ranging from catalysis to molecular recognition. This aspect of our program draws inspiration from bioanalytical chemistry, atomic, molecular and optical (AMO) physics, and chemical reaction dynamics.