Electrospray ionization (ESI) represents an inter-phase between a solution and the gas-phase environment of a mass spectrometer. ESI-MS is increasingly used for investigation of reaction intermediates in organic and organometallic chemistry. The key questions are what is the relationship of the detected ions to the actual intermediates in solution and what is the structure of the detected ions. We are developing methods to address both of these non-trivial questions.
The first method is “Delayed Reactant Labeling” and its aim is to bridge the condensed phase-gas phase gap. The principle is that a given reaction is set up with isotopically labeled as well as unlabeled reactant(s), one of them being added with a time delay. ESI-MS monitoring of this reaction mixture shows pairs of peaks corresponding to the labeled and unlabeled reactants, intermediates and products. Time evolution of the pairs of peaks corresponding to the labeled and unlabeled reactants, intermediates and products is related to the reaction kinetics in solution. We have developed several models and applied it to the problems in gold, palladium, and copper catalysis.
The method to address the structure of the detected ions is ion spectroscopy. We have developed an approach to measure IR and UV-vis spectra of mass-selected ions at variable temperatures. Based on the spectra we can assign the structures of the ions. The method is particularly important for the investigation of catalytic cycles, because most of the intermediates along the catalytic cycle have the same mass-to-charge ratio. Our approach thus allows us to distinguish which ions are detected and which part of the reaction is possible to monitor by ESI-MS. I will show application of our approach for the investigation of metal complexes.