Understanding the intrinsic properties of molecules, molecular building blocks and aggregates is key to realizing the bottom-up design of functional molecules and materials, and catalysts. We explore such molecular units in isolation, via the pristine gas phase environment of specially modified mass spectrometers.
1. Trapped Ion Mobility Spectrometry
The recently (April 2018) installed Bruker timsTOF combines Trapped Ion Mobility Spectrometry (TIMS) with Time-of-Flight mass spectrometry (TOF). Ion mobility is a powerful extension to mass spectrometry that delivers information about the three dimensional structure of an ion and allows for isomer separation.
2. Fourier Transform Ion Cyclotron Resonance
By trapping ions in the magnetic field of a specially modified Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometer (Bruker Daltonics), we explore the chemistry and photoexcitation of a range of isolated metal-containing complexes, clusters, and other non-covalent species relevant to functional materials. A nano-electrospray ionization (nanoESI) source is used to transfer ionic species of interest from solution to the gas phase, where they can be stored and subsequently: (a) collisionally activated, to study unimolecular fragmentation (b) reacted with neutral species, to study bimolecular behaviors (i.e. selectivity and reaction rate); (b) activated by tunable light (IR and nIR-vis-UV spectral ranges), to generate an optical spectrum.
The ICR cell is temperature variable (100 – 400 K) which also enables e.g. the study of association kinetics and binding energies. These experiments provide fundamental chemical and physical insight into the isolated systems, and are valuable benchmarks for computational models.
3. Travelling Wave Ion Mobility
We also utilize Travelling Wave Ion Mobility (TWIMS) to study the properties of isomer-separated ionic species using a Synapt G2 time of flight mass spectrometer (Waters). Ions are generated in an interchangeable ion source (e.g. nanoESI, ESI, MALDI, or ASAP). Prior to mass analysis, the gas phase ions are separated in the TWIMS cell, based on their collisional cross section (CCS), the property describing a molecule’s characteristic topology or “shape and size”. The CCSs derived from experiment are compared to theoretically predicted cross sections for structural identification. In-house modifications to the commercial instrument allow neutral gas introduction in the “trap” and “transfer” regions. As they are separated in time, the isomer-resolved species may have their individual chemistry probed, e.g. (i) via collisional activation to compare fragmentation or (ii) by reaction with neutral species (utilizing the instrumental modification).
We also carry out methodological development on the separation and CCS measurement of metal containing species, large clusters and species possessing non-covalent interactions; systems which currently represent an analytical challenge for the TWIMS technique.
Detection of Intermediates in Dual Gold Catalysis
We have probed for reaction intermediates involved in the dual-gold-catalyzed activation of a conjugated 1,5-diyne substrate and its further coupling to benzene in the liquid phase. This was done by sampling the reaction mixture by electrospray ionization followed by high-resolution ion mobility mass spectrometry—under conditions allowing for the resolution of structural isomers differing in their collision cross sections by less than 0.5%. For the cationic mass corresponding to catalyst + diyne (activation stage) we resolve four isomers. At the mass corresponding to catalyst + diyne + benzene, two isomers are observed. By comparing the experimentally obtained cross sections to those inferred for model structures derived from density functional computations, we find our measurements to be consistent with the proposed solution mechanism. Organometallics 2018, 37,1493-1500
Binding of O2 and CO to Metal Porphyrin Anions in the Gas Phase
The binding energies of O2 and CO to iron(II) and manganese(II) porphyrin anions has been determined in the gas phase. Low-pressure ion–molecule equilibria have been measured in a cryogenically cooled trap of an FT-ICR mass spectrometer, and binding energies of (40.8±1.3) kJ mol−1 and (66.3±2.6) kJ mol−1 have been obtained for oxygen and carbon monoxide, respectively, with a heme-analogue FeII porphyrin complex. Angew. Chem. Int. Ed. 52, 10374–10377 (2013)
Penetrating the Elusive Mechanism of Copper Mediated Fluoromethylation
TWIMS isomer-separation was exploited in order to react the particularly well-defined ionic species, [LCuO]+ (L=1,10-phenanthroline), with the neutral fluoromethane substrates, CH(4-n)Fn (n = 1 - 3), in the gas phase. Experimentally, the monofluoromethane substrate (n = 1) undergoes both hydrogen-atom transfer, forming the copper hydroxide complex [LCuOH]*+ and concomitantly a CH2F* radical; and oxygen-atom transfer, yielding the observable ionic product, [LCu]+, plus the neutral oxidized substrate. DFT calculations reveal that the mechanism for both product channels relies on the initial C-H bond activation of the substrate. Compared to non-fluorinated methane, the addition of fluorine to the substrate assists the reactivity via a lowering of the C-H bond energy and reaction pre-organization (via non-covalent interaction in the encounter complex). A two-state reactivity scenario is mandatory for the oxidation, which competitively results in the unusual fluoromethanol product, CH2FOH, or the decomposed products, CH2O and HF, the latter channel being kinetically disfavored. Overall, CH(3-n)Fn* and CH(3-n)FnOH formation occur under relatively gentle energetic conditions, which sheds light on their potential as reactive intermediates in fluoromethylation reactions mediated by copper in the presence of oxygen. J. Am. Chem. Soc., 2016, Article ASAP
Effect of Adduct Formation of Transition Metal–1,10-Phenanthroline Complexes
The number of separations and analyses of molecular species using traveling wave ion-mobility spectromrtry-mass spectrometry (TWIMS-MS) is increasing, including those extending the technique to analytes containing metal atoms. A critical aspect of such applications of TWIMS-MS is the validity of the collisional cross sections (CCS) measured. Target species were generated via electrospray ionization (ESI), analyzed using TWIMS in N2 drift gas, and the drift time trends compared along with theoretically derived CCSs. For metal containing species in this size regime, reaction with molecular nitrogen has a dramatic effect on measured drift times and must not be ignored when comparing and interpreting TWIMS arrival time distributions. Density-functional theory (DFT) calculations are employed to analyze the periodic differences due to the metal's interaction with nitrogen (and background water) in detail. Anal. Chem., 87, 9769–9776 (2015)