Monitoring Reaction Intermediates to Predict Enantioselectivity Using Mass Spectrometry**

Enantioselective reactions are at the core of chemical synthesis. Their development mostly relies on prior knowledge, laborious product analysis and post‐rationalization by theoretical methods. Here, we introduce a simple and fast method to determine enantioselectivities based on mass spectrometry. The method is based on ion mobility separation of diastereomeric intermediates, formed from a chiral catalyst and prochiral reactants, and delayed reactant labeling experiments to link the mass spectra with the reaction kinetics in solution. The data provide rate constants along the reaction paths for the individual diastereomeric intermediates, revealing the origins of enantioselectivity. Using the derived kinetics, the enantioselectivity of the overall reaction can be predicted. Hence, this method can offer a rapid discovery and optimization of enantioselective reactions in the future. We illustrate the method for the addition of cyclopentadiene (CP) to an α,β‐unsaturated aldehyde catalyzed by a diarylprolinol silyl ether.

Electrification of water interface

The surface charge of a water interface determines many fundamental processes in physical chemistry and interface science, and it has been intensively studied for over a hundred years. We summarize experimental methods to characterize the surface charge densities developed so far: electrokinetics, double-layer force measurements, potentiometric titration, surface-sensitive nonlinear spectroscopy, and surface-sensitive mass spectrometry. Then, we elucidate physical ion adsorption and chemical electrification as examples of electrification mechanisms. In the end, novel effects on surface electrification are discussed in detail. We believe that this clear overview of state of the art in a charged water interface will surely help the fundamental progress of physics and chemistry at interfaces in the future.

Collision Cross Sections of Phosphoric Acid Cluster Anions in Helium Measured by Drift Tube Ion Mobility Mass Spectrometry

In the last years, ion mobility mass spectrometry (IMS-MS) has improved structural analysis and compound identification by giving access to the collision cross section (CCS). An increasingly wide and accurate database of CCS values is now available but often without assessment of the influence of different instrumental settings on CCS values. Here, we present 75 CCS values in helium (^DTCCS_He) for phosphoric acid cluster anions [(H₃PO₄)_n - zH]^z- with charge state (z) up to 4-. The CCS values, noted ^DTCCS_He, were obtained with a commercial drift tube ion mobility mass spectrometer, in helium, by applying a classic multifield approach. Phosphoric acid clusters are fragile structures that allow to evaluate the effect of different experimental conditions on the retention of weak bonds and their effect on CCS values. We probed harsh and soft voltage gradients in the electrospray (ESI) source before the IMS and two different voltage gradients in the post-IMS region. The variations in the ion mobility and mass spectra consisted in a change in the distribution of the cluster anions aggregation numbers (n) and charge states (z), with a higher amount of multiply charged species for the soft pre-IMS voltage gradient and a lower proportion of cluster dissociation for soft post-IMS conditions. However, the CCS values did not change with experimental conditions for a given cluster, as long as it stays intact from the IMS to the mass analyzer. The ^DTCCS_He were found in good agreement among 3 to 10 replicated values, with a relative standard deviation between 0.1 and 1.7%.

Temperature-dependent luminescence spectroscopic and mass spectrometric investigations of U(VI) complexation with aqueous silicates in the acidic pH-range

In this study the complexation of U(VI) with orthosilicic acid (H₄SiO₄) was investigated between pH 3.5 and 5 by combining electrospray ionization mass spectrometry (ESI-MS) and laser-induced luminescence spectroscopy. The ESI-MS experiments performed at a total silicon concentration of 5 · 10^-3M (exceeding the solubility of amorphous silica at both pH-values) revealed the formation of oligomeric sodium-silicates in addition to the UO₂OSi(OH)₃⁺ species. For the luminescence spectroscopic experiments (25 °C), the U(VI) concentration was fixed at 5 · 10^-6M, the silicon concentration was varied between 1.3 · 10^-4-1.3 · 10^-3M (reducing the formation of silicon oligomers) and the ionic strength was kept constant at 0.2 M NaClO₄. The results confirmed the formation of the aqueous UO₂OSi(OH)₃⁺ complex. The conditional complexation constant at 25 °C, log *β = -(0.31 ± 0.24), was extrapolated to infinite dilution using the Davies equation, which led to log *β⁰ = -(0.06 ± 0.24). Further experiments at different temperatures (1-25 °C) allowed the calculation of the molal enthalpy of reaction Δ_rH_m⁰ = 45.8 ± 22.5 kJ·mol^-1 and molal entropy of reaction Δ_rS_m⁰ = 152.5 ± 78.8 J·K^-1·mol^-1 using the integrated van't Hoff equation, corroborating an endothermic and entropy driven complexation process.

The chemical reactions in electrosprays of water do not always correspond to those at the pristine air-water interface

The recent application of electrosprays to characterize the air-water interface, along with the reports on dramatically accelerated chemical reactions in aqueous electrosprays, have sparked a broad interest. Herein, we report on complementary laboratory and in silico experiments tracking the oligomerization of isoprene, an important biogenic gas, in electrosprays and isoprene-water emulsions to differentiate the contributions of interfacial effects from those of high voltages leading to charge-separation and concentration of reactants in the electrosprays. To this end, we employed electrospray ionization mass spectrometry, proton nuclear magnetic resonance, ab initio calculations and molecular dynamics simulations. We found that the oligomerization of isoprene in aqueous electrosprays involved minimally hydrated and highly reactive hydronium ions. Those conditions, however, are non-existent at pristine air-water interfaces and oil-water emulsions under normal temperature and pressure. Thus, electrosprays should be complemented with surface-specific platforms and theoretical methods to reliably investigate chemistries at the pristine air-water interface.

Mode-Selective Laser Control of Palladium Catalyst Decomposition

It is generally assumed that molecules behave ergodically during chemical reactions, that is, reactivities depend only on the total energy content and not on the initial state of the molecule. While there are a few examples of nonergodic behavior in small (usually electronically excited) species, to date there have been no reports of such behavior in larger covalently bound species composed of several tens of atoms. Here, we demonstrate vibrational mode-selective behavior in a series of palladium catalysts. When we excite solvent-tagged gas-phase Pd catalysts with an infrared laser that is tuned to be resonant with specific molecular vibrations, depending on which vibration we excite, we can select different reaction pathways. We also demonstrate that this behavior can be "turned off" via chemical substitution.

ESI activity of Br⁻, BF₄⁻ , ClO₄⁻ and BPh₄⁻ anions in the presence of Li⁺ and NBu⁴⁺ counter-ions

To improve our understanding of the electrospray ionization (ESI) process, we have subjected equimolar mixtures of salts A⁺ X^- (A⁺  = Li⁺ , NBu₄⁺ ; X^-  = Br^- , ClO₄^- , BF₄^- , BPh₄^- ) in different solvents (CH₃ CN, tetrahydrofuran, CH₃ OH, H₂ O) to negative-ion mode ESI and analyzed the relative ESI activity of the different anionic model analytes. The ESI activity of the large and hydrophobic BPh₄^- ion greatly exceeds that of the smaller and more hydrophilic anions Br^- , ClO₄^- and BF₄^- , which we ascribe to its higher surface activity. Moreover, the ESI activity of the anions is modulated by the action of the counter-ions and their different tendency toward ion pairing. The tendency toward ion pairing can be reduced by the addition of the chelating ligands 12-crown-4 and 2.2.1 cryptand and is, although to a smaller degree, further influenced by the variation of the solvent. Complementary electrical conductivity measurements afford additional information on the interactions of the ionic constituents of the sample solutions. Copyright © 2017 John Wiley & Sons, Ltd.

Reaction Intermediates Kinetics in Solution Investigated by Electrospray Ionization Mass Spectrometry: Diaurated Complexes

A new method to investigate the reaction kinetics of intermediates in solution by electrospray ionization mass spectrometry is presented. The method, referred to as delayed reactant labeling, allows investigation of a reaction mixture containing isotopically labeled and unlabeled reactants with different reaction times. It is shown that we can extract rate constants for the degradation of reaction intermediates and investigate the effects of various reaction conditions on their half-life. This method directly addresses the problem of the relevance of detected gaseous ions toward the investigated reaction solution. It is demonstrated for geminally diaurated intermediates formed in the gold mediated addition of methanol to alkynes. Delayed reactant labeling allows us to directly link the kinetics of the diaurated intermediates with the overall reaction kinetics determined by NMR spectroscopy. It is shown that the kinetics of protodeauration of these intermediates mirrors the kinetics of the addition of methanol demonstrating they are directly involved in the catalytic cycle. Formation as well as decomposition of diaurated intermediates can be drastically slowed down by employing bulky ancillary ligands at the gold catalyst; the catalytic cycle then proceeds via monoaurated intermediates. The reaction is investigated for 1-phenylpropyne (Ph-CC-CH3) using [AuCl(PPh3)]/AgSbF6 and [AuCl(IPr)]/AgSbF6 as model catalysts. Delayed reactant labeling is achieved by using a combination of CH3OH and CD3OH or Ph-CC-CH3 and Ph-CC-CD3.

Mass spectrometric behavior of phenolic acids standards and their analysis in the plant samples with LC/ESI/MS system

Liquid chromatography coupled to mass spectrometry (MS) with electrospray ionization (ESI) is one of analytical techniques to obtain accurate results of low molecular weight aromatic compounds in biological samples of different origin. The interpretations of mass spectra of these aromatic compounds in the negative spectra registered in the full scan MS mode may be uneasy due to presence of deprotonated molecules [M-H](-) from different co-eluting entities, fragment ions created after the break-up of precursor ions and also ions representing modified molecules clusters. Thus, the first aim of this study was to evaluate general parameters during analysis performed in the full scan MS or MS/MS mode. Secondly, to set general fragmentation rules for aromatic compounds and entities in a complex biological matrix. We established that different groups of low molecular weight phenolic acids form unique adduct ions and additionally registration LC/MS/MS spectra with two different collision energies may allow for differentiating isomeric or isobaric molecules. These findings together with some general fragmentation rules can facilitate identifications of aromatic acids as we outlined in the sample of cold-pressed rose-hip oil and lupine leaves extract.

Traveling wave ion mobility mass spectrometry and ab initio calculations of phosphoric acid clusters

Positive and negative ion electrospray mass spectra obtained from 50 mM phosphoric acid solutions presented a large number of phosphoric acid clusters: [(H3PO4)n + zH](z+) or [(H3PO4)n - zH](z-), with n up to 200 and z up to 4 for positively charged clusters, and n up to 270 and z up to 7 for negatively charged cluster ions. Ion mobility experiments allowed very explicit separation of the different charge states. Because of the increased pressures involved in ion mobility experiments, dissociation to smaller clusters was observed both in the trap and transfer areas. Voltages along the ion path could be optimized so as to minimize this effect, which can be directly associated with the cleavage of hydrogen bonds. Having excluded the ion mobility times that resulted from dissociated ions, each cluster ion appeared at a single drift time. These drift times showed a linear progression with the number of phosphoric atoms for cluster ions of the same charge state. Cross section calculations were carried out with MOBCAL on DFT optimized geometries with different hydrogen locations and with three types of atomic charges. DFT geometry optimizations yielded roughly spherical structures. Our results for nitrogen gas interaction cross sections showed that values were dependent on the atomic charges definition used in the MOBCAL calculation. This pinpointed the necessity to define a clear theoretical framework before any comparative interpretations can be attempted with uncharacterized compounds.

Mass spectrometric characterization and quantification of Pu(VI) hydrolysis products

In the present work the first direct measurement of hydrolysis products of Pu(VI) was achieved by means of nano-electrospray ionization time-of-flight mass-spectrometry. The results indicate that monomeric PuO2(OH) + and dimeric (PuO2)2(OH) 2 2+ species are present in solution. A trimeric species does not appear within the detection limit of the experiment, in contrast to U(VI) hydrolysis. The relative abundances of the Pu(VI) hydrolysis species in the ESI mass spectra are in good agreement with the published formation constants.

A quantitative approach of the interaction between metal triflates and organic ligands using electrospray mass spectrometry

The interaction between two Lewis "superacid" catalysts Zn(OTf)(2) and In(OTf)(3) and series of amide and phosphate ligands is quantitatively characterized by electrospray ionization mass spectrometry (ESI-MS). A specific feature of the ESI-MS spectra of the mixture of metal triflates and Lewis bases is the formation of ionic adducts resulting from the displacement of one triflate anion by two neutral ligands. A ligand competition model is developed, which describes the relative intensities of the ionic adducts as a function of relative ligand concentrations. The relative affinities deduced from the ligand competition method are combined in an affinity scale for the metal triflate.

Electrospray ionization mass spectrometric investigations of the complexation behavior of macrocyclic thiacrown ethers with bivalent transitional metals (Cu, Co, Ni and Zn)

RATIONALE

Heavy metals are both a problem for the environment and an important resource for industry. Their selective extraction by means of organic ligands therefore is an attractive topic. The coordination of three thiacrown ethers to late 3d-metal ions was investigated by a combination of electrospray ionization mass spectrometry (ESI-MS) and electron paramagnetic resonance (EPR).

METHODS

The mass spectrometric experiments were carried out in an ion trap mass spectrometer with an ESI source. Absolute binding constants were estimated by comparison with data for 18-crown-6/Na(+). EPR spectroscopy was used as a complementary method for investigating the Cu(I) /Cu(II) redox couple.

RESULTS

The study found that thiacrown ethers preferentially bind traces of copper even at an excess of other metal ions (Co(II), Ni(II), and Zn(II)). The absolute association constants of the Cu(I) complexes were about 10(8) M(-1), and about two orders of magnitude lower for the other 3d-metal cations. The EPR spectra demonstrated that the reduction from Cu(II) to Cu(I) upon formation of the [(thiacrown)Cu](+) species takes place in solution.

CONCLUSIONS

ESI-MS demonstrated that the three thiacrown ligands examined had high binding constants as well as good selectivities for copper(I) at low concentrations, and in the presence of other metal ions. By a combination of ESI-MS and EPR spectrometry it was shown that the reduction from Cu(II) to Cu(I) occurred in solution.

Applications of electrospray ionization mass spectrometry in mechanistic studies and catalysis research

Mechanistic studies form the basis for a better understanding of chemical processes, helping researchers develop more sustainable reactions by increasing the yields of the desired products, reducing waste production, and lowering the consumption of resources and energy overall. Conventional methods for the investigation of reaction mechanisms in solution include kinetic studies, isotope labeling, trapping of reactive intermediates, and advanced spectroscopic techniques. Within the past decade, electrospray ionization mass spectrometry (ESI-MS) has provided an additional tool for mechanistic studies because researchers can directly probe liquid samples by mass spectrometry under gentle conditions. Specifically, ESI-MS allows researchers to identify the molecular entities present in solution over the course of a chemical transformation. ESI-MS is particularly useful for investigations of organic reactions or metal catalysis that involve ionic intermediates. Accordingly, researchers are increasingly using ESI-MS in mechanistic studies and catalyst development. However, a further understanding of the ESI process and how it can facilitate mechanistic studies has not accompanied this increased use of the technique. Therefore, at least in part the ESI-MS method not only has offered great promise for the elucidation of reaction mechanisms but also became a black box with the occasional risk of misinterpretation. In this Account, we summarize applications of ESI-MS for synthetic and mechanistic research. Recently researchers have established direct linkages between gas-phase data obtained via ESI-MS and processes occurring in solution, and these results reveal qualitative and quantitative correlations between ESI-MS measurements and solution properties. In this context, time dependences, concentration series, and counterion effects can serve as criteria that allow researchers assess if the gas-phase measurements correlate with the situation in the solution. Furthermore, we report developments that bridge the gap between gas-phase and solution-phase studies. We also describe predictions derived from ESI-MS that have been verified with solution-phase chemistry experiments.