Cara C. Manning

Powerful Insight into Catalytic Mechanisms through Simultaneous Monitoring of Reactants, Products, and Intermediates

Electrospray ionization mass spectrometry (ESIMS) has become a valuable tool in the mechanistic study of organometallic catalytic reactions. Analysis is fast, intermediates at low concentrations can be detected, and complex mixtures are tractable. The family of palladium-catalyzed C C bondforming reactions are the most studied by ESIMS. Although the majority of these investigations have focused on the structural identification of short-lived or low-concentration intermediates, some recent studies have monitored the intensities of intermediates or reactants and products over time. 14] However, no one has yet shown this technique to be capable of providing robust kinetic information for reactants, products, by-products, and low-abundance intermediates simultaneously and under standard reaction conditions. We show herein how powerful this information can be in leading reaction design. The copper-free Sonogashira (Heck alkynylation) reaction is widely used in the synthesis of natural products, pharmaceuticals, and novel materials, but the mechanism is not well understood. Ideally, the reaction should be observed under typical reaction conditions for meaningful information to be obtained about the mechanism, because under such conditions anions and bases 18] as well as alkynes are thought to act as ligands for palladium, with complex effects on the reaction efficiency. In most cases, a large excess of an amine base is required to promote reaction; however, the exact role of the base is in question. Dieck and Heck and Amatore et al. suggested a carbopalladation mechanism in which the terminal alkyne undergoes carbopalladation and the base consumes the H formed during the b-hydride elimination that forms the product. Ljundahl et al. prefer a deprotonation mechanism in which deprotonation of the terminal alkyne by the amine occurs from the cationic intermediate [Pd(Ar)(PR3)(NR’3)(HC CR’’)] or the neutral intermediate [Pd(Ar)(PR3)(X)(HC CR’’)], depending on the electronic nature of the alkyne. An anionic mechanism has also been proposed in which [Pd(PR3)2X] and [Pd(PR3)(X)(Ar)(CCR’’)] intermediates feature. The identity of palladium-containing intermediates has been proposed on the basis of electrochemical or NMR spectroscopic data but not through direct observation. Charged tags are required for the detection of species otherwise invisible to ESIMS, 24] an idea first introduced by Adlhart and Chen; we used an aryl iodide functionalized with a phosphonium hexafluorophosphate salt, [p-IC6H4CH2PPh3] [PF6] . This tag provides very low detection limits owing to its high surface activity, and the noncoordinating counterion reduces ion pairing. The bulky nature of the charged group ensures that the ionization efficiency is largely insensitive to the remaining structure of the ion, so the intensity of the various ions correspond very closely to their real concentration (see the Supporting Information). ESIMS data on reaction progress collected under typical reaction conditions by using pressurized sample infusion (PSI) compare well with H NMR and UV/Vis spectroscopic data (Figure 1). The number of data points is much higher for

Continuous Measurements of Dissolved Ne, Ar, Kr, and Xe Ratios with a Field-deployable Gas Equilibration Mass Spectrometer

Noble gases dissolved in natural waters are useful tracers for quantifying physical processes. Here, we describe a field-deployable gas equilibration mass spectrometer (GEMS) that provides continuous, real-time measurements of Ne, Ar, Kr, and Xe mole ratios in natural waters. Gas is equilibrated with a membrane contactor cartridge and measured with a quadrupole mass spectrometer, after in-line purification with reactive metal alloy getters. We use an electron energy of 35 V for Ne to eliminate isobaric interferences, and a higher electron energy for the other gases to improve sensitivity. The precision is 0.7% or better and 1.0% or better for all mole ratios when the instrument is installed in a temperature-controlled environment and a variable-temperature environment, respectively. In the lab, the accuracy is 0.9% or better for all gas ratios using air as the only calibration standard. In the field (and/or at greater levels of disequilbrium), the accuracy is 0.7% or better for Ne/Kr, Ne/Ar, and Ar/Kr, and 2.5% or better for Ne/Xe, Ar/Xe, and Kr/Xe using air as the only calibration standard. The field accuracy improves to 0.6% or better for Ne/Xe, Ar/Xe, and Kr/Xe when the data is calibrated using discrete water samples run on a laboratory-based mass spectrometer. The e-folding response time is 90-410 s. This instrument enables the collection of a large number of continuous, high-precision and accuracy noble gas measurements at substantially reduced cost and labor compared to traditional methods.

Quantifying air-sea gas exchange using noble gases in a coastal upwelling zone

The diffusive and bubble-mediated components of air-sea gas exchange can be quantified separately using time-series measurements of a suite of dissolved inert gases. We have evaluated the performance of four published air-sea gas exchange parameterizations using a five-day time-series of dissolved He, Ne, Ar, Kr, and Xe concentration in Monterey Bay, CA. We constructed a vertical model including surface air-sea gas exchange and vertical diffusion. Diffusivity was measured throughout the cruise from profiles of turbulent microstructure. We corrected the mixed layer gas concentrations for an upwelling event that occurred partway through the cruise. All tested parameterizations gave similar results for Ar, Kr, and Xe; their air-sea fluxes were dominated by diffusive gas exchange during our study. For He and Ne, which are less soluble, and therefore more sensitive to differences in the treatment of bubble-mediated exchange, the parameterizations gave widely different results with respect to the net gas exchange flux and the bubble flux. This study demonstrates the value of using a suite of inert gases, especially the lower solubility ones, to parameterize air-sea gas exchange.

Impact of recently upwelled water on productivity investigated using in situ and incubation-based methods in Monterey Bay

Photosynthetic conversion of CO2 to organic carbon and the transport of this carbon from the surface to the deep ocean is an important regulator of atmospheric CO2. To understand the controls on carbon fluxes in a productive region impacted by upwelling, we measured biological productivity via multiple methods during a cruise in Monterey Bay, California. We quantified net community production and gross primary production from measurements of O2/Ar and O2 triple isotopes (17Δ), respectively. We simultaneously conducted incubations measuring the uptake of 14C, 15NO3- and 15NH4+, and nitrification, and deployed sediment traps. At the start of the cruise (Phase 1) the carbon cycle was at steady state and the estimated net community production was 35(10) and 35(8) mmol C m−2 d−1 from O2/Ar and 15N incubations respectively, a remarkably good agreement. During Phase 1, net primary production was 96(27) mmol C m−2 d−1 from C uptake, and gross primary production was 209(17) mmol C m−2 d−1 from 17Δ. Later in the cruise (Phase 2), recently upwelled water with higher nutrient concentrations entered the study area, causing 14C and 15NO3- uptake to increase substantially. Continuous O2/Ar measurements revealed submesoscale variability in water mass structure and likely productivity in Phase 2 that was not evident from the incubations. These data demonstrate that O2/Ar and 15N incubation-based NCP estimates can give equivalent results in an N-limited, coastal system, when the non-steady state O2 fluxes are negligible or can be quantified. This article is protected by copyright. All rights reserved.

Oxygen/Argon, Triple Oxygen Isotope, And Ctd Data From Bras D'Or Lake, Nova Scotia, Canada

Oxygen/argon, triple oxygen isotope, and CTD data from Bras d’Or Lake, Nova Scotia, Canada collected in March and April 2013.

Revising Estimates of Aquatic Gross Oxygen Production by the Triple Oxygen Isotope Method to Incorporate the Local Isotopic Composition of Water

Measurement of the triple oxygen isotope (TOI) composition of O2 is an established method for quantifying gross oxygen production (GOP) in natural waters. A standard assumption to this method is that the isotopic composition of H2O, the substrate for photosynthetic O2, is equivalent to Vienna standard mean ocean water (VSMOW). We present and validate a method for estimating the TOI composition of H2O based on mixing of local meteoric water and seawater H2O end-members, and incorporating the TOI composition of H2O into GOP estimates. In the ocean, GOP estimates based on assuming the H2O is equivalent to VSMOW can have systematic errors of up to 48% and in low-salinity systems, errors can be a factor of 2 or greater. In future TOI-based GOP studies, TOI measurements of O2 and H2O should be paired when the H2O isotopic composition is expected to differ from VSMOW.