Monitoring Anhydride and Acid Conversion in Supercritical/Hydrothermal Water by in Situ Fiber-Optic Raman Spectroscopy

Diamond growth from organic compounds in hydrous fluids deep within the Earth

At subduction zones, most diamonds form by carbon saturation in hydrous fluids released from lithospheric plates on equilibration with mantle rocks. Although organic molecules are predicted among dissolved species which are the source for carbon in diamonds, their occurrence is not demonstrated in nature, and the physical model for crustal diamond formation is debated. Here, using Raman microspectroscopy, I determine the structure of carbon-based phases inside fluid inclusions in diamond-bearing rocks from the Alps. The results provide direct evidence that diamond surfaces are coated by sp²-, and sp³-bonded amorphous carbon and functional groups of carboxylic acids (e.g., carboxyl, carboxylate, methyl, and methylene), indicating the geosynthesis of organic compounds in deep hydrous fluids. Moreover, this study suggests diamond nucleation via metastable molecular precursors. As a possible scenario, with carbon saturation by reduction of carboxylate groups, I consider tetrahedral H-terminated C groups as templates for the growth of sp³-structured carbon.

Diamonds can give us clues to the processes regulating deep carbon transport within the Earth. Here, the author discovers evidence from diamond coatings that organic compounds exist at great depth in Earth’s interior, and furthermore, that organic molecules may provide scaffolds for diamond nucleation and growth.

Online monitoring of the kinetic isotope effect in chemical reactions with 1H and 19F low-field NMR spectroscopy

The kinetic isotope effect (KIE) describes the change in the rate of a chemical reaction by substituting one of the atoms in the reactants with one of its isotopes. Investigating the KIE and its temperature dependency in reactions renders information for reconstructing chemical processes and confirming the rate-determining step. However, conventional methods to study the KIE, e.g. by calorimetry, conductivity, titration, Raman spectroscopy etc., require calibration and sophisticated handling of the reaction calorimeter, and the data are obtained at irregular and sparse intervals. This current study employs a compact NMR system as an alternative means to determine the temperature dependency of the reaction rate and, thus, the KIE, as well as the activation energy, enthalpy, and entropy of each reaction. Here the neutral hydrolysis of acetic anhydride and ethyl trifluoroacetate was studied in H₂O, D₂O and H₂O-D₂O mixtures with ¹H and ¹⁹F NMR spectroscopy. The activation energies for the hydrolysis of acetic anhydride with D₂O and H₂O were found to be 45 ± 2 kJ mol^-1 and 40 ± 2 kJ mol^-1, respectively. The activation energies of ethyl trifluoroacetate hydrolysis via¹⁹F NMR spectroscopy were determined to 46.7 ± 1 kJ mol^-1 and 54.9 ± 1 kJ mol^-1 for the reaction with H₂O and D₂O, respectively, and via¹H NMR spectroscopy to 48 ± 3 kJ mol^-1 and 55.8 ± 1 kJ mol^-1. The differences in rate constants and activation energies for both reactions in H₂O and D₂O are due to the kinetic isotope effect, involving the breakage and formation of O-H and O-D bonds during the rate-determining step. The proton inventory studies were performed for both the reactions for determining the isotopic fractionation factors for the given transition states of the reactions which help to predict the reaction mechanisms of other similar reactions. The compact NMR system is a relevant and practical tool to unmask precise reaction pathways, by tracing the KIE in real time with densely sampled data, which are essential for obtaining accurate rate constants.

Driving Chemical Reactions in Plasmonic Nanogaps with Electrohydrodynamic Flow

Nanoparticles from colloidal solution-with controlled composition, size, and shape-serve as excellent building blocks for plasmonic devices and metasurfaces. However, understanding hierarchical driving forces affecting the geometry of oligomers and interparticle gap spacings is still needed to fabricate high-density architectures over large areas. Here, electrohydrodynamic (EHD) flow is used as a long-range driving force to enable carbodiimide cross-linking between nanospheres and produces oligomers exhibiting sub-nanometer gap spacing over mm² areas. Anhydride linkers between nanospheres are observed via surface-enhanced Raman scattering (SERS) spectroscopy. The anhydride linkers are cleavable via nucleophilic substitution and enable placement of nucleophilic molecules in electromagnetic hotspots. Atomistic simulations elucidate that the transient attractive force provided by EHD flow is needed to provide a sufficient residence time for anhydride cross-linking to overcome slow reaction kinetics. This synergistic analysis shows assembly involves an interplay between long-range driving forces increasing nanoparticle-nanoparticle interactions and probability that ligands are in proximity to overcome activation energy barriers associated with short-range chemical reactions. Absorption spectroscopy and electromagnetic full-wave simulations show that variations in nanogap spacing have a greater influence on optical response than variations in close-packed oligomer geometry. The EHD flow-anhydride cross-linking assembly method enables close-packed oligomers with uniform gap spacings that produce uniform SERS enhancement factors. These results demonstrate the efficacy of colloidal driving forces to selectively enable chemical reactions leading to future assembly platforms for large-area nanodevices.

In situ attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy monitoring of 1,2-butylene oxide polymerization reaction by using iterative concentration-guided classical least squares

There has been rapid growth in the application of in situ optical spectroscopy techniques for reaction and process monitoring recently in both academia and industry. Vibrational spectroscopies such as mid-infrared, near-infrared spectroscopy, and Raman spectroscopy have proven to be versatile and informative. Accurate determination of concentrations, based on highly overlapped spectra, remains a challenge. As an example, 1,2-butylene oxide (BO) polymerization, an important industrial reaction, initiated by propylene glycol (PG) and catalyzed by KOH, is studied in this work in a semi-batch fashion by using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR) monitoring. The weak BO absorbance, the constantly changing interference from the product oligomers throughout the course of the reaction, and the change in BO spectral features with system polarity posed challenges for quantitative spectral analysis based on conventional methods. An iterative concentration-guided classical least-squares (ICG-CLS) method was developed to overcome these challenges. Taking advantage of the concentration-domain information, ICG-CLS enabled the estimation of the pure oligomer product spectra at different stages of the semi-batch process, which in turn was used to construct valid CLS models. The ICG-CLS algorithm provides an in situ calibration method that can be broadly applied to reactions of known order. Caveats in its applications are also discussed.

Study of thermal dynamics of defatted bovine serum albumin in D2O solution by Fourier transform infrared spectra and evolving factor analysis

Fourier transform infrared (FT-IR) spectra have been measured for defatted bovine serum albumin (BSA) in D(2)O with a concentration of 2.0 wt % over a temperature range of 26-90 degrees C and the corresponding difference spectra have been calculated by subtracting the contribution of D(2)O at the same temperature. Evolving factor analysis (EFA) by selecting two factors and three factors has been employed to analyze the temperature-dependent difference IR spectra in the 1700-1600 cm(-1) spectral region of the defatted BSA in D(2)O solution. Three-factor EFA has been employed to determine the distinction of the three protein species involved in the process of temperature elevation: native, transitional, and denatured protein. The temperature profiles obtained from three-factor EFA indicate that heat-induced conformational change in the secondary structures of defatted BSA in D(2)O undergoes two two-state transitions, a drastic transition and a slight transition, which occur in the temperature ranges of 68-82 degrees C and 56-76 degrees C, respectively.

Characterization and quantitation of a tertiary mixture of salts by Raman spectroscopy in simulated hydrothermal vent fluid

This article will demonstrate that Raman spectroscopy can be a useful tool for monitoring the chemical composition of hydrothermal vent fluids in the deep ocean. Hydrothermal vent systems are difficult to study because they are commonly found at depths greater than 1000 m under high pressure (200-300 bar) and venting fluid temperatures are up to 400 degrees C. Our goal in this study was to investigate the use of Raman spectroscopy to characterize and quantitate three Raman-active salts that are among the many chemical building blocks of deep ocean vent chemistry. This paper presents initial sampling and calibration studies as part of a multiphase project to design, develop, and deploy a submersible deep sea Raman instrument for in situ analysis of hydrothermal vent systems. Raman spectra were collected from designed sets of seawater solutions of carbonate, sulfate, and nitrate under different physical conditions of temperature and pressure. The role of multivariate analysis techniques to preprocess the spectral signals and to develop optimal calibration models to accurately estimate the concentrations of a set of mixtures of simulated seawater are discussed. The effects that the high-pressure and high-temperature environment have upon the Raman spectra of the analytes were also systematically studied. Information gained from these lab experiments is being used to determine design criteria and performance attributes for a deployable deep sea Raman instrument to study hydrothermal vent systems in situ.

Product Screening of Fast Reactions in IR-Laser-Heated Liquid Water Filaments in a Vacuum by Mass Spectrometry

In the present article a novel approach for rapid product screening of fast reactions in IR-laser-heated liquid microbeams in a vacuum is highlighted. From absorbed energies, a shock wave analysis, high-speed laser stroboscopy, and thermodynamic data of high-temperature water the enthalpy, temperature, density, pressure, and the reaction time window for the hot water filament could be characterized. The experimental conditions (30 kbar, 1750 K, density approximately 1 g/cm3) present during the lifetime of the filament (20-30 ns) were extreme and provided a unique environment for high-temperature water chemistry. For the probe of the reaction products liquid beam desorption mass spectrometry was employed. A decisive feature of the technique is that ionic species, as well as neutral products and intermediates may be detected (neutrals as protonated aggregates) via time-of-flight mass spectrometry without any additional ionization laser. After the explosive disintegration of the superheated beam, high-temperature water reactions are efficiently quenched via expansion and evaporative cooling. For first exploratory experiments for chemistry in ultrahigh-temperature, -pressure and -density water, we have chosen resorcinol as a benchmark system, simple enough and well studied in high-temperature water environments much below 1000 K. Contrary to oxidation reactions usually present under less extreme and dense supercritical conditions, we have observed hydration and little H-atom abstraction during the narrow time window of the experiment. Small amounts of radicals but no ionic intermediates other than simple proton adducts were detected. The experimental findings are discussed in terms of the energetic and dense environment and the small time window for reaction, and they provide firm evidence for additional thermal reaction channels in extreme molecular environments.

Near infrared study of water-benzene mixtures at high temperatures and pressures

Near-infrared absorption of water-benzene mixtures has been measured at temperatures and pressures in the ranges of 473-673 K and 100-400 bar, respectively. Concentrations of water and benzene in the water-rich phase of the mixtures were obtained from the integrated absorption intensities of the OH stretching overtone transition of water and the CH stretching overtone transition of benzene, respectively. Using these concentrations, the densities of the water-rich phase were estimated and compared with the average densities before mixing, which were calculated from literature densities of neat water and neat benzene. It is found that anomalously large volume expansion on the mixing occurs in the region enclosed by an extended line of the three-phase equilibrium curve and the one-phase critical curve of the mixtures, and the gas-liquid equilibrium curve of water. Furthermore, magnitude of the relative volume change increases with decreasing molar fraction of benzene in the present experimental range. It is suggested that dissolving a small amount of benzene in water induces a change in the fluid density from a liquidlike condition to a gaslike condition in the vicinity of the critical region.

Temperature-dependent near-infrared spectra of bovine serum albumin in aqueous solutions: spectral analysis by principal component analysis and evolving factor analysis

Fourier transform near-infrared (FT-NIR) spectra have been measured for bovine serum albumin (BSA) in an aqueous solution (pH 6.8) with a concentration of 5.0 wt% over a temperature range of 45-85 degrees C. Not only conventional spectral analysis methods, such as second-derivative spectra and difference spectra, but also chemometrics, such as principal component analysis (PCA) and evolving factor analysis (EFA), have been employed to analyze the temperature-dependent NIR spectra in the 7500-5500 and 4900-4200 cm-1 regions of the BSA aqueous solution. Intensity changes of bands in the 7200-6600 cm-1 and 4650-4500 cm-1 regions in the difference spectra indicate variations of the hydration and secondary structure of BSA in the aqueous solution, respectively. The plot of a band intensity at 7080 cm-1 in the different spectra shows a clear turning point at 63 degrees C, revealing that a significant change in the hydration occurs at about 63 degrees C. The forward and backward eigenvalues (EVs) from EFA suggest that marked changes in the hydration and secondary structure of BSA take place in the temperature ranges of 61-65 degrees C and 59-63 degrees C, respectively. In addition, the temperature of 71 degrees C marked in the EFA plots may correspond to the onset temperature of increase in the intermolecular beta-sheet structure.