The sequential bond energies for complexes of Mg + with CO, CO 2 , NH 3 , CH 4 , CH 3 OH, and C 6 H 6 are determined by collision-induced dissociation (CID) with xenon or argon in a guided ion beam tandem mass spectrometer. The kinetic energy dependence of the CID and ligand exchange cross sections are analyzed to yield 0 and 298 K bond energies for Mg + -L after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Bond energies (in eV) to Mg + at 0 K are determined for L ) Ar (0.10 ( 0.07), Xe (0.32 ( 0.12), 1-2 CO (0.43 ( 0.06 and 0.40 ( 0.03), 1-3 CO 2 (0.60 ( 0.06, 0.50 ( 0.03, and 0.46 ( 0.06), 1-5 NH 3 (1.60 ( 0.12, 1.27 ( 0.07, 0.99 ( 0.09, 0.45 ( 0.11, and 0.58 ( 0.12), 1-2 CH 4 (0.29 ( 0.07 and 0.15 ( 0.07), 1-3 CH 3 OH (1.51 ( 0.07, 1.25 ( 0.07, and 0.95 ( 0.09), and one C 6 H 6 (1.39 ( 0.10 eV). As expected for largely electrostatic interactions, the sequential bond energies generally decrease monotonically with increasing number of ligands. These values are in good agreement with theoretical values in the literature and ab initio calculations performed here, but the agreement is mixed for comparison with results of photodissociation measurements. Qualitatively, geometries of these complexes are controlled by interactions of the ligands with the single polarized valence electron on Mg + .
In part I, we discussed the chain-propagating and possible competing mechanisms of low-temperature (300−1000 K) dimethyl ether (DME) combustion. Here we consider the chain-branching mechanism that results in explosive combustion, initiated by O2 addition to the ·CH2OCH2OOH intermediate formed in the earlier chain-propagation step. Ideally, chain-branching leads to the formation of two highly reactive ·OH radicals from the ·OOCH2OCH2OOH precursor. Each of these two ·OH radicals can initiate a chain-reaction “branch” with another DME molecule, which, ideally, leads to the formation of four more ·OH, and so on. This exponential increase in ·OH concentration causes an exponential increase in the DME oxidation rate, leading to explosive combustion. Here we show that although the pathway to create the first ·OH from ·OOCH2OCH2OOH in a hydrogen-transfer isomerization step is unambiguous, the formation of the second ·OH from the remaining hydroperoxyformate (HPMF or HOOCH2OC(O)H) fragment is potentially very complicated. HPMF has many possible fates, including HĊO + formic acid (HC(O)OH) + ·OH; H2O + formic acid anhydride (HC(O)OC(O)H); the Criegee intermediate (·CH2OO·) + formic acid; peroxyformic acid (HC(O)OOH) + H2 + CO; dihydroxymethylformate ((HO)2HCOC(O)H); ·OCH2OC(O)H + ·OH; and quite possibly others. The first and last of these products derived from HPMF directly produce ·OH and thus can complete the chain-branching step. Activation energies of 42−44 kcal/mol are needed to overcome barriers to form these two sets of products from HPMF. While these pathways directly form ·OH, they may not be the most favorable. The formation of a Criegee intermediate (·CH2OO·)−formic acid hydrogen-bonded adduct requires ∼15 kcal/mol less enthalpy than paths directly producing ·OH. Formation of the Criegee intermediate has never been considered as an intermediate in DME combustion before, but its formation (along with formic acid) appears to be the most favorable unimolecular path for HPMF decomposition. In atmospheric chemistry, decomposition of vibrationally excited ·CH2OO· can potentially lead to ·OH formation. Thus, we propose ·CH2OO· as a new intermediate that may significantly contribute to dimethyl ether's chain-branching mechanism.
The crystallography of transition Al2O3 has been extensively studied in the past, because of the advantageous properties of the oxide in catalytic and a range of other technological applications. However, existing crystallographic models are insufficient to describe the structure of many important Al2O3 polymorphs, because of their highly disordered nature. In this work, we investigate structure and disorder in high-temperature-treated transition Al2O3 and provide a structural description for θ-Al2O3 by using a suite of complementary imaging, spectroscopy, and quantum calculation techniques. Contrary to current understanding, our high-resolution imaging shows that θ-Al2O3 is a disordered composite phase of at least two different end-members. By correlating imaging and spectroscopy results with density functional theory (DFT) calculations, we propose a model that describes θ-Al2O3 as a disordered intergrowth of two crystallographic variants at the unit-cell level. One variant is based on β-Ga2O3, and the other on a monoclinic phase that is closely related to δ-Al2O3. The overall findings and interpretations afford new insight into the origin of poor crystallinity in transition Al2O3, and we also provide new perspectives on structural complexity that can emerge from intergrowth of closely related structural polymorphs.
We investigate the controlled deposition of Keggin polyoxometalate (POM) anions, PMo 12 O 40 3-and PMo 12 O 40 2-, onto different self-assembled monolayer (SAM) surfaces via soft landing of mass-selected ions. Utilizing in situ infrared reflection absorption spectroscopy (IRRAS), ex situ cyclic voltammetry (CV) and electronic structure calculations, we examine the structure and charge retention of supported multiply-charged POM anions and characterize the redox properties of the modified surfaces. SAMs of alkylthiol (HSAM), perfluorinated alkylthiol (FSAM), and alkylthiol terminated with NH 3 + functional groups (NH 3 + SAM) are chosen as model substrates for soft landing to examine the factors which influence the immobilization and charge retention of multiply charged anionic molecules. The distribution of charge states of POMs on different SAM surfaces are determined by comparing the IRRAS spectra with vibrational spectra calculated using density functional theory (DFT). In contrast to the results obtained previously for multiply charged cations, soft landed anions are found to retain charge on all three SAM surfaces. This charge retention is attributed to the substantial electron binding energy of the POM anions. Investigation of redox properties by CV reveals that, while surfaces prepared by soft landing exhibit similar features to those prepared by adsorption of POM fromsolution, the soft landed POM 2-has a pronounced shift in oxidation potential compared to POM 3-for one of the redox couples. These results demonstrate that ion soft landing is uniquely suited for precisely controlled preparation of substrates with specific electronic and chemical properties that cannot be achieved using conventional deposition techniques.
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In the liquid-phase catalytic processing of molecules using heterogeneous catalystsan important strategy for obtaining renewable chemicals from biomassmany of the key reactions occur at solid–liquid interfaces. In particular, glucose isomerization occurs when glucose is adsorbed in the micropores of a zeolite catalyst. Since solvent molecules are coadsorbed, the catalytic activity depends strongly and often nonmonotonically on the solvent composition. For glucose isomerization catalyzed by NaX and NaY zeolites, there is an initial steep decline when water is mixed with a small amount of the organic cosolvent γ-valerolactone (GVL), followed by a recovery as the GVL content in the mixed solvent increases. Here we elucidate the origin of this complex solvent effect using operando solid-state NMR spectroscopy. The glucopyranose tautomers immobilized in the zeolite pores were observed and their transformations into fructose and mannose followed in real time. The microheterogeneity of the solvent system, manifested by a nonmonotonic trend in the mixing enthalpy, influences the mobility and adsorption behavior of the carbohydrates, water, and GVL, which were studied using pulsed-field gradient (PFG) NMR diffusivity measurements. At low GVL concentrations, glucose is depleted in the zeolite pores relative to the solution phase, and changes in the local structure of coadsorbed water serve to further suppress the isomerization rate. At higher GVL concentrations, this lower intrinsic reactivity is largely compensated by strong glucose partitioning into the pores, resulting in dramatic (up to 32×) enhancements in the local sugar concentration at the solid–liquid interface.
We present a joint experimental and computational study of the hexacyanoferrate aqueous complexes at equilibrium in the 250 meV to 7.15 keV regime. The experiments and the computations include the vibrational spectroscopy of the cyanide ligands, the valence electronic absorption spectra, and Fe 1s core hole spectra using element-specific-resonant X-ray absorption and emission techniques. Density functional theory-based quantum mechanics/molecular mechanics molecular dynamics simulations are performed to generate explicit solute-solvent configurations, which serve as inputs for the spectroscopy calculations of the experiments spanning the IR to X-ray wavelengths. The spectroscopy simulations are performed at the same level of theory across this large energy window, which allows for a systematic comparison of the effects of explicit solute-solvent interactions in the vibrational, valence electronic, and core-level spectra of hexacyanoferrate complexes in water. Although the spectroscopy of hexacyanoferrate complexes in solution has been the subject of several studies, most of the previous works have focused on a narrow energy window and have not accounted for explicit solute-solvent interactions in their spectroscopy simulations. In this work, we focus our analysis on identifying how the local solvation environment around the hexacyanoferrate complexes influences the intensity and line shape of specific spectroscopic features in the UV/vis, X-ray absorption, and valence-to-core X-ray emission spectra. The identification of these features and their relationship to solute-solvent interactions is important because hexacyanoferrate complexes serve as model systems for understanding the photochemistry and photophysics of a large class of Fe(II) and Fe(III) complexes in solution.
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