The local electronic structure of glycine in neutral, basic, and acidic aqueous solution is studied experimentally by X-ray photoelectron spectroscopy and theoretically by molecular dynamics simulations accompanied by first-principle electronic structure and spectrum calculations. Measured and computed nitrogen and carbon 1s binding energies are assigned to different local atomic environments, which are shown to be sensitive to the protonation/deprotonation of the amino and carboxyl functional groups at different pH values. We report the first accurate computation of core-level chemical shifts of an aqueous solute in various protonation states and explicitly show how the distributions of photoelectron binding energies (core-level peak widths) are related to the details of the hydrogen bond configurations, i.e. the geometries of the water solvation shell and the associated electronic screening. The comparison between the experiments and calculations further enables the separation of protonation-induced (covalent) and solvent-induced (electrostatic) screening contributions to the chemical shifts in the aqueous phase. The present core-level line shape analysis facilitates an accurate interpretation of photoelectron spectra from larger biomolecular solutes than glycine.
We have performed a density functional theory investigation of hydroformylation of ethylene for monosubstituted rhodium-carbonyl catalysts, HRh(CO)3L, where the modifying ligand, L, is a phosphite (L = P(OMe)3, P(OPh)3, or P(OCH2CF3)3), a phosphine (L = PMe3, PEt3, PiPr3, or PPh3), or a N-heterocyclic carbene (NHC) based on the tetrahydropyrimidine, imidazol, or tetrazol ring, respectively. The study follows the Heck and Breslow mechanism. Excellent correspondence between our calculations and existing experimental information is found, and the present results constitute the first example of a realistic quantum chemical description of the catalytic cycle of hydroformylation using ligand-modified rhodium carbonyl catalysts. This description explains the mechanistic and kinetic basis of the contemporary understanding of this class of reaction and offers unprecedented insight into the electronic and steric factors governing catalytic activity. The insight has been turned into structure-activity relationships and used as guidelines when also subjecting to calculation phosphite and NHC complexes that have yet to be reported experimentally. The latter calculations illustrate that it is possible to increase the electron-withdrawing capacity of both phosphite and NHC ligands compared to contemporary ligands through directed substitution. Rhodium complexes of such very electron-withdrawing ligands are predicted to be more active than contemporary catalysts for hydroformylation.
X-ray photoemission spectroscopy is used in a great variety of research fields; one observable is the sample's stoichiometry. The stoichiometry can be deduced based on the expectation that the ionization cross sections for innershell orbitals are independent of the molecular composition. Here we used chlorine-substituted ethanes in the gas phase to investigate the apparent carbon stoichiometry. We observe a nonstoichiometric ratio for a wide range of photon energies, the ratio exhibits x-ray-absorption fine structure spectroscopy (EXAFS)-like oscillations and hundreds of eV above the C1s ionization approaches a value far from 1. These effects can be accounted for by considering the scattering of the outgoing photoelectron, which we model by multiple-scattering EXAFS calculations, and by considering the effects of losses due to monopole shakeup and shakeoff and to intramolecular inelastic scattering processes.
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