The subtle energetic differences underpinning adjacent lanthanide discrimination are explored with diglycolamide ligands. Our approach converges liquid-liquid extraction experiments with solution-phase X-ray absorption spectroscopy (XAS) and density functional theory (DFT) simulations, spanning the lanthanide series. The homoleptic [(DGA)Ln] complex was confirmed in the organic extractive solution by XAS, and this was modeled using DFT. An interplay between steric strain and coordination energies apparently gives rise to a nonlinear trend in discriminatory lanthanide ion complexation across the series. Our results highlight the importance of optimizing chelate molecular geometry to account for both coordination interactions and strain energies when designing new ligands for efficient adjacent lanthanide separation for rare-earth refining.
The production of polyisobutylene with Lewis acid catalysts has been in widespread use for over 60 years, but no validated molecular-level understanding of the reaction mechanism exists. We have computed initiation and propagation reaction pathways for isobutylene polymerization under industrially relevant conditions with an AlCl3/H2O initiator from density functional theory calculations. The initiator/catalyst complex we identified is fundamentally different from the putative complex identified in the literature, which typically assumes that the AlCl3OH2 complex is the active catalyst. We found that the reaction pathway with the AlCl3OH2 complex is infeasible due to unreasonably high energy barriers. Our calculations indicate that a minimum of two AlCl3 groups and one H2O molecule is required to initiate the reaction and that the complex must produce a highly acidic proton. It is the extreme acidity of the complex that is crucial for successful initiation of the reaction. The active catalyst moiety we identified produces low-energy-barrier pathways for both initiation and propagation steps. This complex was identified using the growing-string method to identify possible reaction pathways with various AlCl3/H2O complexes. The initiation reaction with our proposed complex was observed to occur naturally in an ab initio molecular dynamics simulation under typical operating conditions, confirming the activity of the complex.
Thermodynamic and kinetic properties of molecular adsorption and transport in metal–organic frameworks (MOFs) are crucially important for many applications, including gas adsorption, filtration, and remediation of harmful chemicals. Using the in situ 1H nuclear magnetic resonance (NMR) isotherm technique, we measured macroscopic thermodynamic and kinetic properties such as isotherms and rates of mass transfer while simultaneously obtaining microscopic information revealed by adsorbed molecules via NMR. Upon investigating isopropyl alcohol adsorption in MOF UiO-66 by in situ NMR, we obtained separate isotherms for molecules adsorbed at distinct environments exhibiting distinct NMR characteristics. A mechanistic view of the adsorption process is obtained by correlating such resolved isotherms with the cage structure effect on the nucleus-independent chemical shift, molecular dynamics such as the crowding effect at high loading levels, and the loading level dependence of the mass transfer rate as measured by NMR and elucidated by classical Monte Carlo simulations.
We present a formalism for accurate estimation of dipole moments using quantum mechanics for complex molecules having conformational degrees of freedom. Dipole moments of complex molecules are often needed for use in correlations for estimating viscosities and other thermophysical properties. However, experimentally measured dipole moments are not available for many molecules, especially those used in proprietary industrial processes and products. Many complex molecules have dipole moments that change significantly in response to the conformation of the molecules. We show that proper accounting of the conformation-dependent dipole moment may be required to achieve an acceptably accurate estimate of the experimental dipole moment and provide recommendations on efficient estimation techniques. We also demonstrate that for molecules with dipole moments above about 1.3 D reasonably accurate estimation of the dipole moment is required for reliable prediction of vapor phase viscosity, whereas estimation of thermal conductivity is less sensitive to the dipole moment.
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