Universität Stuttgart
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Item Open Access Probing self-diffusion of guest molecules in a covalent organic framework : simulation and experiment(2024) Grunenberg, Lars; Keßler, Christopher; Teh, Tiong Wei; Schuldt, Robin; Heck, Fabian; Kästner, Johannes; Groß, Joachim; Hansen, Niels; Lotsch, Bettina V.Covalent organic frameworks (COFs) are a class of porous materials whose sorption properties have so far been studied primarily by physisorption. Quantifying the self-diffusion of guest molecules inside their nanometer-sized pores allows for a better understanding of confinement effects or transport limitations and is thus essential for various applications ranging from molecular separation to catalysis. Using a combination of pulsed field gradient nuclear magnetic resonance measurements and molecular dynamics simulations, we have studied the self-diffusion of acetonitrile and chloroform in the 1D pore channels of two imine-linked COFs (PI-3-COF) with different levels of crystallinity and porosity. The higher crystallinity and porosity sample exhibited anisotropic diffusion for MeCN parallel to the pore direction, with a diffusion coefficient of Dpar = 6.1(3) × 10-10 m2 s-1 at 300 K, indicating 1D transport and a 7.4-fold reduction in self-diffusion compared to the bulk liquid. This finding aligns with molecular dynamics simulations predicting 5.4-fold reduction, assuming an offset-stacked COF layer arrangement. In the low-porosity sample, more frequent diffusion barriers result in isotropic, yet significantly reduced diffusivities (DB = 1.4(1) × 10-11 m2 s-1). Diffusion coefficients for chloroform at 300 K in the pores of the high- (Dpar = 1.1(2) × 10-10 m2 s-1) and low-porosity (DB = 4.5(1) × 10-12 m2 s-1) samples reproduce these trends. Our multimodal study thus highlights the significant influence of real structure effects such as stacking faults and grain boundaries on the long-range diffusivity of molecular guest species while suggesting efficient intracrystalline transport at short diffusion times.Item Open Access A real-time algorithm for FCIQMC(2020) Guther, Kai-Simon; Alavi, Ali (Prof. Ph.D.)New developments of the Full Configuration Interaction Quantum Monte Carlo (FCIQMC) method for solution of the time-dependent Schrödinger equation are investigated and a systematically improvable algorithm to obtain spectral functions via the FCIQMC framework is presented. Further, the implementation of an explicitly correlated method within the conventional FCIQMC method and its application is discussed.Item Open Access Development and application of embedded methods to strongly and weakly correlated systems(2022) Vitale, Eugenio; Alavi, Ali (Prof. Dr.)Coupled cluster (CC) theory is a popular and reliable tool in quantum chemistry due to its improvable hierarchy of methods able to rapidly converge to the full configuration interaction (FCI) limit in weakly correlated systems. Although it represents one of the most efficient single reference methods to treat many-body correlations with high accuracy and reliable outcomes, it yields qualitatively erroneous results when applied to strongly correlated systems. Within this thesis, the Distinguishable Cluster (DC) method (i.e., a small modification of CC amplitude equations able to qualitatively describe strongly correlated systems), is combined with FCI Quantum Monte Carlo (FCIQMC) in order to present a new tailored approach, the tailored DC (TDC), which is more accurate than the corresponding tailored CC and the pure DC. To demonstrate this, the method is first benchmarked with a variety of test cases and then further evaluated with computation of spin-state splittings in a few Fe(II) complexes. The systematic improvability of the TDC method is shown as the active space is increased. In the last part of the thesis, a further embedding scheme to treat strong correlation effects is evaluated. Specifically, the development and application of a screened Coulomb formalism is discussed. This simple approach inspired by Random Phase approximation (RPA) shows to be extremely efficient in the dissociation of one- and two-dimensional hydrogen systems.Item Open Access Linker-cluster cooperativity in confinement of proline-functionalized Zr-based metal-organic frameworks and its effect on the organocatalytic aldol reaction(2025) Dilruba, Zarfishan; Yeganeh, Ardeshir D.; Kolin, Sofia; Noor, Sadia; Shatla, Hassan; Wieland, Carl; Yu, Bo-Hung; Gugeler, Katrin; Zens, Anna; Kästner, Johannes; Estes, Deven P.; Pluhackova, Kristyna; Krause, Simon; Laschat, SabineMetal organic frameworks (MOFs) provide unique opportunities for molecular heterogeneous catalysis by mimicking the active sites of enzymes. However, understanding and controlling the interaction between the metal node and the organic linker carrying the catalytic unit and the resulting confinement effects remain challenging. Here, in a combined theoretical and experimental approach, Zr-UiO-67-MOFs with ortho-N -acylproline-functionalized biphenyl-dicarboxylate linkers were prepared and compared with the corresponding MOFs with regioisomeric meta -linkers. As benchmark catalysis, the organocatalytic aldol reaction of p -nitrobenzaldehyde and cyclohexanone was studied. Experimental results revealed that the ortho -linker accelerated the aldol reactions, whereas the regioisomeric meta -linker decreased the reaction rate, which was rationalized by pore blocking of the meta -linker via molecular dynamics simulations. Moreover, the acid modulator used in the MOF preparation also played a critical role in the formation of acetal byproducts through competing acid catalysis. Our study provides novel insights into the cooperative catalysis between the linker-attached organocatalyst and the MOF metal center.