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A series of oligomers and polymers having dimethylsilylene-spacedalternating 4-aminostyrene donor and stilbene acceptor chromophores(two to one) are regioselectively synthesized, and the two donor chromophoresare separated by different bridges between two donors. Photophysicaltools have been used to examine the folding behavior of these copolymers.Both steady-state and time-resolved fluorescence spectroscopic measurementswere examined. The relative intensities (ICT/ILE) between emission from charge-separatedstate (CT emission) and local excited emission of acceptor chromophore(LE emission) increase with increasing number of repeating units,and reach a plateau, when the linkers between the two aminostyrenechromophores are trimethylene bridges. Replacements of these by dimethyleneor tetramethylene linker reduce the relative intensities of CT emissionof the polymers, owing to the different folding behavior of thesepolymers. The CT emission intensity of the polymer with rigid piperazinelinkers is much lower than that with trimethylene-bridged copolymerof the same degree of polymerization. Slight conformational changeof these polymers would lead to slight variation of the distance betweendonor and acceptor chromophores so that the nonadiabatic interactionsin the excited state between donor–acceptor pairs in theseoligomers and polymers would be perturbed by such change of conformations. [ABSTRACT FROM AUTHOR]

Diamine-appended Mg 2 (dobpdc) (dobpdc 4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks are a promising class of CO 2 adsorbents, although their stability to SO 2 ─a trace component of industrially relevant exhaust streams─remains largely untested. Here, we investigate the impact of SO 2 on the stability and CO 2 capture performance of dmpn-Mg 2 (dobpdc) (dmpn = 2,2-dimethyl-1,3-propanediamine), a candidate material for carbon capture from coal flue gas. Using SO 2 breakthrough experiments and CO 2 isobar measurements, we find that the material retains 91% of its CO 2 capacity after saturation with a wet simulated flue gas containing representative levels of CO 2 and SO 2 , highlighting the robustness of this framework to SO 2 under realistic CO 2 capture conditions. Initial SO 2 cycling experiments suggest dmpn-Mg 2 (dobpdc) may achieve a stable operating capacity in the presence of SO 2 after initial passivation. Evaluation of several other diamine-Mg 2 (dobpdc) variants reveals that those with primary , primary (1°,1°) diamines, including dmpn-Mg 2 (dobpdc), are more robust to humid SO 2 than those featuring primary , secondary (1°,2°) or primary , tertiary (1°,3°) diamines. Based on the solid-state 15 N NMR spectra and density functional theory calculations, we find that under humid conditions, SO 2 reacts with the metal-bound primary amine in 1°,2° and 1°,3° diamine-appended Mg 2 (dobpdc) to form a metal-bound bisulfite species that is charge balanced by a primary ammonium cation, thereby facilitating material degradation. In contrast, humid SO 2 reacts with the free end of 1°,1° diamines to form ammonium bisulfite, leaving the metal-diamine bond intact. This structure-property relationship can be used to guide further optimization of these materials for CO 2 capture applications.

Ru nanoparticles are highly active catalysts for the Fischer-Tropsch and the Haber-Bosch processes. They show various types of surface sites upon CO adsorption according to NMR spectroscopy. Compared to terminal and bridging η 1 adsorption modes on terraces or edges, little is known about side-on η 2 CO species coordinated to B 5 or B 6 step-edges, the proposed active sites for CO and N 2 cleavage. By using solid-state NMR and DFT calculations, we analyze 13 C chemical shift tensors (CSTs) of carbonyl ligands on the molecular cluster model for Ru nanoparticles, Ru 6 (η 2 -μ 4 -CO) 2 (CO) 13 (η 6 -C 6 Me 6 ), and show that, contrary to η 1 carbonyls, the CST principal components parallel to the C-O bond are extremely deshielded in the η 2 species due to the population of the C-O π* antibonding orbital, which weakens the bond prior to dissociation. The carbonyl CST is thus an indicator of the reactivity of both Ru clusters and Ru nanoparticles step-edge sites toward C-O bond cleavage.

Molybdenum-based molecular alkylidyne complexes of the type [MesC≡Mo{OC(CH 3 ) 3-x (CF 3 ) x } 3 ] (MoF 0 , x = 0; MoF 3 , x = 1; MoF 6 , x = 2; MoF 9 , x = 3; Mes = 2,4,6-trimethylphenyl) and their silica-supported analogues are prepared and characterized at the molecular level, in particular by solid-state NMR, and their alkyne metathesis catalytic activity is evaluated. The 13 C NMR chemical shift of the alkylidyne carbon increases with increasing number of fluorine atoms on the alkoxide ligands for both molecular and supported catalysts but with more shielded values for the supported complexes. The activity of these catalysts increases in the order MoF 0 < MoF 3 < MoF 6 before sharply decreasing for MoF 9 , with a similar effect for the supported systems (MoF 0 ≈ MoF 9 < MoF 6 < MoF 3 ). This is consistent with the different kinetic behavior (zeroth order in alkyne for MoF 9 derivatives instead of first order for the others) and the isolation of stable metallacyclobutadiene intermediates of MoF 9 for both molecular and supported species. Detailed solid-state NMR analysis of molecular and silica-supported metal alkylidyne catalysts coupled with DFT/ZORA calculations rationalize the NMR spectroscopic signatures and discernible activity trends at the frontier orbital level: (1) increasing the number of fluorine atoms lowers the energy of the π*(M≡C) orbital, explaining the more deshielded chemical shift values; it also leads to an increased electrophilicity and higher reactivity for catalysts up to MoF 6 , prior to a sharp decrease in reactivity for MoF 9 due to the formation of stable metallacyclobutadiene intermediates; (2) the silica-supported catalysts are less active than their molecular analogues because they are less electrophilic and dynamic, as revealed by their 13 C NMR chemical shift tensors.

A library of 29 homologous Ru-based olefin metathesis catalysts has been tested for ethenolysis of cyclic olefins toward the goal of selectively forming α,ω-diene using cis-cyclooctene as a prototypical substrate. Dissymmetry at the N-heterocyclic carbene (NHC) ligand was identified as a key parameter for controlling the selectivity. The best-performing catalyst bearing an N-CF 3 group significantly outperformed the benchmark second-generation Grubbs catalyst in the ethenolysis of cis-cyclooctene. Application of this optimal catalyst to the ethenolysis of various norbornenes allows the efficient synthesis of valuable diene intermediates in good yields. The observed ligand effect trends could be rationalized through univariate and multivariate parameter analysis involving steric and electronic descriptors of the NHC ligand in the form of the buried volume and the 77 Se NMR chemical shift, in particular the σ yy component of the shielding tensor of [Se(NHC)] model compounds, respectively. Natural chemical shift analysis of this chemical shielding tensor shows that σ yy probes the π-acceptor property of the NHC ligand, the essential electronic parameter that drives the relative rate of degenerate metathesis and selectivity in ethenolysis with catalysts bearing dissymmetric NHC ligands.

Development of well-defined heterogeneous catalysts requires detailed structural characterization of active sites, an essential step toward establishing structure-activity relationships and promoting rational designs of catalysts. Solid-state NMR has emerged as a powerful approach to provide key molecular-level information about active-site structures and dynamics in heterogeneous catalysis. Here, we describe how one can apply solid-state NMR, ranging from 1D chemical shift assignments (and additional parameters, C Q and η, for quadrupolar nuclei, I > 1 / 2 ) to 2D correlations, to analysis of chemical shift anisotropy, providing unprecedented structural information about a broad range of materials. We also describe how modern hyperpolarization techniques like dynamic nuclear polarization can be used to improve the sensitivity of NMR, and make various challenging 1D/2D NMR experiments feasible, thus paving the way to determine the structures of surface sites.

Quantitative metabolomics has become much more important in clinical research in recent years. Individual differences in matrix effects (MEs) and the injection order effect are two major factors that reduce the quantification accuracy in liquid chromatography-electrospray ionization-mass spectrometry-based (LC-ESI-MS) metabolomics studies. This study proposed a postcolumn infused-internal standard (PCI-IS) combined with a matrix normalization factor (MNF) strategy to improve the analytical accuracy of quantitative metabolomics. The PCI-IS combined with the MNF method was applied for a targeted metabolomics study of amino acids (AAs). D8-Phenylalanine was used as the PCI-IS, and it was postcolumn-infused into the ESI interface for calibration purposes. The MNF was used to bridge the AA response in a standard solution with the plasma samples. The MEs caused signal changes that were corrected by dividing the AA signal intensities by the PCI-IS intensities after adjustment with the MNF. After the method validation, we evaluated the method applicability for breast cancer research using 100 plasma samples. The quantification results revealed that the 11 tested AAs exhibit an accuracy between 88.2 and 110.7%. The principal component analysis score plot revealed that the injection order effect can be successfully removed, and most of the within-group variation of the tested AAs decreased after the PCI-IS correction. Finally, targeted metabolomics studies on the AAs showed that tryptophan was expressed more in malignant patients than in the benign group. We anticipate that a similar approach can be applied to other endogenous metabolites to facilitate quantitative metabolomics studies.