Describing and predicting molecular properties via vibrational spectroscopy in combination with electron density analysis

The major aim of the present work is the correlation of electron density investigations with vibrational spectroscopic studies. In particular, Raman spectroscopy was applied to qualitatively approve DFT-calculated changes in the electron density distribution induced by structural modi¯cations. Moreover, a method was derived to predict properties of the electron density distribution quantitatively via combinations of vibrational spectroscopic and NMR spectroscopic data. Furthermore, the initial changes in the electron density distribution upon photoexcitation and related changes in the molecular structure were investigated via resonance Raman spectroscopy. The results of all these studies are shortly summarized in the following. After the impact and the limitations of electron density studies are outlined in the "Introduction" (chapter 1), the basic tools to calculate and analyze the electron density distribution ½(r) are summarized in chapter 2 "Theoretical details". In section 4.1 an il- lustrative example of ½(r)-studies in the life sciences was discussed in detail. This example is related to an investigation of Schirmeister and Luger, who studied the selectivity of an inhibition reaction of an aziridine derivative functioning as a protease inhibitor. For that purpose they determined the electron densities at the carbon atoms within the aziridine ring via high resolution x-ray measurements of an aziridine single crystal and via DFT-calculations of an isolated aziridine molecule. Continuing the work of Schirmeister and Luger the in°uences of neighboring molecules on ½(r) and therewith on the electrophilicity of the aziridine carbons were studied to shed light on the impact of intermolecular interactions on the electron density distribution ½(r). It turned out that NHN-hydrogen bridges and intermolecular interactions between dimethylmalonate moieties exhibit opposite in°uences on the aziridine ring. In particular, an electrophilic attack to a protease enzyme would occur at C2 if hydrogen bridges at the aziridine-N are ruling. In contrast, C1 would be more electrophilic than C2 if intermolecular interactions of the dimethylmalonate group are dominating. The results and the study of an aziridine in a simulated aqueous environment support the assumption of Schirmeister and Luger, who supposed the electrophilic attack occurs via C1

XPS investigations of MOCVD tin oxide thin layers on Si nanowires array

Tin oxide thin layers were grown by metal-organic chemical vapor deposition technique on the top-down nanostructured silicon nanowires array obtained by metal-assisted wet-chemical technique from single crystalline silicon wafers. The composition of the formed layers were studied by high-resolution X-ray photoelectron spectroscopy of tin (Sn 3d) and oxygen (O 1 s) atoms core levels. The ion beam etching was applied to study the layers depth composition profiles. The composition studies of grown tin oxide layers is shown that the surface of layers contains tin dioxide, but the deeper part contains intermediate tin dioxide and metallic tin phases

Self-Assembled Graphene/MWCNT Bilayers as Platinum- Free Counter Electrode in Dye-Sensitized Solar Cells

We describe the preparation and properties of bilayers of graphene- and multi-walled carbon nanotubes (MWCNTs) as an alternative to conventionally used platinum-based counter electrode for dye-sensitized solar cells (DSSC). The counter electrodes were prepared by a simple and easy-to-implement double self-assembly process. The preparation allows for controlling the surface roughness of electrode in a layer-by-layer deposition. Annealing under N2 atmosphere improves the electrode's conductivity and the catalytic activity of graphene and MWCNTs to reduce the I3 − species within the electrolyte of the DSSC. The performance of different counter-electrodes is compared for ZnO photoanode-based DSSCs. Bilayer electrodes show higher power conversion efficiencies than monolayer graphene electrodes or monolayer MWCNTs electrodes. The bilayer graphene (bottom)/MWCNTs (top) counter electrode-based DSSC exhibits a maximum power conversion efficiency of 4.1 % exceeding the efficiency of a reference DSSC with a thin film platinum counter electrode (efficiency of 3.4 %). In addition, the double self-assembled counter electrodes are mechanically stable, which enables their recycling for DSSCs fabrication without significant loss of the solar cell performance. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

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Describing and predicting molecular properties via vibrational spectroscopy in combination with electron density analysis Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) vorgelegt dem Rat der Chemisch-Geowissenschaftlichen Fakultät der Friedrich-Schiller-Universität Jena vo

Metal-mediated reaction modeled on nature: the activation of isothiocyanates initiated by zinc thiolate complexes

On the basis of detailed theoretical studies of the mode of action of carbonic anhydrase (CA) and models resembling only its reactive core, a complete computational pathway analysis of the reaction between several isothiocyanates and methyl mercaptan activated by a thiolate-bearing model complex [Zn(NH ) SMe]+ was performed at a high level of density functional theory (DFT). Furthermore, model reactions have been studied in the experiment using relatively stable zinc complexes and have been investigated by gas chromatography/mass spectrometry and Raman spectroscopy. The model complexes used in the experiment are based upon the well-known azamacrocyclic ligand family ([12]aneN , [14]aneN , i-[14]aneN , and [15]aneN ) and are commonly formulated as ([Zn- ([X]aneN )(SBn)]ClO . As predicted by our DFT calculations, all of these complexes are capable of insertion into the heterocumulene system. Raman spectroscopic investigations indicate that aryl-substituted isothiocyanates predominantly add to the CdN bond and that the size of the ring-shaped ligands of the zinc complex also has a very significant influence on the selectivity and on the reactivity as well. Unfortunately, the activated isothiocyanate is not able to add to the thiolate-corresponding mercaptan to invoke a CA analogous catalytic cycle. However, more reactive compounds such as methyl iodide can be incorporated. This work gives new insight into the mode of action and reaction path variants derived from the CA principles. Further, aspects of the reliability of DFT calculations concerning the prediction of the selectivity and reactivity are discussed. In addition, the presented synthetic pathways can offer a completely new access to a variety of dithiocarbamates. © 2011 American Chemical Society. © 2011 American Chemical Society

Interfacial Electrochemistry of Catalyst-Coordinated Graphene Nanoribbons

The immobilization of molecular electrocatalysts on conductive electrodes is an appealing strategy for enhancing their overall activity relative to analogous molecular compounds. In this study we report on the interfacial electrochemistry of self-assembled 2D nanosheets of graphene nanoribbons (GNR-2DNS) and analogs containing a Rh-based hydrogen evolution reaction (HER) catalyst (RhGNR-2DNS) immobilized on conductive electrodes. Proton-coupled electron transfer (PCET) taking place at N-centers of the nanoribbons was utilized as an indirect reporter of the interfacial electric fields experienced by the monolayer nanosheet located within the electric double layer. The experimental Pourbaix diagrams were compared with a theoretical model which derives the experimental Pourbaix slopes as a function of parameter f, a fraction of the interfacial potential drop experienced by the redox-active group. Interestingly, our study revealed that GNR- 2DNS was strongly coupled to glassy carbon electrodes (f = 1), while RhGNR-2DNS was not (f = 0.15). We further investigated the HER mechanism by RhGNR-2DNS using electrochemical and X-ray absorption spectroelectrochemical methods and compared it to the homogeneous molecular model compounds. RhGNR-2DNS was found to be an active HER electrocatalyst over a broader set of aqueous pH conditions than its molecular analogs. We find that the improved HER performance in the immobilized catalyst arises due to two factors. First, redox-active bipyrimidine- based ligands were shown to dramatically alter the activity of Rh sites by increasing the electron density at the active Rh center and providing RhGNR-2DNS the improved catalysis. Second, the catalyst immobilization was found to prevent catalyst aggregation that was found to occur for the molecular analog in the basic pH. Overall, this study provides valuable insights into the mechanistic by which catalyst immobilization can affect the overall electrocatalytic performance

Quantum chemical insights into the dependence of porphyrin basicity on the meso-aryl substituents: thermodynamics, buckling, reaction sites and molecular flexibility

The chemical and sensing properties of porphyrins are frequently tuned via the introduction of peripheral substituents. In the context of the exceptionally fast second protonation step in the case of 5,10,15,20- tetraphenylporphyrin (TPP), as compared to porphin and 5,10,15,20-tetramesitylporphyrin (TMesP), we investigated the macrocycle-substituent interactions of these three porphyrin derivatives in detail. Using quantum chemical thermodynamics calculations, the analysis of geometric structures, torsional profiles, electrostatic potential distributions, and particularly the analysis of molecular flexibilities via ab initio molecular dynamics simulations, we obtained a comprehensive picture of the reactivities of the studied porphyrins and how these are influenced by the meso-substituents. As compared to porphin and TMesP the second protonation of TPP is energetically more favorable and is particularly energetically comparable to its first protonation, instead of being significantly less favorable like in the case of porphyrin and TMesP. Additionally, the second TPP protonation is facilitated by an interplay between out-of-plane (oop) distortion of the protonation site and a pronounced electrostatic binding spot at the protonation site. Furthermore, the second protonation is particularly facilitated in the case of TPP by the large oop-flexibility of the diprotonated species as unraveled by ab initio molecular dynamics simulations

Intermolecular exciton-exciton annihilation in phospholipid vesicles doped with [Ru(bpy)(2)dppz](2+)

The ultrafast photophysics of [Ru(bpy)(2)dppz](2+) (dppz = dipyrido[3,2-a:2',3'-c]-phenazine) embedded into the walls of phospholipid vesicles has been studied by femtosecond time-resolved pump-probe spectroscopy. While [Ru(bpy)(2)dppz](2+) has been studied intensively with respect to its intramolecular charge transfer processes, which are associated with the well known light-switch effect, this study focuses on intermolecular energy transfer processes taking place upon dense packing of the complexes into a phospholipid membrane composed of dipalmitoyl-t-a-phosphatidylglycerol, which can be thought of as a simplistic model of a cellular membrane. The data indicate additional quenching of excited [Ru(bpy)(2)dppz](2+) upon increasing the pump-pulse intensity. Hence, the observed photophysics, which is assigned to the presence of intermolecular exciton-exciton annihilation at high pump-intensities, might be related to the ultrafast photophysics of [Ru(bpy)(2)dppz](2+) when used as a chromophore to stain cells, an effect that may be taken into account during the employment of novel cellular markers based on Ru polypyridine complexes. (C) 2015 Elsevier B.V. All rights reserved