540 Chemie und zugeordnete Wissenschaften
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- time-dependent density functional theory (1)
- time-resolved optical spectroscopy (1)
- titanium (1)
- transient absorption spectroscopy (1)
- transiente Absorption (1)
- transiente Absorptionsspektroskopie (1)
- transition metal complexes (1)
- trap (1)
- truxenone (1)
- van der Waals Clustern (1)
- van der Waals clusters (1)
- verzögerte Fluoreszenz (1)
- vibrational spectroscopy (1)
- vier Wellen Mischen Spektroskopie (1)
- vinylidene complexes (1)
- wallpainting (1)
- warhead (1)
- weak coupling regime (1)
- xylylene (1)
- zeitabhängige Dichtefunktionaltheorie (1)
- zeitaufgelöste Spektroskopie (1)
- Übergangszustand (1)
- ätherische Öle (1)
- π-complex (1)
- π-conjugated systems (1)
Institute
- Institut für Physikalische und Theoretische Chemie (116) (remove)
Sonstige beteiligte Institutionen
ResearcherID
- B-1911-2015 (1)
- N-3741-2015 (1)
Light-induced excitation of matter proceeds within femtoseconds, resulting in excited states. Originating from these states chemical reaction mechanisms, like isomerization or bond formation, set in. Photophysical mechanisms like energy distribution and excitonic delocalization also occur. Thus, the reaction scheme has to be disentangled by assessing the importance of each process. Spectroscopic methods based on fs laser pulses have emerged as a versatile tool to study these reactions. Within this thesis time-resolved experiments with fs laser pulses on various molecular systems were performed. Novel photosystems, with possible applications ranging from ultrathin molecular wires to molecular switches, were extensively characterized. To resolve the complex kinetics of the investigated systems, time-resolved techniques had to be newly developed. By combining a visible excitation pulse pair with an additional pulse and a continuum probe electronic triggered-exchange two-dimensional spectroscopy (TE2D) was demonstrated for the first time. This goal was accomplished by combining a three-color transient-absorption setup with a pulse shaper. Hence, 2D spectroscopy with a continuum probe was also implemented. Using these methods two different molecular systems in solution were characterized in a comprehensive manner. (ZnTPP)2, a directly beta,beta’-linked Zn-metallated bisporphyrin, and a spiropyran-merocyanine photosystem, 6,8-dinitro BIPS, were characterized. (ZnTPP)2 is a homodimer, featuring strong excitonic effects. These manifest themselves in a twofold splitting of the Soret band (S2). 6,8-Dinitro BIPS exists in one of two possible conformations. The ring closed spiropyran absorbs only in the UV, while the ring open merocyanine also absorbs in the visible. For both molecular systems photodynamics upon illumination were monitored using transient-absorption. However, the obtained results were ambiguous, necessitating more complex methods. In the case of (ZnTPP)2 first the monomeric building block was characterized. There, population transfer from the S2 state into S1 within 2 ps was identified. Afterwards, intersystem crossing proceeds within 2 ns. For (ZnTPP)2 similar pathways were found, albeit the relaxation is faster. The intersystem crossing with 1.5 ns was not only indirectly deduced but directly measured by probing in the NIR spectral range. The excitonic influence of was investigated by coherent 2D spectroscopy in the Soret band. Population transfer within S2 was directly visualized on a time-scale of 100 fs. Calculation of the 2D spectra of a simple homodimer confirmed the results. After this analysis of the distinct excitonic character, this molecule may serve as a building block for larger porphyrin arrays with applications ranging from asymmetric catalysis over biomimicry of electron-transfer to organic optical devices. The second photosystem was the molecular switch 6,8-dinitro BIPS, existing in two conformations. Merocyanine is the more stable form in thermal equilibrium. Transient-absorption measurements uncovered that the sample consisted of a mixture of two merocyanine isomers, referred to as TTC and TTT. However, both isomers are capable of ring-closure forming spiropyran. The remaining excited molecules return to the ground state radiatively. Conducting 2D measurements utilizing a continuum probe the differing photochemistry of both isomers was examined in a single measurement. No isomerization between these conformations was detected. Therefore, 6,8-dinitro BIPS performs a concerted switching without long-living intermediates. This was confirmed by a pump-repump-probe scan. 6,8-DinitroBIPS can be closed by visible and opened by UV pulses using subsequent pulses and vice versa. These mechanisms via singlet pathways satisfy an important criterion for a unimolecular switching device. A second pump-repump-probe experiment showed that the sample is ionized, resulting in a merocyanine radical cation, when the first excited state is resonantly excited. Furthermore, by implementing TE2Dspectroscopy, it was elucidated that only TTC was ionized. Taking all this into account new techniques were developed and complex molecular systems were characterized within this thesis. Deeper insight into the photodynamics of (ZnTPP)2and 6,8-dinitro BIPS was gained by adapting transient absorption for the NIR spectral range, constructing a 2D setup in pump-probe geometry, and combining it with multipulse excitation to coherent TE2D. All techniques solved the questions for which they were constructed, but they are not limited to these cases. Especially TE2D opens new roads in photochemistry. By connecting reactant, product and the corresponding intermediates, a chemical reaction can be tracked through all stages, making unambiguous identification of the reactive states feasible. Thus, fundamental insight into the photochemistry of molecular compounds is gained.
Im ersten Teil wurde die Dynamik des ersten angeregten Zustandes von drei Truxenonen untersucht. Nach Anregung im sichtbaren Bereich findet ein Elektrontransfer zwischen den Triarylamin-Donor und dem Truxenon-Akzeptor statt. Um die Abhängigkeit der Rate für den Rücktransfer von der elektronischen Kopplung zu untersuchen, wurde diese zum einen über den Abstand zwischen Donor und Akzeptor und zum anderen über die Position der Verknüpfung eingestellt. In einer ersten Studie wurde Truxenon 1, bei dem der direkt über das Stickstoff-Atom an den Akzeptor gekuppelt ist, mit dem System 2 verglichen, bei den die Einheiten über einen Phenyl-Spacer verbunden sind. Der Rücktransfer sollte dabei für das System 1 schneller sein, da ein kurzer Abstand mit einer starken elektronischen Kopplung einhergeht und damit auch mit einem schnellen Elektronentransfer. Allerdings wird die große Rate für das System mit dem größeren Abstand beobachtet (2). Dieses Ergebnis kann mit der Geometrie der Moleküle und der größeren sterischen Hinderung in 1 erklärt werden, aus der eine geringere elektronische Kopplung resultiert. In einem weiteren Experiment wurde die Stärke der elektronischen Kopplung in Abhängigkeit von der Position der Verknüpfung in Bezug auf den Phenyl-Spacer untersucht. Zu diesem Zweck wurden die Systeme 2 und 3 miteinander verglichen. Während in 2 die Einheiten in para-Position verknüpft sind, sind Donor und Akzeptor in 3 in meta-Position an den Phenyl-Spacer gekuppelt. Letzteres System zeichnet sich dabei durch eine geringere Resonanzstabilisierung aus. Dies hat eine geringere elektronische Kopplung zur Folge, was sich auch in den UV/Vis-Spektren zeigt. Die langwelligste Absorption ist hier bei höheren Energien zu beobachten. Zudem deuten die transienten Spektren an, dass in erster Linie nicht der ladungsgetrennte Zustand abgeregt wird sondern vielmehr die Truxenon-Einheit selbst. Im zweiten Teil wurden die Resonanz-Raman-Spektren vier verschiedener Borole aufgenommen. Dabei wurden zwei signifikanten Moden beobachtet, die beim pi –pi∗ -Übergang in ihrer Intensität verstärkt werden. Eine Bande bei 1598 cm-1 wird der symmetrischen Ringatmung zugeordnet, die aus einer Expansion des Borol-Rings resultiert. Eine zweite Schwingung bei 1298 cm-1 resultiert aus einer B-R Streckschwingung. Für System 5 wird diese Schwingung mit einer hohen Intensität beobachtet, während die Bande bei den Systemen 6-8, die mit einem Aryl-Rest substituiert sind, mit sehr geringer Intensität auftritt und deshalb lediglich mit einem hochauflösendem Setup detektiert werden kann. Aufgrund der schwachen Resonanzverstärkung kann von einer schwachen Wechselwirkung zwischen dem Bor und dem Aryl-Rest ausgegangen werden. In Borol 5, in dem eine Ferrocen-Einheit an das Bor gebunden ist, ist die Situation eine andere: nach Anregung des pi-pi*-Übergangs wird die Population im BC_4-Ring verschoben. Dadurch kann vom Eisen keine Elektronendichte mehr in das p_z-Orbital des Bors verschoben werden, die Fe-B-Wechselwirkung wird geschwächt und der Fe-B-Abstand wird vergrößert. Zusammenfassend konnte gezeigt werden, dass die Eigenschaften des Substituenten großen Einfluss auf die elektronische Struktur eines dreifach-substituierten Bor-Atoms hat, das in einer p_z-pi-Konjugation beteiligt ist.
A series of combustion relevant species like radicals, carbenes and polycyclic aromatic hydrocarbons were characterized in the gas phase by vacuum UV synchrotron radiation and their ionization energies (IE) and further spectroscopic details of the respective cations were retrieved from threshold photoelectron spectra. The reactive intermediates were generated by flash vacuum pyrolysis from stable precursor molecules. Furthermore three polycyclic aromatic hydrocarbons were investigated by threshold photoelectron spectroscopy, too. The experiment was performed at the VUV beamline of the Swiss Light Source in Villigen/Switzerland and the iPEPICO (imaging photoelectron photoion coincidence) setup was applied to correlate ions and electrons from the same ionization event. From the threshold photoelectron spectra and from quantum chemical computations the vibrational structure of the molecule cations and the geometry changes upon ionization were assigned. The ionization energies of the two C4H5 isomers 2-butyn-1-yl and 1-butyn-3-yl were assigned to 7.94±0.02 eV and 7.97±0.02 eV, respectively. The isomerization between the two isomers was computed to have a barrier of 2.20 eV, so a rearrangement between the two radicals cannot be excluded. From the threshold photoelectron spectra of the two constitutional C4H7 isomers 1-methylallyl and 2-methylallyl the ionization energies were assigned to 7.48±0.02 eV and to 7.59±0.02 eV for 1-E-methylallyl and 1-Z-methylallyl, as well as to 7.88±0.01 eV for 2-methylallyl. The two radicals 9-fluorenyl, C13H9, and benzhydryl, C13H11, were observed to ionize at 7.01±0.02 eV and 6.7 eV. The threshold photoelectron spectrum of benzhydryl also incorporated the signal of the diphenylmethyl carbene, C13H10, which has an IE at 6.8 eV. In addition, the head-to-head dimers of 9-fluorenyl and benzhydryl were observed as products in the pyrolysis. C26H18 has an IE at 7.69±0.04 eV and C26H22 has an IE at 8.13±0.04 eV. The three polycyclic aromatic hydrocarbon DHP (C14H16) 1-PEN (C18H22) and THCT (C22H16) were investigated in an effusive beam. The ionization energies were determined to IE(DHP)= 7.38±0.02 eV, IE(1-PEN)=7.58±0.05 eV and IE(THCT)=6.40±0.02 eV. Furthermore the thermal decomposition and the dissociative photoionization of diazomeldrum’s acid was investigated. The pyrolysis products yielded beside several other products the two not yet (by photoelectron spectroscopy) characterized molecules E-formylketene, C3O2H2 and 2-diazoethenone, N2C2O. The dissociative photoionization showed the Wolff rearrangement to occur at higher internal energies.
This work focuses on theoretical approaches for predicting the valence and core excited states of aggregate systems. For the valence excitations, TD-HF and TD-DFT with different functionals have been tested at the Perylene bisimide (PBI) system. A simple character analysis method based on the calculated transition dipole moments is proposed. However, this method does not work for excited states without any transition dipole moment. Thus, we proposed a more general and more valid method based on a calculated CIS type wavefunction for the character analysis. Furthermore, a model Hamiltonian method is derived from a localized picture. The energies of the diabatic states and the corresponding coupling parameters were also determined on the basis of ab initio calculations. For the core excitation, three different methods were validated for C 1s-excited and ionized states if several small molecules. Also we tested the basis sets dependence of these core excited states. Based on those results, we chose the frozen core approximation method to evaluate the core excited states of NTCDA molecules. In order to explain the findings in the experiments, we developed an algorithm to evaluate the exciton coupling parameter where non-orthogonal MOs are used.
The goal of this thesis was the development and application of higher-order spectroscopic techniques. In contrast to ordinary pump–probe (PP) and two-dimensional (2D) spectroscopy, higher-order coherently detected spectroscopic methods measure a polarization that has an order of nonlinearity higher than three. The key idea of the techniques in this thesis is to isolate the higher-order signals from the lower-order signals either by their excitation frequency or by their excitation intensity dependence. Due to the increased number of interactions in higher-order spectroscopy, highly excited states can be probed. For excitonic systems such as aggregates and polymers, the fifth-order signal allows one to directly measure exciton–exciton annihilation (EEA). In polymers and aggregates, the exciton transport is not connected to a change of the absorption and can therefore not be investigated with conventional third-order techniques. In contrast, EEA can be used as a probe to study exciton diffusion in these isonergetic systems. As a part of this thesis, anisotropy in fifth-order 2D spectroscopy was investigated and was used to study geometric properties in polymers.
In 2D spectroscopy, the multi-quantum signals are separated from each other by their spectral position along the excitation axis. This concept can be extended systematically to higher signals. Another approach to isolate multi-quantum signals in PP spectroscopy utilizes the excitation intensity. The PP signal is measured at specific excitation intensities and linear combinations of these measurements result in different signal contributions. However, these signals do not correspond to clean nonlinear signals because the higher-order signals contaminate the lower-order multi-quantum signals. In this thesis, a correction protocol was derived that uses the isolated multiquantum signals, both from 2D spectroscopy and from PP spectroscopy, to remove the contamination of higher-order signals resulting in clean nonlinear signals. Using the correction on the third-order signal allows one to obtain annihilation-free signals at high excitation intensities, i.e., with high signal-to-noise ratio. Isolation and correction in PP and 2D spectroscopy were directly compared by measuring the clean third-order signals of squaraine oligomers at high excitation intensities. Furthermore, higher-order PP spectroscopy was used to isolate up to the 13th nonlinear order of squaraine polymers.
The demonstrated spectroscopic techniques represent general procedures to isolate clean signals in terms of perturbation theory. The technique of higher-order PP spectroscopy needs only small modifications of ordinary PP setups which opens the field of higher-order spectroscopy to the broad scientific community. The technique to obtain clean nonlinear signals allows one to systematically increase the number of interacting (quasi)particles in a system and to characterize their interaction energies and dynamics.
Nonlinear frequency conversion of low-energy fs laser pulses was investigated in solid-state media. Raman conversion in the white-light-free regime of impulsive stimulated Raman scattering was achieved by pumping KGW crystal with Bessel beam. Efficient supercontinuum generation was demonstrated for sub-microjule pulses focused into microstructure fiber. Application of four-wave mixing techniques to monitoring of the excited-state dynamics in polyatomic molecules was demonstrated. Time constants of the processes related to vibrational energy redistribution upon the initial photoexcitation of stilbene-3 were determined by means of pump-CARS technique, where CARS process served as an effective mode-selective filter. Spectral as well as temporal properties of electronic relaxation pathway in azulene derivatives were explored by using transient population gratings and pump-probe transient absorption techniques.
We introduce a new approach to transient spectroscopy, fluorescence‐detected pump–probe (F‐PP) spectroscopy, that overcomes several limitations of traditional PP. F‐PP suppresses excited‐state absorption, provides background‐free detection, removes artifacts resulting from pump–pulse scattering, from non‐resonant solvent response, or from coherent pulse overlap, and allows unique extraction of excited‐state dynamics under certain conditions. Despite incoherent detection, time resolution of F‐PP is given by the duration of the laser pulses, independent of the fluorescence lifetime. We describe the working principle of F‐PP and provide its theoretical description. Then we illustrate specific features of F‐PP by direct comparison with PP, theoretically and experimentally. For this purpose, we investigate, with both techniques, a molecular squaraine heterodimer, core–shell CdSe/ZnS quantum dots, and fluorescent protein mCherry. F‐PP is broadly applicable to chemical systems in various environments and in different spectral regimes.
Femtosekunden-zeitaufgelöste transiente Absorptionsspektroskopie einer neutralen organischen gemischtvalenten Verbindung In einem Femtosekunden-zeitaufgelösten Anrege-Abfrage-Experiment wurde die Dynamik des Elektronentransfers einer neutralen organischen gemischtvalenten Verbindung untersucht. Neben der Abhängigkeit des Rückelektronentransfers von der Solvenspolarität wurde auch die Rotationsdiffusion in n-Hexan, Toluol, Dibutylether, tert-Butylmethylether und Benzonitril studiert. Die transiente Dynamik lässt sich mit einer Lebensdauer beschreiben, verursacht durch einen Rückelektronentransfer. Während dieser in unpolaren Lösemitteln relativ langsam verläuft, beobachtet man deutlich schnellere ET-Raten mit steigender Polarität des Lösemittels. Die Lebensdauer variiert von 1.2 ps für Benzonitril bis 260 ps für n-Hexan. Rotationsdiffusion konnte nicht beobachtet werden. Die gemessenen Raten wurden mit theoretischen Raten verglichen. Für unpolare Lösemittel konnte eine gute Übereinstimmung gefunden werden. In polaren Lösemitteln bewirkt eine Korrektur, die die Solvensrelaxationszeit berücksichtigt, eine sehr gute Übereinstimmung von berechneten und gemessenen Rückelektronentransferraten. Zeit- und frequenzaufgelöste Photoionisation des C 2A2-Zustandes des Benzylradikals Die Lebensdauer des C 2A2-Zustandes des Benzylradikals wurde in Abhängigkeit der Überschussenergie bestimmt. Die zeitabhängigen Ionensignale konnten dabei mit einer biexponentiellen Dynamik beschrieben werden. Bei einer Anregung am Ursprung (305nm) betragen die Lebensdauern τ1= 400 fs und τ2 = 4.5 ps. Die kürzere Lebensdauer τ1 beschreibt die interne Konversion vom C-Zustand zu den stark koppelnden A/B-Zuständen, die längere Lebensdauer τ2 die interne Konversion von den A/B-Zuständen in den elektronischen Grundzustand. Mit steigender Anregungsenergie beobachtet man eine stete Abnahme beider Lebensdauern. Bei einer Anregung mit einem Puls der Wellenlänge von 301 nm beobachtet man deutlich kürzere Lebensdauern mit τ1 = 350 fs und τ2 = 2.8 ps. Erfolgt die Anregung mit einem Puls der Wellenlänge von 298 nm, betragen die Zeitkonstanten τ1 = 180 fs und τ2 = 2.1 ps. Desweiteren konnte ein zeitabhängiges Ionensignal für eine Spezies mit der Zusammensetzung C7H5 beobachtet werden. Der Träger des Signals ist das Fulvenallenylradikal.
In this work, the laser control of molecules was investigated theoretically. In doing so, emphasis was layed on entering vectorial properties and in particular the orientation in the laboratory frame. Therefore, the rotational degree of freedom had to be included in the quantum mechanical description. The coupled vibrational and rotational dynamics was examined, which is usually not done in coherent control theory. Local control theory was applied, where the field is determined from the dynamics of a system, which reacts with an instantaneous response to the perturbation and, in turn, determines the field again. Thus, the field is entangled with the quantum mechanical motion and the presented examples document, that this leads to an intuitive interpretation of the fields in terms of the underlying molecular dynamics. The limiting case of a classical treatment was shown to give similar results and hence, eases to understand the complicated structure of the control fields. In a different approach, the phase- and amplitude shaping of laser fields was systematically studied in the context of controlling population transfer in molecules.
The first is via direct dissociation and the second likely involves a barrier slowing down dissociation.
Chlorine-containing hydrocarbons pose a great risk for the environment and especially for the atmosphere. In this thesis I present the photodissociation dynamics of multiple chlorine-containing molecules. The method of velocity map imaging was utilized for gaining information on the kinetic energy distribution of the fragments generated in the photodissociation reactions.
First, the photodissociation of benzoyl chloride after excitation to the S1, S2 and the S3 state between 279 nm and 237 nm was studied. This stable molecule was an ideal candidate for demonstrating a new ionization scheme for chlorine atoms. It was shown that benzoyl chloride dissociates statistically from the ground state.
Afterwards, the results from experiments on the radicals trichloromethyl and dichlorocarbene are presented in the range of 230 to 250 nm. These radicals remain after the dissociation of carbon tetrachloride and have not been studied in detail because of their instability. Trichlormethyl dissociates via two paths: The loss of a chlorine atom to dichlorocarbene and by decaying to CCl and a chlorine molecule. The dissociation to dichlorocarbene involves a barrier. If the photon exciting the molecule has enough energy to surpass the barrier, which is the case starting at around 235 nm, trichlormethyl dissociates rapidly resulting in an anisotropic VMI. However, if the the excitation energy is lower, the dissociation takes longer than a rotational period and the anisotropy is lost.The path to CCl is a statistical dissociation.
Dichlorocarbene dissociates to CCl and Cl via to separate channels. The first is via direct dissociation and the second likely involves a barrier slowing down dissociation.
Ziel dieser Arbeit war es, die Möglichkeiten zur Verwendung des Auges bzw. der Augenvorderkammer als spektroskopische Zelle für nicht-invasive In-vivo-Messungen zu untersuchen. Dabei stand vor allem die Geräte-technische Umsetzung und die Entwicklung geeigneter Auswertestrategien im Vordergrund. In dieser Arbeit konnte gezeigt werden, dass die spektroskopische Untersuchung von Kammerwasser-Substanzen möglich ist. Durch den Einsatz der UV/VIS-Spektroskopie konnte Fluorescein in vivo bestimmt werden. Die Anwendung der NIR-Spektroskopie eignet sich vor allem zur Bestimmung von Glucose im Kammerwasser. Die Güte der Glucose-Bestimmung ist von verschiedenen Faktoren abhängig, wie z. B. dem verwendeten Gerät, den Parameter der Auswertung und der Temperatur. Für weitergehende Studien sollten daher die in dieser Arbeit aufgedeckten Probleme und Strategien beachtet werden.
The present studies which have been performed in the work-group C-2 (Prof. W. Kiefer) within the program of the Sonderforschungsbereichs 347, deal with the FT-Raman and –IR spectroscopy on new organometallic complexes, synthesized in the work-groups B-2 (Prof. W. Malisch), B-3 (Prof. W. A. Schenk), D-1 (Prof. H. Werner) and D-4 (Prof. D. Stalke). The FT-Raman spectra recorded at 1064 nm led to very useful and interesting information. Furthermore, the DFT calculations which are known to offer promise of obtaining accurate vibrational wavenumbers, were successfully used for the assignment of the vibrational spectra. For the first time it has been possible to ascribe exactly the n(RhC) stretching mode in the vinylidene rhodium(I) complex trans-[RhF(=C=CH2)(PiPr3)2] by using isotopic substitution, in conjunction with theoretical calculations. This is also true for the complexes trans-[RhF(CO)(PiPr3)2], trans-[RhF(C2H4)(PiPr3)2], trans-[RhX(=C=CHPh)(PiPr3)2] (X = F, Cl, Br, I, Me, PhCºC) and trans-[RhX(CN-2,6-xylyl)(PiPr3)2] (X = F, Cl, Br, I, CºCPh). In addition, the comparison between the n(RhC) wavenumbers of the complexes trans-[RhF(=13C=13CH2)(PiPr3)2] and trans-[RhF(CO)(PiPr3)2], containing the isoelectronic ligands 13C=13CH2 and CO, which have the same reduced mass, indicated that the Rh-C bond is stronger in the carbonyl than in the vinylidene complex. Besides, the n(RhF) stretching mode, which has been observed at higher wavenumbers in the FT-Raman and -IR spectra of trans-[RhF(CO)(PiPr3)2], showed that the carbonyl ligand is a better p-acceptor and a less effective s-donor than the vinylidene one. Moreover, the comparison of the n(CºC) and n(Rh-C) modes from the FT-Raman spectrum of the complexes trans-[Rh(CºCPh)(L)(PiPr3)2] (L = C=CHPh, CO, CN-2,6-xylyl) point out that the p-acceptor ability of the ligand trans to CºCPh should rise in the order C=CH2 < CO < CN-2,6-xylyl £ C=CHPh. The investigated sensitivity of the n(RhC), n(CC), n(CO) and n(CN) vibrational modes to the electronic modifications occuring in the vinylidene, carbonyl, ethylene and isonitrile complexes, should allow in the future the examination of the p-acceptor or p-donor properties of further ligands. Likewise, we were able to characterize the influence of various X ligands on the RhC bond by using the n(RhC) stretching mode as a probe for the weakening of this. The calculated wavenumbers of the n(RhC) for the vinylidene complexes trans-[RhX(=C=CHR)(PiPr3)2], where R = H or Ph, suggested that the strength of the Rh=C bond increases along the sequence X = CºCPh < CH3 < I < Br < Cl < F. For the series of carbonyl compounds trans-[RhX(CO)(PiPr3)2], where X = F, Cl, Br and I, analogous results have been obtained and confirmed from the model compounds trans-[RhX(CO)(PMe3)2]. Since, the calculated vibrational modes for the ethylene complex trans-[RhF(C2H4)(PiPr3)2] were in good agreement with the experimental results and supported the description of this complex as a metallacyclopropane, we were interested in getting more information upon this class of compounds. In this context, we have recorded the FT-Raman and -IR spectra of the thioaldehyde complexes mer-[W(CO)3(dmpe)(h2-S=CH2)] and mer-[W(CO)3(dmpe)(h2-S=CD2)] which have been synthezised by B-3. The positions of the different WL vibrational modes anticipated by the DFT calculations, were consistent with the experimental results. Indeed, the analysis of the band shifts in the FT-Raman and –IR spectra of the isotopomer mer-[W(CO)3(dmpe)(h2-S=CD2)] confirmed our assignment. The different stereoisomers of complex mer-[W(CO)3(dmpe)(h2-S=CH2)] were investigated too, since RMN and IR-data have shown that complex mer-[W(CO)3(dmpe)(h2-S=CH2)] lead in solution to an equilibrium. Since the information on the vibrational spectra of the molybdenum and tungsten complexes Cp(CO)2M-PR2-X (M = Mo, W; R = Me, tBu, Ph; X = S, Se) is very scarce, we extended our research work to this class of compounds. We have tried to elucidate the bonding properties in these chalcogenoheterocycle complexes by taking advantage of the mass effect on the different metal atoms (W vs. Mo). Thus, the observed band shifts allowed to assign most of the ML fundamental modes of these complexes. This project and the following one were a cooperation within the work-group B-2. The Raman and IR spectra of the matrix isolated photoproducts expected by the UV irradiation of the iron silyl complex Cp(CO)2FeSiH2CH3 have been already reported by Claudia Fickert and Volker Nagel in their PhD-thesis. Since no exact assignment was feasible for these spectra, we were interested in the study of the reaction products created by irradiation of the carbonyl iron silyl complex Cp(CO)2FeCH2SiH3. Although the calculated characteristic vibrational modes of the metal ligand unit for the various photoproducts are significantly different in constitution, they are very similar in wavenumbers, which did not simplify their identification. However, the theoretical results have been found to be consistent with the earlier experimental results. Finally, the last part of this thesis has been devoted to the (2-Py)2E- anions which exhibit a high selectivity toward metal-coordination. All di(2-pyridyl) amides and -phosphides which were synthesized by D-4, coordinate the R2Al+ fragment via both ring nitrogen atoms. This already suggests that the charge density in the anions is coupled into the rings and accumulated at the ring nitrogen atoms, but the Lewis basicity of the central nitrogen atom in Et2Al(2-Py)2N is still high enough to coordinate a second equivalent AlEt3 to form the Lewis acid base adduct Et2Al(2-Py)2NAlEt3. Due to the higher electronegativity of the central nitrogen atom in Me2Al(2-Py)2N, Et2Al(2-Py)2N and Et2Al(2-Py)2NAlEt3, compared to the bridging two coordinated phosphorus atom in Me2Al(2-Py)2P and Et2Al(2-Py)2P, the di(2-pyridyl)amide is the hardest Lewis base. In the phosphides merely all charge density couples into the rings leaving the central phosphorus atom only attractive for soft metals. These results were confirmed by using DFT and MP2 calculations. Moreover, a similar behaviour has been observed and described for the benzothiazolyl complex [Me2Al{Py(Bth)P}], where complementary investigations are to be continued. The DFT calculations carried out on the model compounds analysed in these studies supply very accurate wavenumbers and molecular geometries, these being in excellent agreement with the experimental results obtained from the corresponding isolated complexes.
Tetraiododiborane(4) (B\(_2\)I\(_4\)) is a Polymer based on sp\(^3\) Boron in the Solid State
(2020)
Herein we present the first solid‐state structures of tetraiododiborane(4) (B\(_2\)I\(_4\)), which was long believed to exist in all phases as discrete molecules with planar, tricoordinate boron atoms, like the lighter tetrahalodiboranes(4) B\(_2\)F\(_4\), B\(_2\)Cl\(_4\), and B\(_2\)Br\(_4\). Single‐crystal X‐ray diffraction, solid‐state NMR, and IR measurements indicate that B\(_2\)I\(_4\) in fact exists as two different polymeric forms in the solid state, both of which feature boron atoms in tetrahedral environments. DFT calculations are used to simulate the IR spectra of the solution and solid‐state structures, and these are compared with the experimental spectra.
Covalent peptidomimetic protease inhibitors have gained a lot of attention in drug development in recent years. They are designed to covalently bind the catalytically active amino acids through electrophilic groups called warheads. Covalent inhibition has an advantage in terms of pharmacodynamic properties but can also bear toxicity risks due to non-selective off-target protein binding. Therefore, the right combination of a reactive warhead with a well-suited peptidomimetic sequence is of great importance. Herein, the selectivities of well-known warheads combined with peptidomimetic sequences suited for five different proteases were investigated, highlighting the impact of both structure parts (warhead and peptidomimetic sequence) for affinity and selectivity. Molecular docking gave insights into the predicted binding modes of the inhibitors inside the binding pockets of the different enzymes. Moreover, the warheads were investigated by NMR and LC-MS reactivity assays against serine/threonine and cysteine nucleophile models, as well as by quantum mechanics simulations.
In the present contribution, we describe the synthesis of highly dispersed silver nanorods (NRs) of different aspect ratios using a chemical route. The shape and size of the synthesized NRs were characterized by Transmission Electron Microscopy (TEM) and UV-visible spectroscopy. Longitudinal and transverse absorptions bands confirm the rod type structure. The experimentally recorded UV-visible spectra of NRs solutions were fitted by using an expression of the extinction coefficient for rod like nano structures under the dipole approximation. Simulated and experimentally observed UV-visible spectra were compared to determine the aspect ratios (R) of NRs. The average values of R for NR1, NR2 and NR3 solutions are estimated to be 3.0 ± 0.1, 1.8 ± 0.1 and 1.2 ± 0.1, respectively. These values are in good agreement with those obtained by TEM micrographs. The silver NRs of known aspect ratios are used to study antimicrobial activities against B. subtilis (gram positive) and E. coli (gram negative) microbes. We observed that the NRs of intermediate aspect ratio (R = 1.8) have greater antimicrobial effect against both, B. subtilis (gram positive) and E. coli (gram negative). The NRs of aspect ratio, R = 3.0 has better antimicrobial activities against gram positive than on the gram negative.
The SARS virus is the etiological agent of the severe acute respiratory syndrome, a deadly disease that caused more than 700 causalities in 2003. One of its viral proteins, the SARS coronavirus main protease, is considered as a potential drug target and represents an important model system for other coronaviruses. Despite extensive knowledge about this enzyme, it still lacks an effective anti-viral drug. Furthermore, it possesses some unusual features related to its active-site region. This work gives atomistic insights into the SARS coronavirus main protease and tries to reveal mechanistic aspects that control catalysis and inhibition. Thereby, it applies state-of-the-art computational methods to develop models for this enzyme that are capable to reproduce and interpreting the experimental observations. The theoretical investigations are elaborated over four main fields that assess the accuracy of the used methods, and employ them to understand the function of the active-site region, the inhibition mechanism, and the ligand binding. The testing of different quantum chemical methods reveals that their performance depends partly on the employed model. This can be a gas phase description, a continuum solvent model, or a hybrid QM/MM approach. The latter represents the preferred method for the atomistic modeling of biochemical reactions. A benchmarking uncovers some serious problems for semi-empirical methods when applied in proton transfer reactions. To understand substrate cleavage and inhibition of SARS coronavirus main protease, proton transfer reactions between the Cys/His catalytic dyad are calculated. Results show that the switching between neutral and zwitterionic state plays a central role for both mechanisms. It is demonstrated that this electrostatic trigger is remarkably influenced by substrate binding. Whereas the occupation of the active-site by the substrate leads to a fostered zwitterion formation, the inhibitor binding does not mimic this effect for the employed example. The underlying reason is related to the coverage of the active-site by the ligand, which gives new implications for rational improvements of inhibitors. More detailed insights into reversible and irreversible inhibition are derived from in silico screenings for the class of Michael acceptors that follow a conjugated addition reaction. From the comparison of several substitution patterns it becomes obvious that different inhibitor warheads follow different mechanisms. Nevertheless, the initial formation of a zwitterionic catalytic dyad is found as a common precondition for all inhibition reactions. Finally, non-covalent inhibitor binding is investigated for the case of SARS coranavirus main protease in complex with the inhibitor TS174. A novel workflow is developed that includes an interplay between theory and experiment in terms of molecular dynamic simulation, tabu search, and X-ray structure refinement. The results show that inhibitor binding is possible for multiple poses and stereoisomers of TS174.
In the current work, several well-known pharmaceuticals (1,4-dihydrazinophthalazine sulfate, caffeine, and papaverine hydrochloride) and new organometallic compounds (nickel(II) cupferronato complexes NiL2An, L = PhN2O2-, n = 1, A = o-phenanthroline (1), o,o’-bipyridine (2) and n = 2, A = H2O (3), o-NH2Py (4), o-C6H4(NH2)2 (5); silylene-bridged dinuclear iron complexes [Cp(OC)2Fe]2SiX2 (X = H (6), F (7), Cl (8), Br (9), I (10)); 3-silaoxetane 3,3-dimethyl-2,2,4,4-tetraphenyl-1-oxa-3-silacyclobutane (11) and 3-silathietane 3,3-dimethyl-2,2,4,4-tetraphenyl-1-sila-3-thiacyclobutane (12) compounds), which have successfully been characterized by using vibrational spectroscopy in conjunction with accurate density functional theory (DFT) calculations, are presented. The DFT computed molecular geometries of the species of interest reproduced the crystal structure data very well and in conjunction with IR and Raman measurements helped us to clarify the structures of the compounds, for which no experimental data were available; and this, especially for the new organometallic compounds, where the X-Ray analysis was limited by the non-availability of single crystals (3, 5, 10). Furthermore, a natural population analysis (NPA) and natural bond orbital (NBO) calculations together with a detailed analysis of the IR and Raman experimental as well as calculated spectra of the new organometallic compounds, allowed us to study some special bonding situations (1-12) or to monitor the structural changes observed with the change in temperature during the Raman experiments (11, 12). By combining these two methods (DFT and vibrational spectroscopy), the auspicious results obtained on the organometallic compounds 6-12 and overall in literature, made us confident of the power of theoretical calculations in aiding the interpretation of rich SERS spectra by solving some interesting issues. Consequently, the Raman and SERS spectra of well-known pharmaceuticals (1,4-dihydrazinophthalazine sulfate, caffeine, and papaverine hydrochloride) or new potentially biological active organometallic complexes (1-5), that were synthetized by our coworkers, were discussed with the assistance of the accurate results obtained from DFT calculations (structural parameters, harmonic vibrational wavenumbers, Raman scattering activities), and many previous incomplete assignments have been analyzed and improved. This allowed us to establish the vibrational behavior of these biological compounds near a biological artificial model at different pH values or concentrations (Ag substrate), taking into account that information about the species present under particular conditions could be of great importance for the interpretation of biochemical processes. The total electron density of molecules and the partial charges situated on selected atoms, which were determined theoretically by NPA, allowed us to establish the probability of different atoms acting as an adsorptive site for the metal surface. Moreover, a closer examination of the calculated orbitals of molecules brought further arguments on the presence or absence of the photoproducts at the Ag surface during the irradiation (1,4-dihydrazinophthalazine sulfate). Overall, the results provide a benchmark illustration of the virtues of DFT in aiding the interpretation of rich vibrational spectra attainable for larger polyatomic adsorbates by using SERS, as well as in furnishing detailed insight into the relation between the vibrational properties and the nature of the Ag substrate-adsorbate bonding. Therefore, we strongly believe that theoretical calculations will become a matter of rapidly growing scientific and practical interest in SERS.
Infrared (IR) and Raman spectroscopy are among the most widely used techniques in the physical and natural sciences today. Vibrational spectroscopy, including IR and Raman spectroscopy, has both a long and interesting history and an illustrious record of contributions to science. Spectroscopy in the pharmaceutical industry is dominated by techniques such as nuclear magnetic resonance (NMR) and mass spectrometry (MS) for the elucidation of chemical structures. Despite this, the versatility of infrared spectroscopy ensures it still remains a key technique in quality control laboratories, and in applications where solid form characterization or minimal sample preparation is a necessity. Raman spectroscopy has many uses in the pharmaceutical and chemical industry, but its strengths is in solid form analysis. It is regularly used to identify compounds, and results are used in the release of pharmaceutical and chemical products. This work consists of 8 chapters, which cover the vibrational spectroscopy beginning with the theory and instrumentation, continuing with the experimental setup and probes description, and completing with results and discussions of the experiments. The first chapter of this work introduces Raman spectroscopy as a dominant technique used in pharmaceutical and chemical industry. The theoretical background regarding vibrational spectroscopy (IR and Raman) is accounted for in the second chapter of this work, while the samples presentation, the experimental procedures, and the description of the apparatus together with the computational details are briefly specified in the third chapter. The fourth chapter investigates the concentration dependent wavenumber shifts and linewidth changes of tetrahydrofuran in a binary system. Many of the applications in food science rely heavily on Raman spectroscopy, often preceding the biomedical applications. The characterization and identification of food additives using Raman, surface-enhanced Raman spectroscopy, and theoretical calculations is in detail depicted in the fifth chapter, whereas in the sixth and seventh chapters the monitoring of several medicines and various lanthanide complexes with anticancer properties, respectively, employing IR and Raman techniques are treated. These last two chapters address applications of vibrational spectroscopy to pharmaceutical products, and include the use of vibrational spectroscopy in combinatorial chemistry and density functional theory, a modality increasingly used by the pharmaceutical industry for the discovery if new pharmacologically active substances.
In der vorliegenden Arbeit wurden zwei spektroskopische Methoden (Raman- und Circulardichroismus-Spektroskopie) und die Kernspinresonanz zur Untersuchung der Sekundärstruktur von synthetischen Polypeptiden eingesetzt. Dabei wurden die Struktur-Funktions-Beziehungen der dritten extrazellulären Schleife des Gonadotropin-freisetzenden Rezeptors (GnRH-R) untersucht. Die spektroskopischen Ergebnisse belegten, dass die zuvor getroffene Aussage über eine vorhandene helikale Struktur revidiert werden musste. Die Strukturanalysen mit Hilfe der CD-, Raman- und 2D NMR-Experimente an zwei Serien von Polypeptiden lieferten Aussagen über die Sekundärstruktur. Insbesondere die Raman-Untersuchungen in Verbindung mit einer statistischen Datenanalyse lieferten detaillierte Information über subtile Konformationsänderungen, die einerseits durch die Addition und andererseits durch die Substitution einzelner Aminosäuren in den synthetischen Polypeptiden ausgelöst wurden. Anhand der ausgewählten Raman-Linien konnte nachgewiesen werden, dass sowohl die Änderungen der Polypeptidkettenlänge als auch die Änderung der Polypeptidsequenzen mit den beobachteten Intensitäten der Raman-Linien korreliert sind.
The present work presents investigations on energy and charge transport properties in organic crystals. Chapter 4 treats exciton transport in anthracene, which is an example for weakly coupled π-systems. The electronic coupling parameter is evaluated by the monomer transition density approach. With these and the reorganization energy hopping rates are calculated in the framework of the Marcus theory. Together with the knowledge of the crystal structure, these allow us to calculate the experimental accessible exciton diffusion lengths, whose isotropic part fits nicely within the scattering of experimental values found in the literature. Furthermore, the anisotropy of the exciton diffusion lengths is reproduced qualitatively and quantitatively correct. This chapter also contains studies about electron and hole transport in both polymorphs (α and β) of perylene. Reorganization energies as well as diffusion coefficients for both crystal structures and types of charge transport were calculated. The best transport is hole transport in β-perylene, but it is strongly isotropic. The preferred transport direction is along the b-axis of the unit cell with couplings of greater than 100 meV. However, there is no transport along the c-axis. The diffusion constant in b-direction is bigger by two orders of magnitude than in c-direction (62.7•10-6 m2/s vs. 0.4•10-6 m2/s). Charge transport is calculated to be strongly anisotropic for holes as well as electrons in both modifications. To verify these results experimental electron mobilities have been compared to the simulations. Good agreement was found with errors of less than 27%. As it was shown above, the calculation and measurement of transport properties between weakly coupled systems is possible. However, it is difficult to exactly determine the quality of the electronic coupling. For this reason a collaboration about strongly interacting π-systems was started between us and the research group of Prof. Ingo Fischer. There, [2.2]paracyclophanes and its derivates were investigated to show how hydroxyl substitution influences absorption properties. Overall, a combination of SCS-MP2 and SCS-CC2 performs best to address the description of geometric and electronic structures for both ground and excited states of these model systems as well as their parent compounds benzene and phenol. Only [2.2]paracyclophane shows a double minimum potential regarding a twist and shift motion between the benzene/phenol subunits towards each other. All other systems are less flexible due to their substitution pattern. Almost all [2.2]paracyclophanes display minor changes in their geometric structure upon excitation to the S1 state: The inter-ring distance shortens, but qualitatively they keep their shift and twist characteristics, although the extent of these deformations diminishes. The exception is p-DHPC, which turns from a shifted ground state structure into a twisted excited state structure. Consequently, the intensity of the 0-0 transition cannot be observed experimentally due to small Franck-Condon factors and impurities of o-DHPC. In the present thesis, the structures and their changes due to excitation are explained by electrostatic potentials as well as antibonding (bonding) HOMO (LUMO) orbitals. Adiabatic excitation energies have been corrected by ZPEs and result in accuracies with errors smaller than 0.1 eV. Note that corrections on the B3LYP level worsen the results and one has to apply SCS-CC2 to achieve this accuracy. These calculations allow an interpretation of the experimental [1+1]REMPI spectra. Band progressions of the twist, shift and breathing of the [2.2]paracyclophane skeleton vibrations have been identified and show good agreement to the experiment. This work shows that the substitution pattern in [2.2]paracyclophanes can have a significant impact on spectroscopic properties. Because these properties are directly linked to the transport properties of these materials, the hereby gained insight can be used to design materials with customized transport properties. It was shown that the SCS-CC2 method is very appropriate to predict the interaction between the π-systems