757 research outputs found
Intramolecular charge transfer with the planarized 4-cyanofluorazene and its flexible counterpart 4-cyano-N-phenylpyrrole. Picosecond fluorescence decays and femtosecond excited-state absorption.
No full textThe fluorescence spectrum of the rigidified 4-cyanofluorazene (FPP4C) in n-hexane consists of a dual emission from a locally excited (LE) and an intramolecular charge-transfer (ICT) state, with an ICT/LE fluorescence quantum yield ratio of Phi'(ICT)/Phi(LE) = 3.3 at 25 degrees C. With the flexible 4-cyano-N-phenylpyrrole (PP4C) in n-hexane, such an ICT reaction also takes place, with Phi'(ICT)/Phi(LE) = 1.5, indicating that for this reaction, a perpendicular twist of the pyrrole and benzonitrile moieties is not required. The ICT emission band of FPP4C and PP4C in n-hexane has vibrational structure, but a structureless band is observed in all other solvents more polar than the alkanes. The enthalpy difference Delta H of the LE -> ICT reaction in n-hexane, -11 kJ/mol for FPP4C and -7 kJ/mol for PP4C, is determined by analyzing the temperature dependence of Phi'(ICT)/Phi(LE). Using these data, the energy E(FC,ICT) of the Franck-Condon ground state populated by the ICT emission is calculated, 41 (FPP4C) and 40 kJ/mol (PP4C). These large values for E(FC,ICT) lead to the conclusion that with FPP4C and PP4C, direct ICT excitation, bypassing LE, does not take place. FPP4C has an ICT dipole moment of 15 D, similar to that of PP4C (16 D). Picosecond fluorescence decays allow the determination of the ICT lifetime, from which the radiative rate constant k(f)'(ICT) is derived, with comparable values for FPP4C and PP4C. This shows that an argument for a twisted ICT state of PP4C cannot come from k(f)'(ICT). After correction for the solvent refractive index and the energy of the emission maximum (v) over tilde (max)(ICT), it appears that k(f)'(ICT) is solvent-polarity-independent. Femtosecond transient absorption with FPP4C and PP4C in n-hexane reveals that the ICT state is already nearly fully present at 100 fs after excitation, in rapid equilibrium with LE. In MeCN, the ICT state of FPP4C and PP4C is likewise largely developed at this delay time, and the reaction is limited by dielectric solvent relaxation, which shows that the ICT reaction is ultrafast, at the experimental time limit of 50 fs. PP4C and FPP4C have a similar planar ICT structure, without an appreciable twist of the pyrrole and benzonitrile subgroups. Their crystal structure is compared with calculations for the S-0 ground state.Hungarian Science Foundation [76278
Intramolecular charge transfer with 4-fluorofluorazene and the flexible 4-fluoro-N-phenylpyrrole.
No full textWith 4-fluorofluorazene (FPP4F) and its flexible counterpart 4-fluoro-N-phenylpyrrole (PP4F) an intramolecular charge transfer (ICT) reaction occurs in the singlet excited state in sufficiently polar solvents. The ICT reaction begins to appear in tetrahydrofuran (E = 7.4) for FPP4F and in the more polar 1,2-dichloroethane (epsilon = 10.4) with PP4F, showing its presence by dual fluorescence from a locally excited (LE) and an ICT state. Only LE fluorescence is observed in less polar solvents such as n-hexane. The ICT reaction is more pronounced with FPP4F than for PP4F, due to the smaller energy gal) Delta E(S(1),S(2)) of the former molecule, in accordance with the PICT model. The occurrence of an ICT reaction is confirmed by the ICT dipole moments mu(e)(ICT) of 12 D (FPP4F) and 10 D (PP4F), clearly larger than mu(e)(LE) of similar to 4 D for FPP4F and PP4F. Isoemissive points are found in the fluorescence spectra of FPP4F and PP4F in acetonitrile (MeCN), ethyl cyanide (EtCN), and n-propyl cyanide (PrCN) as a function of temperature, confirming the two-state (LE and ICT) reaction mechanism. From plots of the logarithm of the ICT/LE fluorescence quantum yield ratio versus the reciprocal absolute temperature in these solvents, the ICT reaction enthalpies Delta H are determined, with larger -Delta H values for FPP4F than for PP4F: 19.2 as compared with 14.9 kJ/mol in MeCN, as an example. The picosecond fluorescence decay of PP4F at -45 degrees C becomes slower with decreasing solvent polarity, 5.1 ps (MeCN), 14 ps (EtCN), and 35 ps (PrCN), from which the LE -> ICT reaction rate constant is calculated, decreasing from 19 x 10(10) to 2.1 x 10(10) s(-1) between MeCN and PrCN. The femtosecond LE excited-state absorption spectra of FPP4F and PP4F do not undergo any time development in n-hexane (no ICT reaction), but show a fast ICT reaction in MeCN at 22 degrees C, with decay times of 1.1 ps (FPP4F) and 3.3 ps (PP4F). It is concluded that FPP4F and PP4F have a planar ICT state (PICT model), indicating that a perpendicular twist of the donor and acceptor subgroups in a donor/acceptor molecule is not a requirement for fast and efficient ICT. The molecular structures of FPP4F and PP4F obtained from X-ray crystal analysis reveal that the pyrrole group of PP4F is twisted over an angle theta = 25 degrees relative to the fluorophenyl moiety in the ground state, whereas as expected r-PP4F is practically planar (theta = 2 degrees). The pyrrole-phenyl bond length of FPP4F (140.7 pm) is shorter than that for PP4F (141.8 pm).Hungarian Science Foundation [76278
C coupling constants in alkyl-substituted cyclopropenes: experimental and theoretical studies
No full textMeasurements of one-bond carbon-carbon coupling constants, (1)J(C, C), were performed for two series of compounds, alkyl-substituted cyclopropenes and cyclopropanes. The experimental data were complemented by a set of DFT-calculated J couplings for the parent cyclopropene (1), its methyl and silyl derivatives and, additionally, for 1-methylcyclobutene (3), 1-methylcyclopentene (4) and 1-methylcyclohexene (5) and good agreement was observed between the experimental and the calculated data; all the trends are perfectly maintained, including a dramatic decrease in the couplings across endocyclic single bonds in cyclopropene and its derivatives, and a significant decrease in the corresponding couplings in cyclobutene. Using the data obtained, the s characters of the carbon hybrid orbitals involved in the formation of the cyclopropene were calculated. The results clearly show that the ring closure and the related strain exerted upon the cyclopropene molecule only slightly disturb the electron structure of the double bond. The s character of the corresponding carbon orbital is 0.314 in cyclopropene vs the theoretical value of 0.333 in ethene. This is at variance with the endo- and exocyclic single bonds, where the s characters of the orbitals forming the endocyclic single bonds are much smaller than those of the bonds in the open-chain compounds, i.e. 0.229 (C-1 and/or C-2) and 0.166 (C-3). The s values calculated for the exocyclic CH bonds are 0.334 for C-3 and 0.456 for C-1 and/or C-2. Copyright (C) 2002 John Wiley Sons, Ltd
The Power of Language and Silence: Reinhard Jirgl’s Die Stille
No full textReinhard Jirgl (1953–) is an emphatically German author. He insists that German is “die Sprache in der ich denke, spreche und schreibe,”¹ and the award of several prestigious prizes (including the Büchner Prize in 2010) has confirmed his place in the German literary tradition. Yet Jirgl uses the German language in consistently and characteristically iconoclastic ways to challenge the authority of historical, political, and institutional discourse. Precisely because his work went against the ideological prescriptions of the East German state, it remained unpublished in the GDR, where Jirgl lived and worked. Since unification he has become a prolific author,..
ortho-Azobenzenophanes: Photochemical and Thermal Reactions of [2,2][2,2′]Azobenzenophane
No full textReinhard Köhler's scientific production: words, numbers and pictures
No full textThis study draws upon statistical analysis techniques of textual data to examine a corpus composed of 22 research articles published between 1997 and 2010 by Reinhard Köhler as a single author or in collaboration with other scholars. The aim of this article is to draw a representation of the main areas of interest of his research. After having drawn an overall representation of the corpus, Köhler’s latest work – an unpublished volume on Quantitative Syntax Analysis – was analysed to understand its role within the context of his research
Review: Reinhard Müller (2008). Marienthal: Das Dorf - Die Arbeitslosen - Die Studie
No full textDas Buch von Reinhard MÜLLER beschäftigt sich mit der bekannten Studie "Die Arbeitslosen von Marienthal" von Marie JAHODA, Paul F. LAZARSFELD und Hans ZEISEL (1933, 1960, 1975). In seinem sozialhistorischen Zugang gelingt es MÜLLER, interessante Details zur Studie selbst sowie zur Gemeinde und zur Fabrik zu präsentieren. Mehr als die Hälfte des Buches ist der Entwicklung der Arbeitersiedlung Marienthal gewidmet, beginnend bei der Gründung des Dorfes und danach der Textilfabrik. Eingegangen wird auf historische Ereignisse, bedeutende Personen und Entwicklungen in der Gemeinde. Im anderen Teil werden zusätzliche Informationen rund um die Studie präsentiert. Porträts und Erinnerungen der Projektmitarbeiter/innen stehen dabei im Mittelpunkt. Das Buch ermöglicht nicht nur einen tieferen Einblick in die klassische Studie und Marienthal, es veranschaulicht auch (österreichische) Geschichte an einer Gemeinde.The book by Reinhard MÜLLER deals with the well-known study "Marienthal: The Sociography of an Unemployed Community" by Marie JAHODA, Paul F. LAZARSFELD, and Hans ZEISEL (1933, 1971). The author presents interesting socio-historical details about the study itself as well as about the community and factory. More than half of the book is dedicated to the development of the workers' settlement of Marienthal, starting with the founding of the village and then the textile factory. The author describes historical events, important people' and developments in the community. Also additional information - portraits and memories - about the well-known study is presented. The book not only enables a deeper insight into the classical study and the settlement of Marienthal, it also illustrates (Austrian) history by way of a community and a study
Reinhard Köhler's Scientific Production: Words, Numbers and Pictures, di Arjuna Tuzzi
No full textThis study draws upon statistical analysis techniques of textual data to examine a corpus composed of 22 research articles published between 1997 and 2010 by Reinhard Köhler as a single author or in collaboration with other scholars. The aim of this article is to draw a representation of the main areas of interest of his research. After having drawn an overall representation of the corpus, Köhler’s latest work – an unpublished volume on Quantitative Syntax Analysis – was analysed to understand its role within the context of his research
Crystal and solution structures of 7-amino-actinomycin D complexes with d(TTAGBrUT), d(TTAGTT) and d(TTTAGTTT)
No full textSynopsis: In the crystal, actinomycin D intercalates between G-T wobble pairs of DNA involving the sequence d(TTAGTT); in solution, the dominant binding is between guanine and actinomycin.
Abstract: The formation of the complex of 7-amino-actinomycin D with potentially single-stranded DNA has been studied by X-ray crystallography in the solid state, by NMR in solution, and by molecular modeling. The crystal structures of the complex with 5'-TTAG[Br5U]T-3' provide interesting examples of MAD phasing, in which the dispersive component of the MAD signal was almost certainly enhanced by radiation damage. The trigonal and orthorhombic crystal modifications both contain antibiotic molecules and DNA strands in the form of a 2:4 complex; in the orthorhombic form there is one such complex in the asymmetric unit, in the trigonal structure there are four. In both structures the phenoxazone ring of the first drug intercalates between a BrU-G (analogous to T-G) wobble pair and a G-T pair where the T is part of symmetry related molecule. The chromophore of the second actinomycin intercalates between BrU-G and G-BrU wobble pairs of the partially paired third and fourth strands. The base stacking also involves (A*T)*T triplets and Watson-Crick A-T pairs and leads to similar complex three-dimensional networks in both structures with looping-out of unpaired bases. Although the available NOE-constraints of a solution containing the antibiotic and d(TTTAGTTT) strands in the ratio 1:1 are insufficient for determining the structure of the complex from the NMR data alone, they are consistent with the intercalation geometry observed in the crystal structure. Molecular dynamics (MD) trajectories starting from the 1:2 complexes observed in the crystal showed that although the thymines flanking the d(AGT) core are rather flexible and the G-T pairing is not permanently preserved, both strands remain bound to the actinomycin by strong interactions between it and the guanines between which it is sandwiched. Similar strong binding (hemi-intercalation) of the actinomycin to a single guanine was observed in the MD trajectories of a 1:1 complex. The dominant interaction is between the antibiotic and guanine, but the complexes are stabilized further by promiscuous base-pairing
Gott als Vater bei Plotin und Porphyrios
No full textThe metaphor of conception or birth plays a central role in neo-platonism: it is used alongside other to describe the proceeding of the reality of the highest principle and their gradual development. This chapter explores the following questions with reference to two Neoplatonic philosophers, first 'Hour: Plotinus and his student Porphyry: what does this mean for the speech of the gods or the highest principle as a father, if, Father 'a mere abstract relations and function name synonymous with, Author' or 'polluter pays', what aspects of the father image are emphasized and what implications these divine beings for the representation of the relationships between themselves and the relationship between man and God. In their texts, the two go with the father metaphor differently to and present in all structural similarity of the theological system a God each but different stored in its relations with the world and man
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