1,720,967 research outputs found
Characterization of "Free Base" and Metal Complex Thioalkyl Porphyrazines by Magnetic Circular Dichroism and TDDFT Calculations
No full textUV-vis absorption and magnetic circular dichroism (MCD) spectra of octakis thioethyl "free base" porphyrazine H2OESPz and its metal complexes MOESPz (M = Mg, Zn, Ni, Pd, Cu), as well as of [MnOESPz(SH)] were recorded. In the last case, MCD proved to have quite good sensitivity to the coordination of this complex with 1-methylimidazole (1-mim) in benzene. Time-dependent density functional theory (TDDFT) calculations were carried out for the considered porphyrazine complexes and showed good performance on comparing with MCD and UV-vis experimental spectra, even in the open-shell Cu and Mn cases. Calculations accounted for the red shift observed in the thioalkyl compounds and allowed us to reveal the role of sulfur atoms in spectroscopically relevant molecular orbitals and to highlight the importance of the conformations of the thioethyl external groups. Calculated MCD spectra of [MnOESPz(SH)] confirm the Mn(III) → Mn(II) redox process, which leads to the [Mn(OESPz)(1-mim)2] species, and the relevance of the spin state for MCD is revealed
Bilirubin and its congeners: conformational analysis and chirality from metadynamics and related computational methods
No full textThe conformational properties of natural bilirubin, bisbutyricmesobilirubin-XIIIα, and their chiral derivatives (βS,β′S)-
dimethylmesobilirubin-XIIIα and 8,12-(γR,γ′R)- dimethylbisbutyricmesobilirubin-XIIIα were investigated by metadynamics,
an improved molecular dynamics computational methodology useful for conformational analysis. Applied to linear
tetrapyrroles, which may be treated as two blade molecular propellers, our results are represented by conformational energy
hypersurface maps built as functions of the two central dihedral torsion angles (φ1 and φ2) following rotations of two dipyrrinone
units about the connecting CH2,
and are compared successfully with maps previously obtained from molecular
mechanics. With an emphasis on the conformationally more flexible 8,12-(γR,γ′R)-dimethylbisbutyricmesobilirubin-XIIIα,
a conformationally mobile optically active bilirubinoid with butyric acid replacing propionic, metadynamics reveals global
and nearby minima. Its electronic circular dichroism and UV–Vis spectra predicted by DFT calculations correlate well with
the experimental spectra and are explained on the basis of equilibrium between the predicted conformational energy minima
L-Stercobilin-HCl and d-Urobilin-HCl. Analysis of Their Chiroptical and Conformational Properties by VCD, ECD, and CPL Experiments and MD and DFT Calculations
No full textVibrational circular dichroism (CD) and IR spectra of dichloromethane solutions of l-stercobilin and d-urobilin hydrochlorides have been recorded in the mid-IR region. The spectra are best interpreted by combining molecular dynamics calculations and density functional theory (DFT) calculations within the quantum mechanics/molecular mechanics ONIOM-type framework, and the combined predicted results are better and more informative than the more standard analysis provided by DFT calculations. The same approach also sheds light on the Cotton effect sign inversion of room temperature versus low-temperature electronic CD spectra of the same compounds in methanol–glycerol solution. Finally, circularly polarized luminescence spectra for l-stercobilin in chloroform solution provide information on the excited-state geometry of this molecule
Experimental and theoretical aspects of magnetic circular dichroism and magnetic circularly polarized luminescence in the UV, visible and IR ranges: A review
No full textA historical sketch of the MCD (magnetic circular dichroism) spectroscopy is reported in its experimental and theoretical aspects. MCPL (magnetic circularly polarized luminescence) is also considered. The main studies are presented encompassing porphyrinoid systems, aggregates and materials, as well as simple organic molecules useful for the advancement of the interpretation. The MCD of chiral systems is discussed with special attention to new studies of natural products with potential pharmaceutical valence, including Amaryllidaceae alkaloids and related isocarbostyrils. Finally, the vibrational form of MCD, called MVCD, which is recorded in the IR part of the spectrum is also discussed. A final brief note on perspectives is given
On the aggregation of bilirubinoids in solution as evidenced by VCD and ECD spectroscopy and DFT calculations
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
MCD and MCPL Characterization of Luminescent Si(IV) and P(V) Tritolylcorroles: The Role of Coordination Number
Full text linkTwo triarylcorrole complexes, (hydroxy)[5,10,15-
tritolylcorrolato]silicon-(TTC)Si(OH) and (dihydroxy)[5,10,15-
tritolylcorrolato]phosphorous-(TTC)P(OH)2, have been investigated
by magnetic circular dichroism (MCD) and magnetic circularly
polarized luminescence (MCPL). The spectroscopic investigations
have been combined with explicit calculation of MCD response
through time-dependent density functional theory (TD-DFT) formal-
ism. This has allowed us to better define the role of molecular orbitals
in the transitions associated with the Soret and Q bands. Besides and
more importantly, MCD has made it possible to follow the titration
process of (TTC)Si(OH) in dimethyl sulfoxide (DMSO) solution
with NaF and of (TTC)P(OH)2 in dichloromethane solution with
alcohols in a complementary and, we dare say, more sensitive way with
respect to absorption and fluorescence data. Finally, the MCPL spectra and the ancillary TD-DFT calculations have allowed us to characterize the excited state of (TTC)Si(OH)
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
