1,721,315 research outputs found
EPR study of the adsorption of dioxin vapours onto microporous carbons and mesoporous silica
No full textEffect of Orientation of the Peptide-Bridge Dipole Moment on the Properties of Fullerene–Peptide–Radical Systems
No full textWe synthesized two series of compounds in which a nitroxide radical and a fullerene C-60 moiety were kept separated by a 3(10)-helical peptide bridge containing two intramolecular C=O center dot center dot center dot H-N hydrogen bonds. The direction of the resulting molecular dipole moment could be reversed by switching the position of fullerene and nitroxide with respect to the peptide nitrogen and carbon termini. The resulting fullerene-peptide-radical systems were compared to the behaviors of otherwise identical peptides but lacking either C-60 or the free radical moiety. Electrochemical analysis and chemical nitroxide reduction experiments show that the dipole moment of the helix significantly affects the redox properties of both electroactive groups. Besides providing evidence of a folded helical conformation for the peptide bridge, IR and NMR results highlight a strong effect of peptide orientation on the spectral patterns, pointing to a specific interaction of one of the helical orientations with the C-60 moiety. Time-resolved EPR spectra show not only that for both systems triplet quenching by nitroxide induces spin polarization of the radical spin sublevels, but also that the coupling interaction can be either weak or strong depending on the orientation of the peptide dipole. As opposed to the concept of dyads, the molecules investigated are thus better described as fullerene-peptide-radical systems to stress the active role of the bridge as an important ingredient capable of tuning the system's physicochemical properties
Time-Resolved EPR Study of Singlet Oxygen in the Gas Phase
No full textX-band EPR spectra of singlet O2((1)Δg) and triplet O2((3)Σg(-)) were observed in the gas phase under low molecular-oxygen pressures PO2 = 0.175-0.625 Torr, T = 293-323 K. O2((1)Δg) was produced by quenching of photogenerated triplet sensitizers naphthalene C8H10, perdeuterated naphthalene, and perfluoronaphthalene in the gas phase. The EPR spectrum of O2((1)Δg) was also observed under microwave discharge. Integrated intensities and line widths of individual components of the EPR spectrum of O2((3)Σg(-)) were used as internal standards for estimating the concentration of O2 species and PO2 in the EPR cavity. Time-resolved (TR) EPR experiments of C8H10 were the main focus of this Article. Pulsed irradiation of C8H10 in the presence of O2((3)Σg(-)) allowed us to determine the kinetics of formation and decay for each of the four components of the O2((1)Δg) EPR signal, which lasted for only a few seconds. We found that the kinetics of EPR-component decay fit nicely to a biexponential kinetics law. The TR EPR 2D spectrum of the third component of the O2((1)Δg) EPR spectrum was examined in experiments using C8H10. This spectrum vividly presents the time evolution of an EPR component. The largest EPR signal and the longest lifetime of O2((1)Δg), τ = 0.4 s, were observed at medium pressure PO2 = 0.4 Torr, T = 293 K. The mechanism of O2((1)Δg) decay in the presence of photosensitizers is discussed. EPR spectra of O2((1)Δg) evidence that the spin-rotational states of O2((1)Δg) are populated according to Boltzmann distribution in the studied time range of 10-100 ms. We believe that this is the first report dealing with the dependence of O2((1)Δg) EPR line width on PO2 and T
The Turro legacy
No full textThis issue of PPS is dedicated to the memory of Nicholas J. Turro, Professor of Chemistry at Columbia University
Going 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
ENDOR Evidence of Electron–H2 Interaction in a Fulleride Embedding H-2
No full textAn endofulleropyrrolidine, with H-2 as a guest, has been reduced to a paramagnetic endofulleride radical anion. The magnetic interaction between the electron delocalized on the fullerene cage and the guest H-2 has been probed by pulsed ENDOR. The experimental hyperfine couplings between the electron and the H-2 guest were measured, and their values agree very well with DFT calculations. This agreement provides clear evidence of magnetic communication between the electron density of the fullerene host cage and H-2 guest. The ortho-H-2/para-H-2 interconversion is revealed by temperature-dependent ENDOR measurements at low temperature. The conversion of the paramagnetic ortho-H2 to the diamagnetic para-H-2 causes the ENDOR signal to decrease as the temperature is lowered due to the spin catalysis by the paramagnetic fullerene cage of the radical anion fulleride
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