36,257 research outputs found
Conformational studies by dynamic NMR. V. The stereodynamics of hindered aliphatic hydrazones
No full textA number of N-ketiminyl-2,2,6,6-tetramethylpiperidines (TMP-N=CR2) have been synthesized and investigated by 1H NMR spectroscopy. The piperidyl methyls are either nonequivalent in pairs at room temperature or become nonequivalent in pairs at low temperatures. This is shown to be due to restricted rotation about the N-N bond, a phenomenon that is not observed in less hindered hydrazones even at very low temperatures. The free energies of activation for rotation in TMP-N=CR2 were determined by line shape analysis. They increase from a low of 7.6 kcal/mol for R2 = H2 through 11.6, 14.35, and 18.0 kcal/mol for R2 = (CH2)4, (CH2)5, and (CH2)6, respectively. This trend is rationalized in terms of a conformation in which the plane of the N=CR2 moiety is perpendicular to the dynamically averaged plane of the tetramethylpiperidine ring. This conclusion is supported by an x-ray diffraction determination of the structure and conformation of TMP-N=C-(CH3)C6H5. The rotational barriers of some n-nitrosoamines are also reported.Peer reviewed: YesNRC publication: Ye
Measurement of the casimir force between 0.2 and 8 µm: Experimental procedures and comparison with theory
No full textWe present results on the determination of the differential Casimir force between an Au-coated sapphire sphere and the top and bottom of Au-coated deep silicon trenches performed by means of the micromechanical torsional oscillator in the range of separations from 0.2 to 8 µm. The random and systematic errors in the measured force signal are determined at the 95% confidence level and combined into the total experimental error. The role of surface roughness and edge effects is investigated and shown to be negligibly small. The distribution of patch potentials is characterized by Kelvin probe microscopy, yielding an estimate of the typical size of patches, the respective r.m.s. voltage and their impact on the measured force. A comparison between the experimental results and theory is performed with no fitting parameters. For this purpose, the Casimir force in the sphere-plate geometry is computed independently on the basis of first principles of quantum electrodynamics using the scattering theory and the gradient expansion. In doing so, the frequency-dependent dielectric permittivity of Au is found from the optical data extrapolated to zero frequency by means of the plasma and Drude models. It is shown that the measurement results exclude the Drude model extrapolation over the region of separations from 0.2 to 4.8 µm, whereas the alternative extrapolation by means of the plasma model is experimentally consistent over the entire measurement range. A discussion of the obtained results is provided
Substituent effects on the bond dissociation enthalpies of aromatic amines
No full textBond dissociation enthalpy differences, Z-X ΔBDE = BDE(4-YC6H4Z-X) - BDE(C6H5Z-X), for Z = CH2 and O are largely independent of X and are determined mainly by the stabilization/destabilization effect of Y on the 4-YC6H4Z• radicals. The effects of Y are small (≤2 kcal/mol for all Y) for Z = CH2, but they are large for Z = O, where good correlations with σp+(Y) yield ρ+ = 6.5 kcal/mol. For Z = NH, two sets of electrochemically measured N-H ΔBDEs correlate with σp+(Y), yielding ρ+ = 3.9 and 3.0 kcal/mol. However, in contrast to the situation with phenols, these data indicate that the strengthening effect on N-H BDEs of electron-withdrawing (EW) Y's is greater than the weakening effect of electron-donating (ED) Y's. Attempts to measure N-H ΔBDEs in anilines using two nonelectrochemical techniques were unsuccessful; therefore, we turned to density functional theory, Calculations on 15 4-YC6H4NH2 gave N-H ΔBDEs correlating with σp+ (ρ+ = 4.6 kcal/mol) and indicated that EW and ED Y's had comparable strengthening and weakening effects, respectively, on the N-H bonds. To validate theory by connecting it to experiment, the N-H ΔBDEs of four 4,4′-disubstituted diphenylamines and five 3,7-disubstituted phenothiazines were both calculated and measured by the radical equilibration EPR technique. For all compounds, theory and experiment agreed to better than 1 kcal/mol. Dissection of N-H ΔBDEs in 4-substituted anilines and O-H ΔBDEs in 4-substituted phenols into interaction enthalpies between Y and NH2/OH (molecule stabilization/destabilization enthalpy, MSE) and NH•/O• (radical stabilization/destabilization enthalpy, RSE) reveals that for both groups of compounds, ED Y's destabilize the molecule and stabilize the radical, while the opposite holds true for EW Y's. However, in the phenols the effects of substituents on the radical are roughly 3 times as great as those in the molecule, whereas in the anilines the two effects are of comparable magnitudes. These differences arise from the stronger ED character of NH2 vs OH and the weaker EW character of NH• vs O•. The relatively large contributions to N-H BDEs in anilines arising from interactions in the molecules suggested that N-X ΔBDEs in 4-YC6H4NH-X would depend on X, in contrast to the lack of effect of X on O-X and CH2-X ΔBDEs in 4-YC6H4O-X and 4-YC6H4CH2-X. This suggestion was confirmed for X = CH3, H, OH, and F, for which the calculated NH-X ΔBDEs yielded p+ = 5.0, 4.6, 4.0, and 3.0 kcal/mol, respectively
Erratum to: Effect of moderate red wine intake on cardiac prognosis after recent acute myocardial infarction of subjects with Type 2 diabetes mellitus (Diabetic Medicine, (2006), 23, 9, (974-981), 10.1111/j.1464-5491.2006.01886.x)
No full textIn an article by Marfella et al, the author name C. Saron is incorrect and should be listed as C. Sardu. Therefore the correct author list is: R. Marfella, F. Cacciapuoti, M. Siniscalchi, F. C. Sasso, F. Marchese, F. Cinone, E. Musacchio, M. A. Marfella, L. Ruggiero, G. Chiorazzo, D. Liberti, G. Chiorazzo, G. F. Nicoletti, C. Sardu, F. D'Andrea, C. Ammendola, M. Verza and L. Coppola.In an article by Marfella et al, the author name C. Saron is incorrect and should be listed as C. Sardu. Therefore the correct author list is: R. Marfella, F. Cacciapuoti, M. Siniscalchi, F. C. Sasso, F. Marchese, F. Cinone, E. Musacchio, M. A. Marfella, L. Ruggiero, G. Chiorazzo, D. Liberti, G. Chiorazzo, G. F. Nicoletti, C. Sardu, F. D'Andrea, C. Ammendola, M. Verza and L. Coppola
I Żywoty Świętych di Łazarz Baranowicz,
No full textThe Author discusses the Polish written work by the Ukrainian writer L. Baranowicz, focusing on the problems of genre, stylistic features, national and religious identity, similaraties or differencies with contemporary European baroque works belonging to analogous genres
The unusual reaction of semiquinone radicals with molecular oxygen
No full text(Chemical Equation Presented) Hydroquinones (benzene-1,4-diols) are naturally occurring chain-breaking antioxidants, whose reactions with peroxyl radicals yield 1,4-semiquinone radicals. Unlike the 1,2-semiquinone radicals derived from catechols (benzene-1,2-diols), the 1,4-semiquinone radicals do not always trap another peroxyl radical, and instead the stoichiometric factor of hydroquinones varies widely between 0 and 2 as a function of ring-substitution and reaction conditions. This variable antioxidant behavior has been attributed to the competing reaction of the 1,4-semiquinone radical with molecular oxygen. Herein we report the results of experiments and theoretical calculations focused on understanding this key reaction. Our experiments, which include detailed kinetic and mechanistic investigations by laser flash photolysis and inhibited autoxidation studies, and our theoretical calculations, which include detailed studies of the reactions of both 1,4-semiquinones and 1,2-semiquinones with O2, provide many important insights. They show that the reaction of O2 with 2,5-di-tert-butyl-1,4-semiquinone radical (used as model compound) has a rate constant of 2.4 \ub1 0.9
7 105 M -1 s-1 in acetonitrile and as high as 2.0 \ub1 0.9
7 106 M-1 s-1 in chlorobenzene, i.e., similar to that previously reported in water at pH 3c7. These results, considered alongside our theoretical calculations, suggest that the reaction occurs by an unusual hydrogen atom abstraction mechanism, taking place in a two-step process consisting first of addition of O2 to the semiquinone radical and second an intramolecular H-atom transfer concerted with elimination of hydroperoxyl to yield the quinone. This reaction appears to be much more facile for 1,4-semiquinones than for their 1,2-isomers. \ua9 2008 American Chemical Society.Peer reviewed: YesNRC publication: Ye
Letter From Nora G. to Alfred L. Shoemaker, March 17, 1948
No full textA handwritten letter from Nora G. addressed to Alfred L. Shoemaker, dated March 17, 1948. Within, the author provides a list of old weather predictions accredited to Peter Derro, as well as information about special Saints\u27 days and agricultural advice.https://digitalcommons.ursinus.edu/shoemaker_documents/1104/thumbnail.jp
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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