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991.
[reaction: see text] A reinvestigation of the thermolysis of 4,4-dibromotetracyclo[6.2.1.0(2,7).0(3,5)]undec-9-ene (2) affords diene 8 with a completely rearranged hydrocarbon skeleton via the isolable intermediate 4, along with cyclopentadiene and bromobenzene. DFT calculations show that the novel tandem retro-Diels-Alder-Diels-Alder reaction with role reversal is slightly less favored than the overall single-step Cope rearrangement. 相似文献
992.
Henchoz Y Schappler J Geiser L Prat J Carrupt PA Veuthey JL 《Analytical and bioanalytical chemistry》2007,389(6):1869-1878
A rapid and universal capillary zone electrophoresis (CZE) method was developed to determine the dissociation constants (pK
a) of the 20 standard proteogenic amino acids. Since some amino acids are poorly detected by UV, capacitively coupled contactless
conductivity detection (C4D) was used as an additional detection mode. The C4D coupling proved to be very successful on a conventional CE-UV instrument, neither inducing supplementary analyses nor instrument
modification. In order to reduce the analysis time for pK
a determination, two strategies were applied: (i) a short-end injection to reduce the effective length, and (ii) a dynamic
coating procedure to generate a large electroosmotic flow (EOF), even at pH values as low as 1.5. As a result, the analysis
time per amino acid was less than 2 h, using 22 optimized buffers covering a pH range from 1.5 to 12.0 at a constant ionic
strength of 50 mM. pK
a values were calculated using an appropriate mathematical model describing the relationship between effective mobility and
pH. The obtained pK
a values were in accordance with the literature.
Figure
a UV (1) and C4D (2) detectors placed on-line on the CE capillary. b Curve of effective mobility as a function of pH for histidine 相似文献
993.
Rochet-Capellan A Schwartz JL 《The Journal of the Acoustical Society of America》2007,121(6):3740-3754
This paper investigates the coordination between the jaw, the tongue tip, and the lower lip during repetition with rate increase of labial-to-coronal (L(a)C(o)) consonant-vowel-consonant-vowel disyllables (e.g., /pata/) and coronal-to-labial (C(o)L(a)) ones (e.g., /tapa/) by French speakers. For the two types of disyllables: (1) the speeding process induces a shift from two jaw cycles per disyllable to a single cycle; (2) this shift modifies the coordination between the jaw and the constrictors, and (3) comes with a progression toward either a L(a)C(o) attractor [e.g., (/pata/ or /tapa/) --> /patá/ --> /ptá/] or a C(o)L(a) one (e.g., /pata/ or /tapa/ --> /tapá/ --> /tpá/). Yet, (4) the L(a)C(o) attractor is clearly favored regardless of the initial sequencing. These results are interpreted as evidence that a L(a)C(o) CVCV disyllable could be a more stable coordinative pattern for the lip-tongue-jaw motor system than a C(o)L(a) one. They are discussed in relation with the so-called LC effect that is the preference for L(a)C(o) associations rather than C(o)L(a) ones in CV.CV disyllables in both world languages and infants' first words. 相似文献
994.
Meunier T Vink IT van Beveren LH Tielrooij KJ Hanson R Koppens FH Tranitz HP Wegscheider W Kouwenhoven LP Vandersypen LM 《Physical review letters》2007,98(12):126601
We observe an experimental signature of the role of phonons in spin relaxation between triplet and singlet states in a two-electron quantum dot. Using both the external magnetic field and the electrostatic confinement potential, we change the singlet-triplet energy splitting from 1.3 meV to zero and observe that the spin relaxation time depends nonmonotonously on the energy splitting. A simple theoretical model is derived to capture the underlying physical mechanism. The present experiment confirms that spin-flip energy is dissipated in the phonon bath. 相似文献
995.
When confining vacuum fluctuations between two identical walls, the Casimir force manifests itself as a mutual attraction of the walls. When confining concentration fluctuations of a binary liquid mixture, an analogous force should exist near the critical temperature TC; it is called the critical Casimir force. Here we show experimentally that this purely entropic force can be either attractive or repulsive, depending on the boundary conditions for the fluctuations. For symmetrical boundary conditions an attractive force is found while asymmetrical ones lead to a repulsive force. This is observed directly by confining the fluctuations in a thin wetting film. Depending on the boundary conditions either a thinning or a thickening of the film is observed when T→TC. 相似文献
996.
Martinez GR Ravanat JL Cadet J de Medeiros MH Di Mascio P 《Journal of mass spectrometry : JMS》2007,42(10):1326-1332
The main singlet molecular oxygen ((1)O(2)) oxidation products of free 2'-deoxyguanosine (dGuo) in aqueous solution were identified as a pair of diastereomeric spiroiminodihydantoin 2'-deoxyribonucleosides (dSp) together with 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo). In the present work, evidence is provided from (18)[(1)O(2)] and H(2) (18)O labeling experiments, using HPLC-ESI-MS/MS, that the formation of dSp is explained by the addition of water to a reactive quinonoid intermediate, and a second reaction pathway leading to dSp involves (1)O(2) oxidation of initially generated 8-oxodGuo. 相似文献
997.
Schmidt K Brovelli S Coropceanu V Beljonne D Cornil J Bazzini C Caronna T Tubino R Meinardi F Shuai Z Brédas JL 《The journal of physical chemistry. A》2007,111(42):10490-10499
A comprehensive study of the photophysical properties of a series of monoaza[5]helicenes is presented on the basis of joint optical spectroscopy and quantum chemistry investigations. The molecules have been characterized by absorption and CW/time-resolved luminescence measurements. All quantities related to spin-orbit-coupling processes, such as intersystem crossing rates and radiative phosphorescence lifetimes, were found to depend strongly on the nitrogen position within the carbon backbone. Density functional theory and semiempirical quantum-chemical methods were used to evaluate the molecular geometries, the characteristics of the excited singlet and triplet states, and the spin-orbit coupling matrix elements. We demonstrate that the magnitude of spin-orbit coupling is directly correlated with the degree of deviation from planarity. The trends from the calculated photophysical quantities, namely, radiative fluorescence and phosphorescence decay rates and intersystem crossing rates, of the mono-aza-helicenes are fully consistent with experiment. 相似文献
998.
To rationally design and explore a potential energy source based on the highly exothermic oxidation of boron, density functional theory (DFT) was used to characterize small boron clusters with 0-3 oxygen atoms and a total of up to ten atoms. The structures, vibrational frequencies, and stabilities were calculated for each of these clusters. A quantum molecular dynamics procedure was used to locate the global minimum for each species, which proved to be crucial given the unintuitive structure of many of the most stable isomers. Additionally, due to the plane-wave, periodic DFT code used in this study a straightforward comparison of these clusters to the bulk boron and B2O3 structures was possible despite the great structural and energetic differences between the two forms. Through evaluation of previous computational and experimental work, the relevant low-energy structures of all but one of the pure boron clusters can be assigned with great certainty. Nearly all of the boron oxide clusters are described here for the first time, but there are strong indications that the DFT procedure chosen is particularly well suited for the task. Insight into the trends in boron and boron oxide cluster stabilities, as well as the ultimate limits of growth for each, are also provided. The work reported herein provides crucial information towards understanding the oxidation of boron at a molecular level. 相似文献
999.
Lidija Matjacic Martin P. Seah Gustavo F. Trindade Alexander Pirkl Rasmus Havelund Jean-Luc Vorng Ewald Niehuis Ian S. Gilmore 《Surface and interface analysis : SIA》2022,54(4):331-340
In 2017, we introduced the OrbiSIMS instrument that features a dual analyser configuration with a time-of-flight (ToF) mass spectrometer (MS) and an Orbitrap MS, which confer advantages of speed and high-performance mass spectrometry, respectively. The ability to combine the MS performance usually found in a state-of-the-art proteomics and metabolomics MS with 3D imaging at the microscale and from nanolayers of <10 nm of material has proved popular in a broad field of application from organic electronics to drug discovery. There are now several instruments in operation around the world, and metrology is needed to help ensure repeatability and reproducibility of the intensity scale. We conduct a systematic study of two key parameters, the target potential, VT, and the collision cell pressure, P, in the transfer optics on the transmitted secondary ion intensities. We measure VT–P maps of the ions across the mass range for Ag as a representative of inorganic materials and two different organic materials, Irganox 1010 and NPB (N,N′-Di[1-naphthyl]-N,N′-diphenyl-4,4′-biphenyldiamine). The manufacturer's defaults for these values ensure very good transmission for a broad range of analyte classes. However, the maps reveal a sometimes complex behaviour and indicate the possibility for additional separation of ions based on their shape, labile nature and kinetics of formation. Guidance is provided on how to optimise these parameters for sensitivity for different material classes and also the need for optimisation to improve spectral repeatability and reproducibility. 相似文献
1000.
Mahroug Imane Doppiu Stefania Dauvergne Jean-Luc Toutain Jean del Barrio Elena Palomo 《Journal of Thermal Analysis and Calorimetry》2022,147(22):12455-12465
Journal of Thermal Analysis and Calorimetry - LiOH–LiBr binary system is thoroughly investigated by means of DSC and XRD experimental analysis. Observed discrepancies compared to previous... 相似文献