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991.
Jakob Heller Tobias F. Pascher Dr. Christian van der Linde Dr. Milan Ončák Prof. Dr. Martin K. Beyer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(66):16367-16376
Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s–3p excitations of Al+ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S1 to T2 takes place, and photochemical hydrogen formation starts on the T2 surface, which passes through a conical intersection, changing to T1. On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH+(H2O)n-1 and are transparent in the investigated energy range. For n=4–8, a mixture of Al+(H2O)n and HAlOH+(H2O)n-1 is present in the experiment. 相似文献
992.
Eva Blokker Dr. Xiaobo Sun Prof. Dr. Jordi Poater Dr. Ir. J. Martijn van der Schuur Dr. Trevor A. Hamlin Prof. Dr. F. Matthias Bickelhaupt 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(63):15616-15622
We have quantum chemically analyzed element−element bonds of archetypal HnX−YHn molecules (X, Y=C, N, O, F, Si, P, S, Cl, Br, I), using density functional theory. One purpose is to obtain a set of consistent homolytic bond dissociation energies (BDE) for establishing accurate trends across the periodic table. The main objective is to elucidate the underlying physical factors behind these chemical bonding trends. On one hand, we confirm that, along a period (e. g., from C−C to C−F), bonds strengthen because the electronegativity difference across the bond increases. But, down a period, our findings constitute a paradigm shift. From C−F to C−I, for example, bonds do become weaker, however, not because of the decreasing electronegativity difference. Instead, we show that the effective atom size (via steric Pauli repulsion) is the causal factor behind bond weakening in this series, and behind the weakening in orbital interactions at the equilibrium distance. We discuss the actual bonding mechanism and the importance of analyzing this mechanism as a function of the bond distance. 相似文献
993.
994.
Xianwen Lou Christianus M. A. Leenders Arthur H. A. M. van Onzen Ralf A. A. Bovee Joost L. J. van Dongen Jef A. J. M. Vekemans E. W. Meijer 《Journal of the American Society for Mass Spectrometry》2014,25(2):297-300
Tetrahydrofuran (THF) is one of the most frequently used solvents in the MALDI TOF MS analysis of synthetic compounds. However, it should be used with caution because a trace amount of 4-hydroxybutanal (HBA) might be generated and accumulated in THF during storage. Since only a tiny amount of analytes is required in MALDI MS measurements, a trace amount of HBA might have a significant effect on the MS results. It was found that HBA will quickly react with primary and secondary amino compounds, leading to false results about the sample composition with an extra series of ions with additional mass of 70 Da in between. The formation of HBA can be inhibited by butylated hydroxytoluene (BHT) antioxidant. Therefore, when THF is required as the solvent for sample preparation, it is strongly recommended to use a BHT-stabilized one, at least for the analysis of compounds with amino groups. Figure
? 相似文献
995.
Andreas Bornø Carl J. Hulston Gerrit van Hall 《Journal of mass spectrometry : JMS》2014,49(8):674-680
In the present study, different MS methods for the determination of human muscle protein fractional synthesis rate (FSR) using [ring‐13C6]phenylalanine as a tracer were evaluated. Because the turnover rate of human skeletal muscle is slow, only minute quantities of the stable isotopically labeled amino acid will be incorporated within the few hours of a typical laboratory experiment. GC combustion isotope ratio MS (GC‐C‐IRMS) has thus far been considered the ‘gold’ standard for the precise measurements of these low enrichment levels. However, advances in liquid chromatography‐tandem MS (LC‐MS/MS) and GC‐tandem MS (GC‐MS/MS) have made these techniques an option for human muscle FSR measurements. Human muscle biopsies were freeze dried, cleaned, and hydrolyzed, and the amino acids derivatized using either N‐acetyl‐n‐propyl, phenylisothiocyanate, or N‐methyl‐N‐(tert‐butyldimethylsilyl)trifluoroacetamide (MTBSTFA) for GC‐C‐IRMS, LC‐MS/MS, and GC‐MS/MS analysis, respectively. A second derivative, heptafluorobutyric acid (HFBA), was also used for GC‐MS/MS analysis as an alternative for MTBSTFA. The machine reproducibility or the coefficients of variation for delta tracer‐tracee‐ratio measurements (delta tracer‐tracee‐ratio values around 0.0002) were 2.6%, 4.1%, and 10.9% for GC‐C‐IRMS, LC‐MS/MS, and GC‐MS/MS (MTBSTFA), respectively. FSR determined with LC‐MS/MS compared well with GC‐C‐IRMS and so did the GC‐MS/MS when using the HFBA derivative (linear fit Y = 1.08 ± 0.10, X + 0.0049 ± 0.0061, r = 0.89 ± 0.01, P < 0.0001). In conclusion, (1) IRMS still offers the most precise measurement of human muscle FSR, (2) LC‐MS/MS comes quite close and is a good alternative when tissue quantities are too small for GC‐C‐IRMS, and (3) If GC‐MS/MS is to be used, then the HFBA derivative should be used instead of MTBSTFA, which gave unacceptably high variability. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
996.
Single‐Layered Ultrasmall Nanoplates of MoS2 Embedded in Carbon Nanofibers with Excellent Electrochemical Performance for Lithium and Sodium Storage
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Dr. Changbao Zhu Dr. Xiaoke Mu Prof. Peter A. van Aken Prof. Yan Yu Prof. Joachim Maier 《Angewandte Chemie (International ed. in English)》2014,53(8):2152-2156
The preparation and electrochemical storage behavior of MoS2 nanodots—more precisely single‐layered ultrasmall nanoplates—embedded in carbon nanowires has been studied. The preparation is achieved by an electrospinning process that can be easily scaled up. The rate performance and cycling stability of both lithium and sodium storage were found to be outstanding. The storage behavior is, moreover, highly exciting from a fundamental point of view, as the differences between the usual storage modes—insertion, conversion, interfacial storage—are beneficially blurred. The restriction to ultrasmall reaction domains allows for an almost diffusion‐less and nucleation‐free “conversion”, thereby resulting in a high capacity and a remarkable cycling performance. 相似文献
997.
Mixed IR/Vis Two‐Dimensional Spectroscopy: Chemical Exchange beyond the Vibrational Lifetime and Sub‐ensemble Selective Photochemistry
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Dr. Luuk J. G. W. van Wilderen Dr. Andreas T. Messmer Prof. Dr. Jens Bredenbeck 《Angewandte Chemie (International ed. in English)》2014,53(10):2667-2672
Two‐dimensional exchange spectroscopy (2D EXSY) is a powerful method to study the interconversion (chemical exchange) of molecular species in equilibrium. This method has recently been realized in femtosecond 2D‐IR spectroscopy, dramatically increasing the time resolution. However, current implementations allow the EXSY signal (and therefore the chemical process of interest) only to be tracked during the lifetime (T1) of the observed spectroscopic transition. This is a severe limitation, as typical vibrational T1 are only a few ps. An IR/Vis pulse sequence is presented that overcomes this limit and makes the EXSY signal independent of T1. The same pulse sequence allows to collect time‐resolved IR spectra after electronic excitation of a particular chemical species in a mixture of species with strongly overlapping UV/Vis spectra. Different photoreaction pathways and dynamics of coexisting isomers or of species involved in different intermolecular interactions can thus be revealed, even if the species cannot be isolated because they are in rapid equilibrium. 相似文献
998.
Cell Tracking with Caged Xenon: Using Cryptophanes as MRI Reporters upon Cellular Internalization
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Stefan Klippel Jörg Döpfert Dr. Jabadurai Jayapaul Martin Kunth Federica Rossella Matthias Schnurr Dr. Christopher Witte Dr. Christian Freund Dr. Leif Schröder 《Angewandte Chemie (International ed. in English)》2014,53(2):493-496
Caged xenon has great potential in overcoming sensitivity limitations for solution‐state NMR detection of dilute molecules. However, no application of such a system as a magnetic resonance imaging (MRI) contrast agent has yet been performed with live cells. We demonstrate MRI localization of cells labeled with caged xenon in a packed‐bed bioreactor working under perfusion with hyperpolarized‐xenon‐saturated medium. Xenon hosts enable NMR/MRI experiments with switchable contrast and selectivity for cell‐associated versus unbound cages. We present MR images with 103‐fold sensitivity enhancement for cell‐internalized, dual‐mode (fluorescence/MRI) xenon hosts at low micromolar concentrations. Our results illustrate the capability of functionalized xenon to act as a highly sensitive cell tracer for MRI detection even without signal averaging. The method will bridge the challenging gap for translation to in vivo studies for the optimization of targeted biosensors and their multiplexing applications. 相似文献
999.
Back Cover: Cell Tracking with Caged Xenon: Using Cryptophanes as MRI Reporters upon Cellular Internalization (Angew. Chem. Int. Ed. 2/2014)
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1000.
Casey A. Dougherty Joseph C. Furgal Dr. Mallory A. van Dongen Prof. Theodore Goodson III Prof. Mark M. Banaszak Holl Dr. Janet Manono Prof. Stassi DiMaggio 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(16):4638-4645
Fluorescent dyes are commonly conjugated to nanomaterials for imaging applications using stochastic synthesis conditions that result in a Poisson distribution of dye/particle ratios and therefore a broad range of photophysical and biodistribution properties. We report the isolation and characterization of generation 5 poly(amidoamine) (G5 PAMAM) dendrimer samples containing 1, 2, 3, and 4 fluorescein (FC) or 6‐carboxytetramethylrhodamine succinimidyl ester (TAMRA) dyes per polymer particle. For the fluorescein case, this was achieved by stochastically functionalizing dendrimer with a cyclooctyne “click” ligand, separation into sample containing precisely defined “click” ligand/particle ratios using reverse‐phase high performance liquid chromatography (RP‐HPLC), followed by reaction with excess azide‐functionalized fluorescein dye. For the TAMRA samples, stochastically functionalized dendrimer was directly separated into precise dye/particle ratios using RP‐HPLC. These materials were characterized using 1H and 19F NMR spectroscopy, RP‐HPLC, UV/Vis and fluorescence spectroscopy, lifetime measurements, and MALDI. 相似文献