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61.
Ren Tschaggelar Besnik Kasumaj Maria Grazia Santangelo Jrg Forrer Patrik Leger Henry Dube Franois Diederich Jeffrey Harmer Rolf Schuhmann Ins García-Rubio Gunnar Jeschke 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2009,200(1):81-87
The construction and performance of a cryogenic 35 GHz pulse electron nuclear double resonance (ENDOR) probehead for large samples is presented. The resonator is based on a rectangular TE102 cavity in which the radio frequency (rf) B2-field is generated by a two turn saddle ENDOR coil crossing the resonator along the sample axis with minimal distance to the sample tube. An rf power efficiency factor is used to define the B2-field strength per square-root of the transmitted rf power over the frequency range 2–180 MHz. The distributions of the microwave B1- and E1-field, and the rf B2-field are investigated by electromagnetic field calculations. All dielectrics, the sample tube, and coupling elements are included in the calculations. The application range of the probehead and the advantages of using large sample sizes are demonstrated and discussed on a number of paramagnetic samples containing transition metal ions. 相似文献
62.
Christopher Selleck Prof. James A. Larrabee Assoc. Prof. Jeffrey Harmer Assoc. Prof. Luke W. Guddat Dr. Nataša Mitić Waleed Helweh Prof. David L. Ollis Dr. Whitney R. Craig Prof. David L. Tierney Dr. Marcelo Monteiro Pedroso Prof. Gerhard Schenk 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(49):17704-17714
Antibiotic resistance has emerged as a major threat to global health care. This is largely due to the fact that many pathogens have developed strategies to acquire resistance to antibiotics. Metallo‐β‐lactamases (MBL) have evolved to inactivate most of the commonly used β‐lactam antibiotics. AIM‐1 is one of only a few MBLs from the B3 subgroup that is encoded on a mobile genetic element in a major human pathogen. Here, its mechanism of action was characterised with a combination of spectroscopic and kinetic techniques and compared to that of other MBLs. Unlike other MBLs it appears that AIM‐1 has two avenues available for the turnover of the substrate nitrocefin, distinguished by the identity of the rate‐limiting step. This observation may be relevant with respect to inhibitor design for this group of enzymes as it demonstrates that at least some MBLs are very flexible in terms of interactions with substrates and possibly inhibitors. 相似文献
63.
Artificial neural network (ANN) and a hybrid principal component analysis-artificial neural network (PCA-ANN) classifiers have been successfully implemented for classification of static time-of-flight secondary ion mass spectrometry (ToF-SIMS) mass spectra collected from complex Cu–Fe sulphides (chalcopyrite, bornite, chalcocite and pyrite) at different flotation conditions. ANNs are very good pattern classifiers because of: their ability to learn and generalise patterns that are not linearly separable; their fault and noise tolerance capability; and high parallelism. In the first approach, fragments from the whole ToF-SIMS spectrum were used as input to the ANN, the model yielded high overall correct classification rates of 100% for feed samples, 88% for conditioned feed samples and 91% for Eh modified samples. In the second approach, the hybrid pattern classifier PCA-ANN was integrated. PCA is a very effective multivariate data analysis tool applied to enhance species features and reduce data dimensionality. Principal component (PC) scores which accounted for 95% of the raw spectral data variance, were used as input to the ANN, the model yielded high overall correct classification rates of 88% for conditioned feed samples and 95% for Eh modified samples. 相似文献
64.
Zero‐Valent Amino‐Olefin Cobalt Complexes as Catalysts for Oxygen Atom Transfer Reactions from Nitrous Oxide
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Dr. Thomas L. Gianetti Dr. Rafael E. Rodríguez‐Lugo Dr. Jeffrey R. Harmer Dr. Monica Trincado Dr. Matthias Vogt Dr. Gustavo Santiso‐Quinones Prof. Dr. Hansjörg Grützmacher 《Angewandte Chemie (International ed. in English)》2016,55(49):15323-15328
The synthesis and characterization of several zero‐valent cobalt complexes with a bis(olefin)‐amino ligand is presented. Some of these complexes proved to be efficient catalysts for the selective oxidation of secondary and allylic phosphanes, as well as diphosphanes, even with a direct P?P bond. With 5 mol % catalyst loadings the oxidations proceed under mild conditions (25–70 °C, 7–22 h, 2 bar N2O) and afford good to excellent yields (65–98 %). In this process, the greenhouse gas N2O is catalytically converted into benign N2 and added‐value organophosphorus compounds, some of which are difficult to obtain otherwise. 相似文献
65.
Deblon S Liesum L Harmer J Schönberg H Schweiger A Grützmacher H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(3):601-611
The 17-electron complexes [M(tropp(ph))2] (M=Co0, Rh0, Ir0) were prepared and isolated (tropp = tropylidene phosphane). A structural analysis of [Co(tropp(ph))2] revealed this complex to be almost tetrahedral, while the heavier homologues have more planar structures. Partially deuterated tropp complexes [D6][M(tropp(ph))2] were synthesised for M = Rh and Ir in order to enhance the resolution in the EPR spectra. This synthesis involves a four-fold intramolecular C-H activation reaction, whereby alkyl groups are transformed into olefins. Dihydrides were observed as intermediates for M = Ir. The electronic and geometric structures of all complexes [M(tropp(ph))2] (M = Co, Rh, Ir) and [D6][M(tropp(ph))2] (M = Rh, Ir) were investigated by continuous wave (CW) and echo-detected EPR in combination with pulse ENDOR and ESEEM techniques. In accord with their planar structures, cis and trans isomers were detected for [M(tropp(ph))2] (M = Rh0, Ir0) for which a dynamic equilibrium was established. The thermodynamic data show that the cis isomer is slightly preferred by deltaH(o) = -4.7 +/- 0.3 kJ mol(-1) (M = Rh) and delta H(o) = -5.1 +/- 0.5 kJ mol(-1); (M = Ir). The entropies for the process trans-[M(tropp(ph))2] <==> cis-[M(tropp(ph))2] are also negative [deltaS(o) = -5 +/- 1.5 J mol(-1) (M = Rh); deltaS(o) = -17 +/- 3.7 J mol(-1) (M = Rh)], indicating higher steric congestion in the cis isomers. The cobalt(0) and irdium(0) complexes show rather large g anisotropies, while that of the rhodium(0) complex is small (Co: g(parallel) = 2.320, g(perpendicular) = 2.080; cis-Rh: g(parallel) = 2.030, g(perpendicular) = 2.0135; trans-Rh: g(parallel) = 2.050, g(perpendicular) = 2.030; cis-Ir: g(parallel) = 2.030, g(perpendicular) = 2.060; trans-Ir: g(parallel) = 1.980, g(perpendicular) = 2.150). The g matrices of [M(tropp(ph))2] (M = Co, Rh) are axially symmetric with g(parallel) > g(perpendicular), indicating either a distorted square planar structure (SOMO essentially d(x2 - y2) or a compressed tetrahedron (SOMO essentially d(xy)). Interestingly, for [Ir(tropp(ph))2] the inverse ordering, g(perpendicular) > g(parallel) is found; this cannot be explained by simple ligand field arguments and must await a more sophisticated analysis. The hyperfine interactions of the unpaired electron with the metal nuclei, phosphorus nuclei, protons, deuterons and carbon nuclei were determined. By comparison with atomic constants, the spin densities on these centres were estimated and found to be small. However, the good agreement of the distance between the olefinic protons and the metal centres determined from the dipolar coupling parameter indicates that the unpaired electron is primarily located at the metal centre. 相似文献
66.
C. J. Barbé M. A. Harmer G. W. Scherer 《Journal of Sol-Gel Science and Technology》1994,2(1-3):507-512
A sol gel route has been developed to produce stoichiometric, dense and crack free KTiOPO4 (KTP) thin films which exhibit waveguiding. This method involved the deposition and pyrolysis of hydrolyzed solutions of K, Ti and P alkoxides on an appropriate substrate. The final ceramic film morphology is greatly dependent on the preparation chemistry, apparently as a result of the formation of polymetallic complexes. The solution chemistry, processing conditions and their influence on the final ceramic film microstructure will be discussed in detail.Optical properties such as indices of refraction and optical losses have been found to be dependent on the film microstructures. Deposition on different substrates has been undertaken to promote preferential orientation and thus to try to minimize the optical losses in the films. This correlation between the optical losses and microstructures will be discussed in detail. 相似文献
67.
Gajda T Düpre Y Török I Harmer J Schweiger A Sander J Kuppert D Hegetschweiler K 《Inorganic chemistry》2001,40(19):4918-4927
The interaction of Cu(II) with the ligand tdci (1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol) was studied both in the solid state and in solution. The complexes that were formed were also tested for phosphoesterase activity. The pentanuclear complex [Cu(5)(tdciH(-2))(tdci)(2)(OH)(2)(NO(3))(2)](NO(3))(4).6H(2)O consists of two dinuclear units and one trinuclear unit, having two shared copper(II) ions. The metal centers within the pentanuclear structure have three distinct coordination environments. All five copper(II) ions are linked by hydroxo/alkoxo bridges forming a Cu(5)O(6) cage. The Cu-Cu separations of the bridged centers are between 2.916 and 3.782 A, while those of the nonbridged metal ions are 5.455-5.712 A. The solution equilibria in the Cu(II)-tdci system proved to be extremely complicated. Depending on the pH and metal-to-ligand ratio, several differently deprotonated mono-, di-, and trinuclear complexes are formed. Their presence in solution was supported by mass, CW, and pulse EPR spectroscopic study, too. In these complexes, the metal ions are presumed to occupy tridentate [O(ax),N(eq),O(ax)] coordination sites and the O-donors of tdci may serve as bridging units between two metal ions. Additionally, deprotonation of the metal-bound water molecules may occur. The dinuclear Cu(2)LH(-3) species, formed around pH 8.5, provides outstanding rate acceleration for the hydrolysis of the activated phosphodiester bis(4-nitrophenyl)phosphate (BNPP). The second-order rate constant of BNPP hydrolysis promoted by the dinuclear complex (T = 298 K) is 0.95 M(-1) s(-1), which is ca. 47600-fold higher than that of the hydroxide ion catalyzed hydrolysis (k(OH)). Its activity is selective for the phosphodiester, and the hydrolysis was proved to be catalytic. The proposed bifunctional mechanism of the hydrolysis includes double Lewis acid activation and intramolecular nucleophilic catalysis. 相似文献
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69.
Zhongshu Li Jaap E. Borger Fabian Müller Jeffrey R. Harmer Cheng‐Yong Su Hansjrg Grützmacher 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(33):11551-11555
The facile preparation of alkali salts of phosphanyl cyanophosphides [NHP‐PCN]? (NHP=N‐heterocyclic phosphenium) is reported. Their formation is achieved by isoelectronic replacement of O for [N]? in the phosphaketenes NHP‐PCO using alkaline hexamethyldisilazide M[N(SiMe3)2] (M=Na, K) as reagent. The new anionic entities are versatile PCN building blocks which allow the formation of a diversity of new cyanophosphine derivates including the first example of a PCNB hetero‐cumulene and a PCN‐ligated transition metal complex. 相似文献
70.
Shannon J. Lee Juyeon Won Dr. Lin-Lin Wang Dr. Dapeng Jing Colin P. Harmer Dr. Justin Mark Dr. Georgiy Akopov Prof. Kirill Kovnir 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(26):7383-7390
Three novel isostructural equiatomic gold tetrel pnictides, AuSiAs, AuGeP, and AuGeAs, were synthesized and characterized. These phases crystallize in the noncentrosymmetric (NCS) monoclinic space group Cc (no. 9), featuring square-planar Au within cis-[AuTt2Pn2] units (Tt=tetrel, Si, Ge; Pn=pnictogen, P, As). This is in drastic contrast to the structure of previously reported AuSiP, which exhibits typical linear coordination of Au with Si and P. Chemical bonding analysis through the electron localization function suggests covalent two-center two-electron Tt−Pn bonds, and three-center Au−Tt−Au and Au−Pn−Au bonds with 1.6 e− per bond. X-ray photoelectron spectroscopy studies support the covalent and nonionic nature of Au−Pn and Au−Tt bonds. The title materials were found to be n-type narrow-gap semiconductors or semimetals, with nearly temperature-independent electrical resistivities and low thermal conductivities. A combination of the semimetallic properties with tunable NCS structure provides opportunities for the development of materials based on gold tetrel pnictides. 相似文献