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31.
Walderhaug H Knudsen KD 《Langmuir : the ACS journal of surfaces and colloids》2008,24(19):10637-10645
Microemulsion samples of a polyoxyethylene trisiloxane surfactant, water, and 1-decanol are investigated using pulsed field gradient NMR and small-angle neutron scattering (SANS) to determine the solution structure. The surfactant/decanol weight ratio has been kept constant at values of 10:1, 8:1, and 6:1 under variation of water content. The temperature was 32 degrees C for the measurement series at the weight ratio of 10:1 to avoid phase separation at high water content. Also, aqueous surfactant solution samples have been investigated as a function of composition and temperature. Water-rich samples consist of micelles that are close to spherical at very low surfactant concentration and grow into anisometric, that is, oblate formed aggregates, at higher surfactant (or surfactant and decanol) concentration. The aggregates grow with increasing temperature, most probably due to dehydration of the hydrophilic groups. In a concentration range around 50 wt % water, the systems form bicontinuous structures. SANS data are used to estimate surfactant film properties using a model developed for interpretation of neutron scattering data from related systems. 相似文献
32.
Nagatoshi Nishiwaki Kristian Rahbek Knudsen Kurt V. Gothelf Karl Anker Jrgensen 《Angewandte Chemie (International ed. in English)》2001,40(16):2935-2935
The cover picture shows two new catalytic asymmetric reactions in front of an enantiomeric pair of lurs—one of the national symbols of Denmark. Chiral bisoxazoline copper complexes catalyze the reactions. The reaction on the left is the direct enantioselective Mannich reaction of carbonyl compounds with imines to give highly functionalized 4‐oxo‐glutamic esters, while the reaction on the right is the enantioselective nitro‐Mannich reaction of nitro alkanes with imines to give optically active β‐nitro‐α‐amino esters. Why the lur in relation to the present chemistry? Lurs exist as pairs of enantiomers and they probably belong to some of the first man‐made pair of enantiomers, as they are from the Bronze Age (1000–500 BC). The lurs are the oldest musical instruments that can still be played, and are used as enantiomers. Side by side stood two lur‐players, symmetric in every detail, the soft curves of the instruments swaying upwards and outwards at each side. The sonorous notes emphasized the solemn nature of the rituals. It is conceivable that the sound of the lur also accompanied the crackle of the funeral pyre when one of the tribe was cremated and laid to rest in the burial mound of his forefathers. The lurs are made of bronze (copper) and were discovered in lakes; less than 50 are known. The most famous pair of lurs was found in Brudevælte and is the pair shown on the cover. Further information about lurs can be obtained from kaj@chem.au.dk. The picture of the lurs and the accompanying music (click here to listen to the music: www.angewandte.com) are used with permission from The National Museum. For more details about the two new catalytic asymmetric reactions, see Jørgensen et al. on p. 2992 ff. and p. 2995 ff. 相似文献
33.
Antonia Neels Helen Stoeckli-Evans Burhkard Costisella Harald Jancke Kenneth D. Knudsen Philip Pattison 《Helvetica chimica acta》1999,82(1):35-43
The structure of [(dimethylamino)methylene]bis[phosphonic acid] dihydrate (C3H11NO6P2⋅2 H2O; 1 ) was solved ab initio from synchrotron powder X-ray diffraction data. The structure determination was based on direct methods combined with difference Fourier techniques, and the refinement was carried out using the Rietveld method. Using this high-quality diffraction pattern, it was possible to index a second phase which corresponds to the structure of the known [(dimethylamino)methylene]bis[phosphonic acid] monohydrate ( 2 ). [(Dimethylamino)methylene]bis[phosphonic acid] dihydrate ( 1 ) is monoclinic, space group P21/c, Z=4, with a=10.6644(1), b=9.1599(1), c=10.5213(1) Å, and β=98.353(1)°. The structure analysis indicates two non-equivalent P-atoms in the molecule of 1 which are also observed in the corresponding monohydrate 2 and unhydrated form 3 . All three compounds exhibit extended H-bonding networks which result in remarkably different 31P-NMR spectra. The [(dimethylamino)methylene]bis[phosphonic acids] 1 – 3 crystallize in the betaine-type structure which, therefore, contains two nonequivalent P-atoms. The −P(=O)(OH)2 and −P(=O)(OH)O− groups of 1 – 3 are involved in a number of strong H-bonds which can be characterized by the different 31P-NMR chemical shifts of the two P-atoms of 1 – 3 . 相似文献
34.
Loan T. T. Trinh Anna-Lena Kjøniksen Kaizheng Zhu Kenneth D. Knudsen Sondre Volden Wilhelm R. Glomm Bo Nyström 《Colloid and polymer science》2009,287(12):1391-1404
In this work, the salt-induced aggregation of bare and polymer-covered silver particles has been studied with the aid of light
scattering and UV-visible spectroscopy. Light scattering on a suspension of bare silver particles at a low salt concentration
shows that the cluster fractal dimension d
f changes from 1.6 to 2 in the course of the aggregation process, whereas no restructuring of the clusters is observed at a
higher salinity where d
f ≈ 1.6. The growth of the clusters over time can be described by a power law R
h ∝ t
α
, where R
h is the apparent hydrodynamic radius. The UV-visible experiments revealed that increasing the size of the bare silver particles
lead to a significant broadening and red-shift of the absorbance band, whereas for salt-induced growth of Ag clusters, a blue-shift
and broadening was observed. Addition of salt to a suspension of silver particles and hydroxyethylcellulose divulged a slower
broadening of the surface plasmon peak than without polymer. 相似文献
35.
A general approach to N-arylation and N-alkenylation of all five DNA/RNA nucleobases at the nitrogen atom normally attached to the sugar moiety in DNA or RNA has been developed. Various protected or masked nucleobases engaged readily in the copper-mediated Chan-Lam-Evans-modified Ullmann condensation with a range of different boronic acids at room temperature and were subsequently converted to the corresponding deprotected or unmasked adducts. Different N(3)-protecting groups were examined in the case of thymine, where the benzoyl group afforded the highest yields. A 4-alkylthio-substituted pyrimidin-2(1H)-one served as both a cytosine and a uracil precursor and was N-arylated and N-alkenylated in high yields. Adenine was efficiently and selectively N-arylated and N-alkenylated at the N(9) position by employing a bis-Boc-protected adenine derivative, while a bis-Boc-protected 2-amino-6-chloropurine served as guanine precursor and could also be selectively N(9)-arylated and N(9)-alkenylated. 相似文献
36.
Linear Frameworks for Block Ciphers 总被引:1,自引:0,他引:1
Inthis paper we generalize the structure of the ciphers Shark, Square, BKSQ, Crypton and Rijndael. We show that the linearcomponents play an essential role in the effect of the nonlinearS-boxes in providing resistance against differential and linearcryptanalysis and provide upper bounds for the probability ofdifferential characteristics and the correlation of linear approximationsfor the general structure. We show how good linear componentscan be constructed efficiently from Maximum-Distance Separablecodes. The presented block cipher structure can make optimaluse of a wide range of processor word lengths and its parallelismallows very fast dedicated hardware implementations. Cipherswith variable block length can be constructed by varying certainparameters in the presented structure. 相似文献
37.
H. Roy-Poulsen J. M. Knudsen L. Larsen N. O. Roy-Poulsen L. Vistisen 《Hyperfine Interactions》1986,29(1-4):1089-1092
Samples from the two Ni-rich iron meteorites Santa Catharina and Twin City have been investigated by conventional Mössbauer spectroscopy and by conversion electron Mössbauer spectroscopy. The spectra obtained show that the major components of these two meteorites are an ordered FeNi (~50%, ~50%) phase and a paramagnetic Fe?Ni phase with ≤28% Ni. 相似文献
38.
J.P. Merrison H. Bluhme M. Charlton H. Knudsen M.R. Poulsen 《Hyperfine Interactions》1997,109(1-4):313-321
In support of the positronium based reaction scheme for antihydrogen formation an experiment has been performed which has
demonstrated hydrogen formation following proton impact on positronium. The cross section has been experimentally determined
at proton energies of 11.3 keV, 13.3 keV and 15.8 keV, values of σH = 26(±9), 7.8(±2.3) and 7.6(± 4.4) × 10−16 cm2 were obtained. The determination is in reasonable agreement with several recent calculations of the cross section and provides
a well characterised process for antihydrogen formation.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
39.
40.
M. B. Madsen S. Mørup T. V. V. Costa J. M. Knudsen M. Olsen 《Hyperfine Interactions》1988,41(1):827-830