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51.
Paulat F Kuschel T Näther C Praneeth VK Sander O Lehnert N 《Inorganic chemistry》2004,43(22):6979-6994
The spectroscopic properties and the electronic structure of the only nitrous oxide complex existing in isolated form, [Ru(NH(3))(5)(N(2)O)]X(2) (1, X = Br(-), BF(4)(-)), are investigated in detail in comparison to the nitric oxide precursor, [Ru(NH(3))(5)(NO)]X(3) (2). IR and Raman spectra of 1 and of the corresponding (15)NNO labeled complex are presented and assigned with the help of normal coordinate analysis (NCA) and density functional (DFT) calculations. This allows for the identification of the Ru-N(2)O stretch at approximately 300 cm(-)(1) and for the unambiguous definition of the binding mode of the N(2)O ligand as N-terminal. Obtained force constants are 17.3, 9.6, and 1.4 mdyn/A for N-N, N-O, and Ru-N(2)O, respectively. The Ru(II)-N(2)O bond is dominated by pi back-donation, which, however, is weak compared to the NO complex. This bond is further weakened by Coulomb repulsion between the fully occupied t(2g) shell of Ru(II) and the HOMO of N(2)O. Hence, nitrous oxide is an extremely weak ligand to Ru(II). Calculated free energies and formation constants for [Ru(NH(3))(5)(L)](2+) (L = NNO, N(2), OH(2)) are in good agreement with experiment. The observed intense absorption at 238 nm of 1 is assigned to the t(2g) --> pi(*) charge transfer transition. These data are compared in detail to the spectroscopic and electronic structural properties of NO complex 2. Finally, the transition metal centered reaction of nitrous oxide to N(2) and H(2)O is investigated. Nitrous oxide is activated by back-donation. Initial protonation leads to a weakening of the N-O bond and triggers electron transfer from the metal to the NN-OH ligand through the pi system. The implications of this mechanism for biological nitrous oxide reduction are discussed. 相似文献
52.
Weissbuch I Zepik H Bolbach G Shavit E Tang M Jensen TR Kjaer K Leiserowitz L Lahav M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(8):1782-1794
A possible role that might have been played by ordered clusters at interfaces for the generation of homochiral oligopeptides under prebiotic conditions has been probed by a catalyzed polymerization of amphiphilic activated alpha-amino acids, in racemic and chiral non-racemic forms, which had self-assembled into two-dimensional (2D) ordered crystallites at the air-aqueous solution interface. As model systems we studied N(epsilon)-stearoyl-lysine thioethyl ester (C(18)-TE-Lys), gamma-stearyl-glutamic thioethyl ester (C(18)-TE-Glu), N(alpha)-carboxyanhydride of gamma-stearyl-glutamic acid (C(18)-Glu NCA) and gamma-stearyl-glutamic thioacid (C(18)-thio-Glu). According to in-situ grazing incidence X-ray diffraction measurements on the water surface, (R,S)-C(18)-TE-Lys, (R,S)-C(18)-TE-Glu, and (R,S)-C(18)-Glu-NCA amphiphiles self-assembled into ordered racemic 2D crystallites. Oligopeptides 2-12 units long were obtained at the air-aqueous solution interface after injection of appropriate catalysts into the water subphase. The experimental relative abundance of oligopeptides with homochiral sequence generated from (R,S)-C(18)-TE-Lys and (R,S)-C(18)-TE-Glu, as determined by mass spectrometry on enantioselectively deuterium-labeled samples, was found to be significantly larger than that obtained from (R,S) C(18)-thio-Glu which polymerizes randomly. An efficient chiral amplification was obtained in the polymerization of non-racemic mixtures of C(18)-Glu-NCA since the monomer molecules in the racemic 2D crystallites are oriented such that the reaction occurs between heterochiral molecules related by glide symmetry to yield heterochiral oligopeptides whereas the enantiomer in excess, in the enantiomorphous crystallites, yield oligopeptides of a single handedness. 相似文献
53.
Gustav Bojesen Torben Breindahl Ulla N. Andersen 《Journal of mass spectrometry : JMS》1993,28(12):1448-1452
The decomposition of 59 different cluster ions (generated by fast atom bombardment) consisting of two different amino acids and a sodium ion was analysed. The only fragment ions of significant abundance could be assigned to sodium ion-bound amino acids. Assuming that the most abundant ion in the fragment ion spectrum corresponds to the amino acid with the highest sodium ion affinity (SIA), the 20 common α-amino acids could be ordered with increasing sodium ion affinity as follows: Gly, Ala, Cys, Val, (Leu, Ile), Ser, Met, Thr, (Phe, Pro), Asp, Tyr, (Glu, Lys), Trp, Asn, Gln, His, Arg. Quantitative determinations were carried out by comparison of the lithium ion affinity (LIA) of Ala with that of dimethylformamide (DMF) in a fragment ion scan of the ion-bound dimer Ala—Li+—DMF. LIA(Ala) was calculated from LIA(Ala) = LIA(DMF) – (1/C)ln[I(AlaLi+)/I(DMF—Li+)], where the constant C was estimated from measurements of proton-bound amine–amino acid clusters. From fragment ion analysis of nine other Li+-bound α-amino acid dimers, the following lithium ion affinities were obtained: Gly 51.0, Ala 52.6, Sar 53.5, α-aminobutyric acid 53.7, glycine methyl ester 54.7 and Val 54.8. SIA(Ala) was estimated to be 75% of the lithium ion affinity and from fragment ion analysis of ten Na+-bound α-amino acid dimers the following sodium ion affinities were obtained: Gly 37.9, Ala 39.4, α-aminobutyric acid 40.3, Val 41.0, glycine methylster 41.0 and Sar 41.2. 相似文献
54.
Incorporation of a novel pyren-1-ylcarbonyl-functionalized 4'-C-piperazinomethyl-DNA monomer into oligodeoxynucleotides leads to increased thermal stability of duplexes with DNA complements but reduced thermal stability of duplexes with RNA complements. This DNA-selective hybridization is explored for recognition of double-stranded DNA by a novel dual strand invasion approach. 相似文献
55.
Torben Maack Bisgaard 《Mathematische Nachrichten》2002,236(1):31-46
The problem of characterizing those semigroups of class ℳ︁ such that every positive definite function factors via the greatest C ‐separative ∗︁‐homomorphic image is shown to be equivalent to several other problems, including that of characterizing flat semigroups. If the latter could be solved then the problem of characterizing perfect semigroups would be reduced to characterizing quasi‐perfect semigroups. 相似文献
56.
57.
Mathevon N Dabelsteen T Blumenrath SH 《The Journal of the Acoustical Society of America》2005,117(1):442-449
Birds often sing from high perches referred to as song posts. However, birds also listen and keep a lookout from these perches. We used a sound transmission experiment to investigate the changes for receiving and sending conditions that a territorial songbird may experience by moving upwards in the vegetation. Representative song elements of the blackcap Sylvia atricapilla were transmitted in a forest habitat in spring using a complete factorial design with natural transmission distances and speaker and microphone heights. Four aspects of sound degradation were quantified: signal-to-noise ratio, excess attenuation, distortion within the sounds determined as a blur ratio, and prolongation of the sounds with "tails" of echoes determined as a tail-to-signal ratio. All four measures indicated that degradation decreased with speaker and microphone height. However, the decrease was considerably higher for the microphone than for the speaker. This suggests that choosing high perches in a forest at spring results in more benefits to blackcaps in terms of improved communication conditions when they act as receivers than as senders. 相似文献
58.
If A is a complex *-algebra with unit then every linear form L on A which is positive in the sense that
for all
vanishes on the Jacobson radical of A. 相似文献
59.
Torben Poulsen 《Applied Acoustics》1982,15(5):363-376
The development of an acoustic traffic signal for blind pedestrians is described. The WALK and WAIT signals used represent a compromise between many contradictory demands, e.g. easy localisation and detection in a background of traffic noise, minimum annoyance, maximum attenuation into dwellings and simple and reliable construction. The WALK sound consists of 200 ms pulses at a repetition rate of 2·5 Hz. The WAIT sound consists of 400 ms pulses at a repetition rate of 0·5 Hz. Both WALK and WAIT pulses are formed from an 880 Hz sawtooth or square wave signal.A weak attention signal is audible (automatic level control) within a distance of 2–3 m from the traffic light pole. The system has been in use for more than 2 years. Twenty-six blind pedestrians tested the system and found it very useful. A questionnaire investigation showed no adverse reactions from neighbours to the system. The attention system is being standardised in Denmark. The system is prepared for extension with a louder guiding signal which guides the blind across the crossing. 相似文献
60.