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1.
After the International Laboratory Accreditation Cooperation (ILAC) had taken in 2004, the resolution to conduct accreditation
of producers of reference materials according to ISO Guide 34 ‘General requirements for the competence of reference material
producers’ in combination with ISO/IEC 17025 ‘General requirements for the competence of testing and calibration laboratories’,
ISO/REMCO, the ISO Committee on Reference Materials, decided in 2005 to revise ISO Guide 34 to align it closer with ISO/IEC
17025 and to clarify certain issues for accreditors and producers seeking accreditation without adding new requirements. Moreover,
the publication in 2007 of ISO/IEC Guide 99 ‘International vocabulary of metrology—Basic and general concepts and associated
terms (VIM)’ triggered additional adaptations of the guide. 相似文献
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This paper is a reply to a recent paper on the isomers of trisulfur oxides (Chem. Commun., 2005, 3712) where the accuracy of the CASPT2 method is called in question. We illustrate that their results are due to an inadequate choice of active orbitals. Some results for related O and S containing molecules are also presented to strengthen the argumentation. It is concluded that the CASPT2 method is accurate to at least 0.1 eV (2 kcal mol(-1)) for this type of systems. 相似文献
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Jarowski PD Wodrich MD Wannere CS Schleyer PV Houk KN 《Journal of the American Chemical Society》2004,126(46):15036-15037
The conjugation stabilization energies of dienes and diynes are considerably larger than estimates based on heat of hydrogenation differences between 1,3-butadiyne and 1-butyne as well as between 1,3-butadiene and 1-butene. Such comparisons do not take into account the counterbalancing hyperconjugative stabilization of the partially hydrogenated products by their ethyl groups. When alkyl hyperconjugation is considered, the conjugation stabilization of diynes ( approximately 9.3 kcal/mol) is found by two methods (involving isomerization of nonconjugated into conjugated isomers and heats of hydrogenation) to be larger than that of dienes ( approximately 8.2 kcal/mol). 相似文献
6.
The remarkable conservation of protein structure, compared to that of sequences, suggests that, in the course of evolution,
residue substitutions which tend to destabilise a particular structure must be compensated by other substitutions that confer
greater stability on that structure. Given the compactness of proteins, spatially close residues are expected to undergo the
compensatory process. Surprisingly, approaches designed to detect such correlated changes have led, until now, only to limited
success in detecting pairs of residues adjacent in the three-dimensional structures. We have undertaken, by simulating the
evolution of DNA sequences including sites mutating in a correlated manner, to analyse whether such poor results can be attributed
to the detection methods or if this failure could result from a compensatory process more complex than that implicitly underlying
the different approaches. Present results show that only methods taking into account the phylogenetic reconstruction can lead
to correct detection.
Received: 24 April 1998 / Accepted: 8 August 1998 / Published online: 11 November 1998 相似文献
7.
Maroulis G 《The Journal of chemical physics》2008,129(4):044314
An extensive conventional ab initio and density functional theory investigation reveals that HXeI is a polar molecule with large multipole moments and highly anisotropic (hyper)polarizability. At the CCSD(T) level of theory our best values for the mean (hyper)polarizability are alphae(2)a(0) (2)E(h) (-1)=101.46, betae(3)a(0) (3)E(h) (-2)=-850.7, and gammae(4)a(0) (4)E(h) (-3)=18.7x10(3). The corresponding anisotropies are Deltaalphae(2)a(0) (2)E(h) (-1)=119.66, Deltabetae(3)a(0) (3)E(h) (-2)=-2518.7, Delta(1)gammae(4)a(0) (4)E(h) (-3)=-249.1x10(3), and Delta(2)gammae(4)a(0) (4)E(h) (-3)=-99.6x10(3). The longitudinal components of the (hyper)polarizability are dominant. Our value for the anisotropy of the dipole polarizability is considerably larger than the recent empirical estimate of 22.9 e(2)a(0) (2)E(h) (-1) [N. H. Nahler et al., J. Chem. Phys. 119, 224 (2003)]. The results of the insertion of Xe into HI are quantified by the calculation of the differential (hyper)polarizability at the MP2 level of theory: alpha(diff) identical withalpha(HXeI)-alpha(HI)-alpha(Xe)=36.29 e(2)a(0) (2)E(h) (-1) and gamma(diff) identical with gamma(HXeI)-gamma(HI)-gamma(Xe)=18.1x10(3) e(4)a(0) (4)E(h) (-3). 相似文献
8.
How important is the back reaction of electrons via the substrate in dye-sensitized nanocrystalline solar cells? 总被引:3,自引:0,他引:3
The role of the conducting glass substrate (fluorine-doped tin oxide, FTO) in the back reaction of electrons with tri-iodide ions in dye-sensitized nanocrystalline solar cells (DSCs) has been investigated using thin-layer electrochemical cells that are analogues of the DSCs. The rate of back reaction is dependent on the type of FTO and the thermal treatment. The results show that this back-reaction route cannot be neglected in DSCs, particularly at lower light intensities, where it is the dominant route for the back transfer of electrons to tri-iodide. This conclusion is confirmed by measurements of the intensity dependence of the photovoltages of DSCs with and without blocking layers. It follows that blocking layers should be used to prevent the back reaction in mechanistic studies in which the light intensity is varied over a wide range. Conclusions based on studies of the intensity dependence of the parameters of DSCs such as photovoltage and electron lifetime in cells without blocking layers, must be critically re-examined. 相似文献
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Ivan Gutman Ned?ad Turkovi? Jelena Jovi?i? 《Monatshefte für Chemie / Chemical Monthly》2004,95(1):1389-1394
Cyclic conjugation in benzo-annelated perylenes is studied by means of the energy-effects of their six-membered rings. Classical theoretical approaches (based on Kekulé structures, Clar formulae, or conjugated circuits) predict that the central ring in benzo-annelated perylenes is empty and thus negligibly contributes to cyclic conjugation. Our calculations show that this is true only to a limited degree. In particular, rings angularly annelated relative to the central ring significantly increase the extent of its cyclic conjugation. 相似文献
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《Chemical physics letters》1986,130(3):190-194
Excitation of the C-H overtones of 1,3-butadiene in the presence of HCl to an energy ≈ 16 kcal mol−1 in excess of the activation energy causes the formation of no detectable addition products. This indicates that the rate constant for the formation of 1-chloro-2-butene is at least slower than 2.1× 10−14 cm3 molecule−1 s−1. 相似文献
13.
Summary. Cyclic conjugation in benzo-annelated perylenes is studied by means of the energy-effects of their six-membered rings. Classical theoretical approaches (based on Kekulé structures, Clar formulae, or conjugated circuits) predict that the central ring in benzo-annelated perylenes is empty and thus negligibly contributes to cyclic conjugation. Our calculations show that this is true only to a limited degree. In particular, rings angularly annelated relative to the central ring significantly increase the extent of its cyclic conjugation. 相似文献
14.
Ab initio calculations illustrate that CH/pi attractions significantly contribute to host-guest complexation, but are not always a direct factor in molecular recognition. 相似文献
15.
Johanne Penafiel Prof. Dr. Laurent Maron Prof. Dr. Sjoerd Harder 《Angewandte Chemie (International ed. in English)》2015,54(1):201-206
Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal–alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca2+. The role of the metal was evaluated by a study using the metal‐free catalysts: [Ph2N?][Me4N+] and [Ph3C?][Me4N+]. These “naked” amides and carbanions can act as catalysts in the conversion of activated double bonds (C?O and C?N) in the hydroamination of Ar? N?C?O and R? N?C?N? R (R=alkyl) by Ph2NH. For the intramolecular hydroamination of unactivated C?C bonds in H2C?CHCH2CPh2CH2NH2 the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study. 相似文献
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Fabio Ganazzoli 《Macromolecular theory and simulations》2000,9(8):682-686
We report theoretical results about amphiphilic random copolymers in a quasi‐ideal conformation with an overall size very close to that of the analogue homopolymers. We found that a few states may coexist with about the same free energy and a similar radius of gyration, but with different intramolecular conformations. We also argue that, in most cases, amphiphilic copolymers may never achieve the unperturbed Θ state, defined thermodynamically by a vanishing second virial coefficient. Thus, we suggest that such copolymers usually show neither an unperturbed conformation nor an unperturbed state from the thermodynamic viewpoint. We also briefly discuss star homopolymers, which show a depression of the Θ temperature with respect to linear chains and a significant, though finite, Θ swelling, as well as linear chains in the Θ state and in the melt. The main general conclusion is that interactions between chain segments do not cancel each other and are non‐negligible. Accordingly, we suggest that the word "unperturbed" be used only with reference to solution thermodynamics and not for the chain size or conformation. 相似文献
17.
Johanne Penafiel Laurent Maron Sjoerd Harder 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(1):203-208
Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal–alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca2+. The role of the metal was evaluated by a study using the metal‐free catalysts: [Ph2N−][Me4N+] and [Ph3C−][Me4N+]. These “naked” amides and carbanions can act as catalysts in the conversion of activated double bonds (CO and CN) in the hydroamination of Ar NCO and R NCN R (R=alkyl) by Ph2NH. For the intramolecular hydroamination of unactivated CC bonds in H2CCHCH2CPh2CH2NH2 the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study. 相似文献
18.
M. Ebihara Y. Oura T. Ishii M. Setoguchi H. Nakahara T. Ohtsuki 《Journal of Radioanalytical and Nuclear Chemistry》2000,244(3):491-496
Chondritic meteorite samples were analyzed nondestructively by photon activation analysis. Powdered samples weighing about 50 mg each were irradiated with photons (-rays) converted from electrons accelerated by a linear electron accelerator at 20 and 30 MeV. With 30-minute and 6-hour irradiations, 11 and 12 elements with duplication of 6 elements could be determined, respectively. Considering that several major elements including Mg, Si and Fe can be determined in addition to Ti, Rb, Sr, Y and Zr, which cannot or hardly be determined by instrumental neutron activation analysis (INAA) with rather high sensitivity, instrumental photon activation analysis is as effective as INAA. 相似文献
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Seredyuk M Gaspar AB Ksenofontov V Galyametdinov Y Kusz J Gütlich P 《Journal of the American Chemical Society》2008,130(4):1431-1439
Three types of interplay/synergy between spin-crossover (SCO) and liquid crystalline (LC) phase transitions can be predicted: (i) systems with coupled phase transitions, where the structural changes associated to the Cr<-->LC phase transition drives the spin-state transition, (ii) systems where both transitions coexist in the same temperature region but are not coupled, and (iii) systems with uncoupled phase transitions. Here we present a new family of Fe(II) metallomesogens based on the ligand tris[3-aza-4-((5-C(n))(6-R)(2-pyridyl))but-3-enyl]amine, with C(n) = hexyloxy, dodecyloxy, hexadecyloxy, octadecyloxy, eicosyloxy, R = hydrogen or methyl (C(n)-trenH or C(n)-trenMe), which affords examples of systems of types i, ii, and iii. Self-assembly of the ligands C(n)-trenH and C(n)-trenMe with Fe(A)2 x xH2O salts have afforded a family of complexes with general formula [Fe(C(n)-trenR)](A)2 x sH2O (s > or = 0), with A = ClO4(-), F-, Cl-, Br- and I-. Single-crystal X-ray diffraction measurements have been performed on two derivatives of this family, named as [Fe(C6-trenH)](ClO4)2 (C(6)-1) and [Fe(C6-trenMe)](ClO4)2 (C(6)-2), at 150 K for C(6)-1 and at 90 and 298 K for C(6)-2. At 150 K, C(6)-1 displays the triclinic space group P, whereas at 90 and at 298 K C(6)-2 adopts the monoclinic P2(1)/c space group. In both compounds the iron atoms adopt a pseudo-octahedral symmetry and are surrounded by six nitrogen atoms belonging to imino groups and pyridines of the ligands C(n)-trenH and C(n)-trenMe. The average Fe(II)-N bonds (1.963(2) A) at 150 K denote that C(6)-1 is in the low-spin (LS) state. For C(6)-2 the average Fe(II)-N bonds (2.007(1) A) at 90 K are characteristic of the LS state, while at 298 K they are typical for the high-spin (HS) state (2.234(3) A). Compound C(6)-1 and [Fe(C18-trenH)](ClO4)2 (C(18)-1) adopts the LS state in the temperature region between 10 and 400 K, while compound C(6)-2 and [Fe(Cn-trenMe)](ClO4)2 (n = 12 (C(12)-2), 18 (C(18)-2)) exhibit spin crossover behavior at T(1/2) centered around 140 K. The thermal spin transition is accompanied by a pronounced change of color from dark red (LS) to orange (HS). The light-induced excited spin state trapping (LIESST) effect has been investigated in compounds C(6)-2, C(12)-2 and C(18)-2. The T(1/2)LIESST is 56 K (C(6)-2), 48 K (C(16)-2), and 56 K (C(18)-2). On the basis of differential scanning calorimetry, optical polarizing microscopy, and X-ray diffraction findings for C(18)-1, C(12)-2, and C(18)-2 at high temperature a smectic mesophase SX has been identified with layered structures similar to C(6)-1 and C(6)-2. The compounds [Fe(C(n)-trenH)](Cl)2 x sH2O (n = 16 (C(16)-3, s = 3.5, C(16)-4, s = 0.5, C(16)-5, s = 0), 18 (C(18)-3, s = 3.5, C(18)-4, s = 0.5, C(18)-5, s = 0), 20 (C(20)-3, s = 3.5, C(20)-4, s = 0.5, C(20)-5, s = 0)) and [Fe(C18-tren)](F)2 x sH2O (C(18)-6, s = 3.5, C(18)-7, s = 0) show a very particular spin-state change, while [Fe(C18-tren)](Br)2 x 3H2O (C(18)-8) together with [Fe(C18-tren)](I)2 (C(18)-9) are in the LS state (10-400 K) and present mesomorphic behavior like that observed for the complexes C(18)-1, C(12)-2, and C(18)-2. In compounds C(n)-3 50% of the Fe(II) ions undergo spin-state change at T(1/2) = 375 K induced by releasing water, and in partially dehydrated compounds (s = 0.5) the Cr-->SA phase transition occurs at 287 K (C(16)-4), 301 K (C(18)-4) and 330 K (C(20)-4). For the fully dehydrated materials C(n)-5 50% of the Fe(II) ions are in the HS state and show paramagnetic behavior between 10 and 400 K. In the partially dehydrated C(n)-4 the spin transition is induced by the change of the aggregate state of matter (solid<-->liquid crystal). For compound C(18)-6 the full dehydration to C(18)-7 provokes the spin-state change of nearly 50% of the Fe(II) ions. The compounds C(n)-3 and C(18)-6 are dark purple in the LS state and become light purple-brown when 50% of the Fe(II) atoms are in the HS state. 相似文献