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11.
Vinzenz Bachler  Franz Mark 《Tetrahedron》1977,33(21):2857-2861
Using intermolecular SCF-perturbation theory, the controlling factors determining the regioselectivity in the acrolein dimerization are derived by a rigorous decomposition of the interaction energy. This partitioning of the interaction energy is performed up to a level where a comparison with frontier orbital models is possible. The regioselectivity is found to be determined by orbital interactions occuring via the terminal atoms and atoms involved in secondary Woodward-Hoffmann interactions.  相似文献   
12.
The metal-organic framework [Co(INA)(2)]x0.5EtOH (INA = isonicotinate, NC(5)H(4)-4-CO(2)(-)), was synthesised under solvothermal conditions. Its X-ray crystal structure shows channels containing ethanol guests which are hydrogen-bonded to carboxylate oxygens of the framework. The pyridyl rings of the framework alternate between 'open' and 'closed' positions along the channels resulting in large variation in the channel cross-sectional area from ca. 1.4 by 2.3 A at the narrowest point to 4.9 by 5.3 A at the widest. Despite the very small windows, the ethanol guests (of van der Waals diameter ca. 4.2-6.1 A) may be reversibly desorbed and sorbed into the structure quantitatively, as shown by in situ variable-temperture IR spectroscopy and XRPD. The single-crystal structure of the desolvated form [Co(INA)(2)] shows that there is no change in the overall connectivity on desolvation, but the rotational positions of the pyridine rings are altered. This suggests that pyridyl rotation may occur to allow guests to pass in and out. When the synthesis was conducted in 1-propanol solvent [Co(INA)(2)]x0.5Pr(n)OHxH(2)O, was obtained, and a single-crystal X-ray structure revealed the same overall connectivity as in but with pyridine rings disordered over closed and open positions. There was no evidence of included guests from X-ray crystallography, suggesting that they are also highly disordered. Variable-temperature XRPD performed on bulk samples showed peaks which were unsymmetrical and exhibited shoulders, suggesting that for each pattern obtained the material actually consisted of several closely-related phases. The movements of the peaks during desolvation showed the presence of intermediate phases before the final desolvated product was formed. The peak positions of the intermediate phases matched more closely with the calculated pattern for than with or, suggesting that they may have disordered structures similar to. The results also suggest that the intermediate phase represents an initial increase in volume before a larger decrease in volume occurs to give the final desolvated material.  相似文献   
13.
A study of managanese and iron associated with particulate matter in seawaters derived from the English Channel (and North Sea). Sequential extraction procedures have been used to identify geochemical processes that govern Mn and Fe coprecipitation/accumulation/distribution in different particulate phases (carbonate; Fe-Mn(hydr)oxides; organic matter and residual phase). The oxidation states of Mn and Fe, some mechanisms of bonding and the magnetic characteristics of particles have been investigated by Mössbauer spectroscopy, electron spin resonance(ESR) spectroscopy, and mass susceptibility balance. The effect of pH and redox potential on stability of different dissolved and particulate Mn species was also examined. In the these calcareous seawaters, manganese (II) is predominantly bound to the carbonate phase (calcite) in the form of MnxCa1-xCO3. However the persistent occurrence of Mn in oxidation states > 2 in these particles has suggested the existence of manganese(III) in the polymeric FeOOH structures as an intermediate entity during Mn(II) oxidation in the sea. The activity coefficient of the entity MnOOH in FeOOH has been evaluated and p-pH diagrams of Mn have been proposed under marine conditions.  相似文献   
14.
The Cr(CO)3(CH3CN)3 complex is found to catalyze the 1,4-addition of hydrogen to 1,3-dienes such as 2-methyl-1,3-butadiene, trans-1,3-pentadiene, and trans, trans-2,4-hexadiene at low temperature (40°) and low H2 pressure (20 psi). For trans, trans-2,4-hexadiene the only product obtained when D2 is used is 2,5-dideuterio-cis-3-hexene. The catalytic 1,4-hydrogenation can be carried out in neat dienes, and turnover numbers for the catalyst of greater than 3000 have been observed.  相似文献   
15.
Implicit solvent methods have become popular tools in the field of protein dynamics simulations, yet evaluation of their validity has been primarily limited to comparisons with experimental and theoretical data for small molecules. In this paper, we use a recently developed hybrid explicit/implicit solvent methodology to evaluate the accuracy of several Poisson-based implicit solvent models. Specifically, we focus on the calculation of electrostatic solvation free energies of various fixed conformations for two proteins. We show that, among various dielectric boundary definitions, the Lee-Richards molecular surface has the best agreement with hybrid solvent results. Furthermore, certain modifications of the molecular surface Poisson protocol provide varied results. For instance, simple modifications of atomic radii on charged residues generally improve absolute errors but do not significantly reduce relative errors among conformations. On the other hand, using a water-probe radius of 1.0 A, as opposed to the standard value of 1.4 A, to generate the molecular surface, moderately improves both absolute and relative results.  相似文献   
16.
LetQ be any rectangle and letK d (d 2) be a continuum which is either symmetric across a hyperplane or symmetric through a pointz K. We show thatK contains the vertices of a rectangle similar toQ which exhibits the same symmetry as doesK.  相似文献   
17.
Hayes ET  Galal A  Mark HB 《Talanta》1995,42(6):873-877
The development of a sensor for the direct and selective determination of atmospheric ethanol is in the initial stages. The sensor takes advantage of the selective chemical reaction between ethanol and vanadium oxinate. This reaction occurs in an organic medium where a red colored complex is the product. This reaction is determined spectrophotometrically where the absorbance maxima is 475 nm. The focus of this paper is to discuss the parameters necessary to develop a solid sorbent sampling-sensor which can be used to determine atmospheric ethanol.  相似文献   
18.
19.
In the Higgs phase we may be left with a residual finite symmetry groupH of the condensate. The topological interactions between the magnetic and electric excitations in these so-called discreteH gauge theories are completely described by the Hopf algebra or quantum groupD(H). In 2+1 dimensional space time we may add a Chern-Simons term to such a model. This deforms the underlying Hopf algebraD(H) into the quasi-Hopf algebraD (H) by means of a 3-cocycle onH. Consequently, the finite number of physically inequivalent discreteH gauge theories obtained in this way are labelled by the elements of the cohomology groupH 3(H,U(1)). We briefly review the above results in these notes. Special attention is given to the Coulomb screening mechanism operational in the Higgs phase. This mechanism screens the Coulomb interactions, but not the Aharonov-Bohm interactions.Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 98. No. 3, pp. 509–523, March, 1994  相似文献   
20.
We formulate a series of conjectures (and a few theorems) on the quotient of the polynomial ring in two sets of variables by the ideal generated by all S n invariant polynomials without constant term. The theory of the corresponding ring in a single set of variables X = {x 1, ..., x n} is classical. Introducing the second set of variables leads to a ring about which little is yet understood, but for which there is strong evidence of deep connections with many fundamental results of enumerative combinatorics, as well as with algebraic geometry and Lie theory.  相似文献   
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