Chemical multicenter

Numerous positive delayed skin tests in patients with contrast medium-induced non-immediate skin reactions have been reported when the patients were tested with the culprit contrast medium [summarized in 1]. In a large European multicenter study, 37% of patients with non-immediate reactions were positive in delayed IDEs and/or patch tests [13]. The majority of the patients also reacted to the culprit contrast medium and also to other, structurally similar RCM. Notably, in more than 30% of those skin test-positive patients a RCM had been administered for the first time. Thus, there is a lack of a sensitization phase. Again it may be hypothesized that these previously non-exposed patients may have already been sensitized. Different patterns of RCM cross-reactivity indicate that several chemical entities could be involved. No positive skin tests have been obtained with other contrast medium excipients, such as ethylenediaminetetraacetic acid (EDTA), and only rarely patients have been found to react to inorganic iodide. 

Relationship Between Personal, Outdoor and Indoor Air Concentrations (RIOPA) The overall goal of the national multicenter (Elizabeth, NJ, Houston, TX, and Los Angeles County, CA) RIOPA study is to establish a scientific foundation for effective, timely, public health intervention strategies. Outdoor, indoor, and personal exposures of adults and children to PM are measured and evaluated by mass, elemental, chemical, and source apportionment analyses in the other research programs. Non-smoking asthmatic and non-asthmatic adults and their children are included. Monitoring occurs... 

One of the advantages of the molecular (one center) and multicenter multipole expansions of the MEP is that, through truncating the series after some terms, one can get an analytical expression. The molecular multipole expansion, in contrast with the multicenter multipole expansion, diverges at distances of chemical interest. The various multicenter multipole expansion [87, 89, 93-96] practically equivalent to each other [97]. 

It may be seen from this superficial summary that there is a wide variety of potentially active single or multicenter sites on oxide catalysts. There will be sites that are distinct with respect to their chemical nature, and for chemically equivalent sites there is a certain energy distribution. 

Iverson, G., Kharakats, Y. I., Kuznetsov, A.M. and Ulstrup, J., (1999) Fluctuation and coherence in long-range and multicenter electron transfer, in Jortner, J. and Bixon M. (eds.) Advances in Chemical Physics. Part 1, John Wiley Sons. NY., pp. 453-514. 

Saturated Hydrocarbons. Dehydrogenation is the reaction of hydrocarbons at many metal surfaces. However, for this reaction to proceed under mild conditions the hydrocarbon must be bound to the surface for a reasonable period of time—the hydrocarbon must be coordinated to the surface. The most chemically reasonable bonding mode is through a multicenter C-H-M interaction. Methane neither reacts nor chemisorbs cn clean metal surfaces at 20°C. In principle, methane could bond through the aegis of one, two, or three such multicenter bonds on a close packed clean metal surface... 

A valid alternative for systems of delocalized electrons is to represent the considered molecules by their canonical forms according to VB-theory. However, this approach does not reflect chemical reality and it is certainly not adequate for molecules with multicenter bonds and many organometallic systems. 

This index, an invariant in the tensor sense <1984ZNA1259>, gives the electronic population along the AB bond and gives values agreeing with chemical expectation. The multicenter bond index <1990STG423>, an extension of /ab to multicenter bonds, involves the total electron population (despite admitting ct-rt-separation). 

An interesting comparison between several aromaticity indices (harmonic oscillator model, nucleus-independent chemical shift, para-delocalization index, aromatic fluctuation index, multicenter indices, atoms-in-molecules theoretical indices and graph-theoretical indices) concluded that the most reliable ones are based on electron delocalization (08JCC1543). 

What functions shall we use for the single-electron MOs We could try the exact MOs for shown in Fignre 6.2. However, these exact MOs are not described by simple eqnations, and they are inconvenient for applications. Therefore, we introdnce the LCAO method to construct approximate MOs directly from the Hartree AOs for the atoms in the molecule, guided by molecular symmetry and chemical intnition. The essential new feature compared with the atomic case is that the (multicenter) approximate MOs are spread around all the nuclei in the molecnle, so the electron density is de-localized over the entire molecule. The approximate MOs therefore differ considerably from the (single-center) AOs nsed in Section 5.2. Constructing the approximate MOs and nsing them in qnalitative descriptions of bonding are the core objectives of this section. 

Although the Mulliken population analysis is useful for the discussion of two-center bonding, it is not sufficient for the description of the three- or four-center bonds that may exist in metal clusters. In order to elucidate the multicenter bonds, we examine contour maps of the charge densities of Na4 and Mg4 as typical tetramers of AM and AE, respectively. We use the differential charge density, Ap defined by eq. (3)., to express the charge redistribution on the formation of chemical bonds. 

Chemically, soretolide (D 2916) is AT-(5-methyl 3-isoxazolyl)-2,6-diinethylbenzamide (64), a compound similar to carbamazepine in its activity profile (229). It was noted that the active hydroxymethyl metabolite, (64a), was formed preferentially in female rats however, it is uncertain whether this species-specificef-fect is noted in humans (247). It is currently undergoing a multicenter study in refractory partial epilepsy (229). 

Heering, H.A., Weiner, J.H., and Armstrong, F.A. (1997) Direct detection and measurement of electron relays in a multicentered enzyme voltammetry of electrode-surface films of E. coli fumarate reductase, an iron-sulfur flavoprotein. Journal of the American Chemical Society, 119,11628-11638. 

Electron-transfer (ET) reactions play a central role in all biological systems ranging from energy conversion processes (e.g., photosynthesis and respiration) to the wide diversity of chemical transformations catalyzed by different enzymes (1). In the former, cascades of electron transport take place in the cells where multicentered macromolecules are found, often residing in membranes. The active centers of these proteins often contain transition metal ions [e.g., iron, molybdenum, manganese, and copper ions] or cofactors as nicotinamide adenine dinucleotide (NAD) and flavins. The question of evolutionary selection of specific structural elements in proteins performing ET processes is still a topic of considerable interest and discussion. Moreover, one key question is whether such stmctural elements are simply of physical nature (e.g., separation distance between redox partners) or of chemical nature (i.e., providing ET pathways that may enhance or reduce reaction rates). 

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