Atom selectivity

In order to allow any multiple chlorination of the biphenyl skeleton, the user may define an atom list (eonsisting of hydrogen and chlorine atoms) and substitute all H-atoms by this list. One may click on the drop-down selection box behind the element icons, select the options Generics. .set the user-defined atom to A1 and quit by the OK button. As a result this atom selection is active for the subsequent drawing steps. After this atom list is drawn ten times as the ten substituents, its composition has to be defined by clicking the A, icon on the left-hand side of the structure editor and by selecting H and Cl in the periodic table (Figure 5-16). 

Name any one-atom, two-atom, three-atom, or four-atom selection with the Select/Select command. 

B. Angle This measures the angle around a central atom. Select ammonia from the molecules on screen, and then select Angle. Click first on H, then on N, then on another H, or on two NH bonds. The angle (in degrees) will be displayed at the bottom of the screen. Repeat the process as necessary, and click on Done when finished. 

C. Dihedral This measures the angle formed by two intersecting planes, the first plane containing the first three atoms and the second plane containing the last three atoms. Select hydrogen peroxide from the molecules on screen, and then select Dihedral. Click... 

Supported bimetallic Re—Pt catalysts are important in selective reforming of petroleum. It is believed that sulhding the catalyst before use gives ReS units which act as inert diluents to reduce the size of a local ensemble of platinum atoms. Selectivity for desirable dehydrocyclization and isomerization reactions... 

The low Ti content (up to 3 wt % in Ti02) makes the extraction of vibrational, energetic, and geometric features specific to Ti04 moieties a difficult task as the experimental data are dominated by the features of the siliceous matrix. This is the reason why the structure of the local environment around Ti(IV) species inside TS-1 was only definitively assessed more than 10 years after the discovery of the material, when the atomic selectivity of X-ray absorption spectroscopies (both XANES and EXAFS) were used [58-60]. 

However, one category of selectivity is largely ignored by organic chemists the atom selectivity , or what is variously called atom economy (Trost, 1991, 1995), atom efficiency, or atom utilization solvents (Sheldon, 1992, 1992a, 1993, 1994, 1996, 1997, 1997a). The complete disregard of this parameter is the root cause of the waste problem in fine chemicals manufacture. 

Again as in the s shells, pAj and pAu have different values depending on the atom selected. Thus to calculate the various ionization cross sections, the contributions from the 2p and 3p spectra depend on the values of M/ and ,4//, which are given by... 

Figure 3. Concept for the ultralow-level counter. Low-level counting facilities could be improved by using pulsed lasers to ionize daughter atoms selectively (when neutral) by using a RIS scheme. Shown here is the electronics logic that could be used to further reduce background for more details, see Ref. 8.

Design acyclic compounds containing groups of C, H and O atoms (select all molecule types with C, H 0 atoms)... 

A third group of interatomic distances (and coordination) is that corresponding to the Cu coordination around Cu atoms. Selecting among the Cu atoms (all equivalent to each other) the atom Cu in A A 0 as the reference atom, the next... 

Fig. 10. Molecular and electronic structure of cation 74. (a) Perspective view on the cation 74. (b) Coordination sphere of the Pb atom, selected interatomic distances (pm) and angles (deg) Pb-(center CC, C7), 281.7 Pb-(center C6 CT), 280.7 Pbl-C2, 227(2) Pbl-C2 221.1(15) Pbl-C8, 231.0(19) C6-C7, 133(2) CC-CT 132(2) (center C6, C7)-Pb-(center C6, C7 163.6. (c) Contour plots of the Laplacian distribution [V p(r)] in the plane containing the atoms Pb, C6 and C7. Solid and dotted lines designate regions of local charge concentration and depletion, respectively. The bond paths are indicated by the solid back lines, bond critical points are marked with a black square. (Reprinted with permission from Ref. 53. Copyright 2003, Wiely-VCH.)...

Where the target analyte contains heteroatoms such as nitrogen, phosphorus and sulfur, atom-selective detectors can provide an ideal detection method. A number of examples appear in the literature of the use of a detector called a thermal energy analyser (TEA) for the measurement of A-nitroso compounds [14-17] and aromatic nitro compounds [18]. This has also been used as an HPLC detector [19, 20], and a modified TEA has been reported to be useful for analysis of amines and other nitrogen-containing compounds [17]. Unfortunately, this technique appears not to have gained in popularity, since no reports have appeared in the literature for over two decades. 

Let us conclude this section with the intriguing observation of the absence of the a (7t(ag, b f)) contribution in the Nls NEXAFS spectra of both neutral TCNQ and TTF-TCNQ. Let us recall the absence of precisely the Og and b u contributions in the CI5 spectra of TTF discussed above. However, the Cls NEXAFS spectrum of TCNQ shows some intensity in the a (7r(ag, b f)) region [between n a , bif) and TT (jr(b3g, af>) + a n b g, 2u))]- This is due to the signihcant 6 -contribution from carbon for neutral TCNQ while nitrogen contributes negligibly. Thus, it seems that in addition to the intra-atomic selection rules there are additional restrictions apparently symmetry-related in MOMs like those discussed here. This unexplained phenomenon certainly calls for both theoretical and experimental future work. 

In the synthesis of fluorinated prostanoids, there are still limited methods to introduce fluorine atoms selectively to the target molecule. It is hoped that the development of new fluorination reactions will contribute to the progress on the prostanoid research, and possibly discovery of novel drugs addressing unmet medical needs in future. 

Nucleophilic displacement of Z-, 4-, and 6-halo substituents by alkoxy or aryloxy ions occurs readily except in the presence of strongly electron-releasing substituents in the ring <1994HC(52)1>. Stepwise reaction can be achieved with di- and trihalo-pyrimidines, with the more reactive 4-position being the first site of reaction. For example, even with the presence of a bulky ortho substituent such as a 5-bromine atom, selective methanolysis at the 4-position was still observed with 5-bromo-2,4-dichloropyrimidine 179 <2006TL4415>. 

Direct bromination of toluene and ethylbenzene form the corresponding benzyl bromides in high yield. The observed selectivity in SC-CO2 is similar to that observed in conventional organic solvents. Also, SC-CO2 is an effective alternative to carbon tetrachloride for use in the classical Ziegler bromination with N-bromosuccinimide. Reaction yields are high, side products are minimized, and bromine-atom selectivities are observed. Thus, SC-CO2 must be useful as a viable, environmentally benign substitute for many of the solvents typically used for free-radical reactions (Tanko and Blackert, 1994). 

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