Structure theoretical calculations

It is well accepted that the high diastereospecificify of aldehyde allylboration reactions is a consequence of the compact cyclic transition structure. Theoretical calculations have shown that the chairlike transition structure shown in Scheme 1 and Fig. 1 is the lowest in energy relative to other possibilities such as the twist-boat conformation. With boronate reagents, it has also been suggested that a weak hydrogen bond between the axial boronate oxygen and the hydrogen of the polarized formyl unit contribntes to the preference for the transition structme with the aldehyde substituent in the psendo-eqnatorial position. ... 

However the sample is prepared, we measure 13C spectra of one or more adsorbates on the catalyst, and then need to interpret the spectra to deduce the structure of adsorption complexes or reactive intermediates formed on the catalyst. In many cases the complexes and intermediates formed are unusual and exotic species for which the interpretation of the spectra may be far less than routine. This is where ab initio chemical shift calculations are essential. In diffraction methods, such as x-ray or neutron diffraction, one can more-or-less easily invert the experimental data to yield molecular structure. There is no straightforward relationship between chemical shift data and structure theoretical calculations provide the bridge between experiment and theory. In a typical study, we model the adsorbates on clusters that represent catalyst active sites, using experience and chemical intuition to create our initial structures. 

Fe—Ni Alloys-. Samples of Fe and Ni in experiments can have nonmagnetic or magnetic soft ferro magnets. For the alloy compound of iron and nickel, they have a much lower saturation magnetization compared to pure samples of each metal. For example, when we have 37% of Ni, it has a low curie point and an FCC structure. Theoretical calculations estimate a more complicated magnetic structure for these types of alloys due to the different combinations. 

The scarcity of Ir(I) dinuclear complexes able to activate H2 has been attributed to the rigidity of such complexes, which prevents the distortion of the square-planar geometry and, consequently, the appearance of the empty metal orbital required for the coordination of the dihydrogen molecule [81-83]. In the particular case of open-book structures, theoretical calculations support a weak metal-metal interaction... 

Nuclear magnetic resonance (NMR) spectroscopic studies ( H, and P), are consistent with the dipolar ylide structure and suggest only a minor contribution fi om the ylene structure. Theoretical calculations support this view, also. ... 

Figure Bl.4.9. Top rotation-tunnelling hyperfine structure in one of the flipping inodes of (020)3 near 3 THz. The small splittings seen in the Q-branch transitions are induced by the bound-free hydrogen atom tiiimelling by the water monomers. Bottom the low-frequency torsional mode structure of the water duner spectrum, includmg a detailed comparison of theoretical calculations of the dynamics with those observed experimentally [ ]. The symbols next to the arrows depict the parallel (A k= 0) versus perpendicular (A = 1) nature of the selection rules in the pseudorotation manifold.

According to early theoretical calculations Kloptnan and I carried out in 1971, the parent molecular ions of alkanes, such as CH4, observed in mass spectrometry, also prefer a planar hypercarbon structure. 

Whereas the proton (H ) can be considered the ultimate Bronsted acid (having no electron), the helium dication (He ) is an even stronger, doubly electron-deficient eleetron aceeptor. In a theoretical, calculational study we found that the helionitronium trication (NOaHe" ) has a minimum structure isoelectronic and isostructural... 

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

IP, isolated pure MI, matrix isolated GP, data from pure gas phase material CE, chemical evidence for existence TH, theoretical calculation XR, X-ray structure MW, microwave structure UV ultraviolet spectrum. 

S-Alkylthiiranium salts, e.g. (46), may be desulfurized by fluoride, chloride, bromide or iodide ions (Scheme 62) (78CC630). With chloride and bromide ion considerable dealkylation of (46) occurs. In salts less hindered than (46) nucleophilic attack on a ring carbon atom is common. When (46) is treated with bromide ion, only an 18% yield of alkene is obtained (compared to 100% with iodide ion), but the yield is quantitative if the methanesulfenyl bromide is removed by reaction with cyclohexene. Iodide ion has been used most generally. Sulfuranes may be intermediates, although in only one case was NMR evidence observed. Theoretical calculations favor a sulfurane structure (e.g. 17) in the gas phase, but polar solvents are likely to favor the thiiranium salt structure. 

The structure refinement program for disordered carbons, which was recently developed by Shi et al [14,15] is ideally suited to studies of the powder diffraction patterns of graphitic carbons. By performing a least squares fit between the measured diffraction pattern and a theoretical calculation, parameters of the model structure are optimized. For graphitic carbon, the structure is well described by the two-layer model which was carefully described in section 2.1.3. 

In this paper, we review progress in the experimental detection and theoretical modeling of the normal modes of vibration of carbon nanotubes. Insofar as the theoretical calculations are concerned, a carbon nanotube is assumed to be an infinitely long cylinder with a mono-layer of hexagonally ordered carbon atoms in the tube wall. A carbon nanotube is, therefore, a one-dimensional system in which the cyclic boundary condition around the tube wall, as well as the periodic structure along the tube axis, determine the degeneracies and symmetry classes of the one-dimensional vibrational branches [1-3] and the electronic energy bands[4-12]. 

Theoretical results of similar quality have been obtained for thermodynamics and the structure of adsorbed fluid in matrices with m = M = 8, see Figs. 8 and 9, respectively. However, at a high matrix density = 0.273) we observe that the fluid structure, in spite of qualitatively similar behavior to simulations, is described inaccurately (Fig. 10(a)). On the other hand, the fluid-matrix correlations from the theory agree better with simulations in the case m = M = S (Fig. 10(b)). Very similar conclusions have been obtained in the case of matrices made of 16 hard sphere beads. As an example, we present the distribution functions from the theory and simulation in Fig. 11. It is worth mentioning that the fluid density obtained via GCMC simulations has been used as an input for all theoretical calculations. 

X-Ray structural data and recent high level theoretical calculations confirm that this neutral, diamagnetic dithiadiazole is an aromatic six k-electron ring system. The gas-phase infra-red and photoelectron spectra of S2N2CO have also been reported. ... 

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