Molecular-orbital calculations 1.2- shifts

Molecular orbital calculations have shown that in all these electron-deficient centres suprafacial [1, 2] shift occurs with retention of configuration through a two electron Huekel type transition state ... 

In addition to this type of empirical approach, there are several other approaches that are related more directly to specific properties of the organic, such as the C-H bond dissociation enthalpies (Heicklen, 1981 Jolly et a.L, 1985), ionization energy (Gaffney and Levine, 1979), or NMR shifts (Hodson, 1988). In addition, molecular orbital calculations (Klamt, 1993) and transition state theory (Cohen and Benson, 1987) have been applied. 

Electron centre, C033 (C3 symmetry) A pyramidal AB3-type molecular ion with 25 electrons is stabilized by a trivalent impurity such as Y3+ at room temperature and forms a complex of Y3+ - C033-. The shift of -factors from the free electron value ge = 2.0023 and the hyperfine (hfs) constant, A can be estimated by molecular orbital calculation of AB325 -type molecule ... 

Tennakone et al.%S) used triphenylmethane type (metallochromic) organic dye (Dye 21, 22), both of which show a large bathochromic shift on complexing with metal ions. The molecular orbital calculation of these dyes in chelating condition with the Tilv ion revealed that the LUMOs of these dyes are localized on the TiIV ion, but the HOMOs are delocalized in the whole dyes. Such MO distribution similar to the LMCT transition in transition metal complexes should contribute to the vectorial electron transfer (high rj ) from the excited dye to Ti02. 

Fe4C(CO)l3, [HFe4C(CO),2] and [Fe4C(CO)12]2, which are very similar in geometry (see Table I), the carbide resonances are found at 469.8,464.2, and 478.0 ppm. The cause of these large shifts is not yet known, but some light will undoubtedly be shed on this phenomenon by molecular orbital calculations, magnetic suceptibility and temperature-dependent solid-state NMR studies. 

Figure 56)155. Compounds investigated included three forms of tetramesityldisilene the solvent-free form 69156, the toluene adduct 69-C7Hs157 and the tetrahydrofuran solvate 69 THF158. Also studied were a second tetraaryldisilene (70)159 dialkyldiaryl-substituted disilene, 71160, and the only tetraalkyldisilene known to be stable as a solid, 72161. Three silyl-substituted disilenes, 73, 74 and 75, were also investigated162,163. To assist in the interpretation of the experimental results, ab initio molecular orbital calculations of the 29Si chemical shift tensors were carried out for model disilene molecules. 

In so far as the decrease in chemical reactivity is an indication of diminished transition metal basicity, it was proposed143 that the thiocarbonyl complex is less basic than its carbonyl analogue. This conclusion is substantiated by the spectral shifts in Table 22 and is also in agreement with molecular orbital calculations which predict the thiocarbonyl complex to be less basic than the carbonyl complex155,156. 

The carbon chemical shifts for steroids are the most readily available data from a routine 13C NMR determination. Since they reflect the electronic and steric environments of the various carbon nuclei, they provide sensitive insights to the configurational and conformational features of such molecules. While much interesting work on ab initio molecular orbital calculations of carbon chemical shifts is now appearing, it is probably true that the difficulties of carrying out such calculations on large molecules will prevent their applications to steroids for some time. We are limited, therefore, to a more empirical approach to steroid carbon chemical shifts. (3, 38)... 

Its absorption spectrum shows one band at 320 nm (e = 2900 M 1cm 1), assigned to the cti - ct2 transition localized in the Au-Tl moiety. The emission spectrum in the solid state at 77 K shows a band at 602 nm, which is attributable to a transition between orbitals that appear as a result of the metal-metal interaction. In this sense, Fenske-Hall molecular orbital calculations indicate that the ground state is the result of the mixing of the empty 6s and 6pz orbitals of gold(I) with the filled 6,v and the empty 6pz orbitals of thallium(I). In frozen solution, this derivative shows a shift of the emission to 536 nm, which has been explained by a higher aggregation of [AuT1(MTP)2] units in the solid state if compared to the situation in solution. 

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