Tetrahedral inversion

Although this mechanistic picture developed from experiments involving optically active alkyl halides chemists speak even of methyl halides as undergoing nucleophilic substitution with inversion By this they mean that tetrahedral inversion of the bonds to carbon occurs as the reactant proceeds to the product... 

When the relative amount of large cations (Fe3 +, Fe2+, Mg) in the octahedral sheet is increased (80 to 100% of the octahedral cations), the layer strain is slightly reduced and the octahedral sheet can extend to 8 or 9 positions before forcing the tetrahedral inversion (sepiolite). It seems likely, that the octahedral sheets of attapulgite and sepiolite have a much wider compositional range than indicated by the present limited analyses and that they occupy the intermediate position between the diocta-hedral and trioctahedral clays. 

Figure 9 also plots the orbital correlations as the change progresses 123,124). It can be seen that for d complexes, not only must a spin be reversed, but an electron must move from one orbital to another. Thus, the transition is forbidden on the grounds of both spin and orbital symmetry. The ground state of the tetrahedron correlates with an excited state of the square plane 125). This in turn means that square-planar isomerizations and tetrahedral inversions, each of which depend on the operation of two of these steps, are not thermally allowed processes. (They are, however, photochemically allowed.)... 

Banfield JF, Bailey SW (1996) Evidenee for formation of regularly interstratified serpentine-ehlorite minerals by tetrahedral inversion in long-period serpentine polytypes. Am Mineral 81 79-91 Banfield JF, Barker WW (1998) Low-temperature alteration in tuffs from Yucea Mountain, Nevada. Clays Clay Minerals 46 27-37... 

ETHANE METHANE TETRAHEDRAL INVERSION, PLANAR ARRANGEMENTS... 

Ammonia is a two-state system [16], in which the two base states lie at a minimum energy. They are connected by the inversion reaction with a small baiiier. The process proceeds upon the spin re-pairing of four electrons (Fig. 15) and has a very low barrier. The system is analogous to the tetrahedral carbon one... 

Stereochemical inversion of the tetrahedral arrangement of bonds to the carbon at which substitution occurs... 

Antineoplastic Drugs. Cyclophosphamide (193) produces antineoplastic effects (see Chemotherapeutics, anticancer) via biochemical conversion to a highly reactive phosphoramide mustard (194) it is chiral owing to the tetrahedral phosphoms atom. The therapeutic index of the (3)-(-)-cyclophosphamide [50-18-0] (193) is twice that of the (+)-enantiomer due to increased antitumor activity the enantiomers are equally toxic (139). The effectiveness of the DNA intercalator dmgs adriamycin [57-22-7] (195) and daunomycin [20830-81-3] (196) is affected by changes in stereochemistry within the aglycon portions of these compounds. Inversion of the carbohydrate C-1 stereocenter provides compounds without activity. The carbohydrate C-4 epimer of adriamycin, epimbicin [56420-45-2] is as potent as its parent molecule, but is significandy less toxic (139). 

Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (o) instead they adopt the inverse spinel structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B)t[AB]o04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects. ... 

Although Fc304 is an inverse spinel it will be recalled that Mn304 (pp. 1048-9) is normal. This contrast can be explained on the basis of crystal field stabilization. Manganese(II) and Fe" are both d ions and, when high-spin, have zero CFSE whether octahedral or tetrahedral. On the other hand, Mn" is a d and Fe" a d ion, both of which have greater CFSEs in the octahedral rather than the tetrahedral case. The preference of Mn" for the octahedral sites therefore favours the spinel structure, whereas the preference of Fe" for these octahedral sites favours the inverse structure. 

For d ions in tetrahedral fields the splitting of the free-ion ground term is the inverse of its splitting in an octahedral field, so that ig(F) lies lowest. In this ca.se three relatively intense bands are to be expected, arising from the transitions ... 

One consequence of tetrahedral geometry is that an amine with three different substituents on nitrogen is chiral, as we saw in Section 9.12. Unlike chiral carbon compounds, however, chiral amines can t usually be resolved because the two enantiomeric forms rapidly interconvert by a pyramidal inversion, much as an alkyl halide inverts in an Sfg2 reaction. Pyramidal inversion occurs by a momentary rehybridization of the nitrogen atom to planar, sp2 geometry, followed by rehybridization of the planar intermediate to tetrahedral, 5p3 geometry... 

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