Tetrahedral jump mechanism

For some transition-metal hydrides of the type HMP4, the ground-state structure is a distorted trigonal bipyramid and can be pictured as a tetrahedral arrangement of the P ligands around M with the H atom on one tetrahedral face. For such hydrides, a tetrahedral jump mechanism has been proposed in which the H atom moves from one tetrahedral face to an edge and then to another face, as shown in Scheme 4.5. 

Fig. 11a and b. Decay of the alignment echo height as a function of the mixing time x2 for different motional mechanisms, a Tetrahedral jumps as a model for conformational changes b Diffusive motion, the solid lines correspond to unrestricted rotational diffusion, the dashed lines to diffusion restricted to an angular region of 8°. Note the strong dependence of the decay curves on the evolution time t, in case of diffusive motion... 

Fig. 3.2.5 [Miill] Deuteron wideline spectra for different motional mechanisms and times tc-The angle between the axis of rotation and C—bond (principal axis Z of the quadfupole coupling tensor) is denoted by d. (a) Twofold jump with 9 — 60° (b) Twofold jump with 9 = 180° (flips of p phenylene). (c) Three-fold jump with 9 = 109° (rotation of a methyl group), (d) Rotational diffusion on a cone 9 = 109°. (e) Tetrahedral jump, (f) Isotropic rotational diffusion.

Due to the tetrahedral arrangement of water its rotational freedom is limited. Also such bound hydrogen and free hydrogen can exchange their positions by the well-known large-amplitude rotational jump mechanism with a time interval of 100 fs [13]. In bulk water the dipole, OH, and HH vector relaxation timescales are 2.05, 2.3, and 2 ps, respectively. But in confined water such relaxation times... 

Cobalt.—Fluxional, Rotational, and Conformational Molecules. A modification of the tetrahedral tunnelling or jump mechanism mentioned above arises from the fluxional behaviour of the series [XCo P(OR)3 4] X = (non-rigid... 

The rate constants and k represent rate constants for a surface reaction and have units m mol s and s respectively. The accelerative effects are about 10 -10 fold. They indicate that both reactants are bound at the surface layer of the micelle (surfactant-water interface) and the enhanced rates are caused by enhanced reactant concentration here and there are no other significant effects. Similar behavior is observed in an inverse micelle, where the water phase is now dispersed as micro-droplets in the organic phase. With this arrangement, it is possible to study anion interchange in the tetrahedral complexes C0CI4 or CoCl2(SCN)2 by temperature-jump. A dissociative mechanism is favored, but the interpretation is complicated by uncertainty in the nature of the species present in the water-surfactant boundary, a general problem in this medium. 

The jump vector. A, wUl obviously depend on the mechanism and the structure. For example, an atom diffusing through the octahedral interstitial sublattice in an FCC metal, with lattice spacing a (Fig. 6.6), must jump the distance between interstitial sites, A = fl/V2. This is, of course, the same distance an atom diffusing by the vacancy mechanism must jump. It will be recalled that for every atom in a close-packed stmcture, there are two tetrahedral interstitial sites and one octahedral interstitial site. The reader might ask if the distances between the tetrahedral sites ate the same. 

The dynamics of substitution on tetrahedral chromium(VI) has been studied by temperature-jump and rapid mixing59 60). The mechanistic interpretation of these results has to account for the acid catalysis. The most satisfactory mechanism involves the rapid protonation of HCrOj, followed by rate-determining water elimination and attachment of the incoming ligand on the threefold coordinated activated complex611 Therefore, the substrate is, at best, weakly coordinated to the metal center in the transition state variations in the observed rate constants are due to ratios of rate constants for the elementary steps in this mechanism. 

Another example of an interesting H jump diffusion mechanism has been reported for hydrogen dissolved in the cubic A15-type compound Nb3Al [87]. In this compound H atoms occupy the tetrahedral 6d sites coordinated by four Nb atoms. The 6d sites form three sets of nonintersecting chains in the <100>, <010> and <001> directions. The distance between the nearest-neighbor d sites in the... 

In a perfect tetrahedral geometry, the length of the proton jump is equal 2rQH sin (10472) = 1.6 A, because the rotation is around the oxygen atom, the distance rOH is 1 A and the angle HOH is around 104°. So, this jump is, more exactly, a hindered rotation of the whole molecule, where the moving H atom remains attached to the same molecule. This is what I call a molecular diffusion mechanism due to rotational jumps. Within this picture, the permanent dipole oscillates with librational motions and has a different orientation at each jump, but the molecule remains neutral. 

Thus, if after jumping from a normal site into an octahedral site, the atom generally makes its next jump into a tetrahedral site, 0 / 0o/i migration will be an interstitial diffusion mechanism, and the atom may travel quite a distance before jumping into another normal site again. On the other hand if 0, rate-limiting step is simply the jump from the normal to the o site, and... 

C. H. Greene State University of New York, Ceramic College) Does not the actual diffusion mechanism involve some cooperative phenomena Before a cation can jump from one normal site to a second normal site the cation occupying this second site must be removed in some way. It is likely that the rate at which cations jump out of such destination sites is increased by the presence of jumping cations at intermediate non-normal tetrahedral or octahedral sites. 

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