Double cross experiment

Thus, although the sticky olefin hypothesis requires, as considered in Section II,D, an unacceptable account for the bonding in the four-membered ring intermediate, it is experimentally not easily distinguishable from the carbene-chain mechanism. However, the distribution of products formed in the double cross experiments should distinguish the two, as shown below, and so should the distribution of products in another version of the experiment that is essentially the reverse of the double cross. 

This analysis suggests that in the double cross experiment the product of [Ci4]/[Ci2]—called —and [Ci4]/[Cig]—called rz—should be 4, and Fig. 1 shows that experimentally this is approximately true. A more accurate measure of this product at zero time can be achieved by multiplying ri and rj at the different times and extrapolating to zero time, as in Fig. 3. In a series of four such experiments, three using cyclooc-tene, c/s-2-butene, and cw-4-octene, and one using cyclooctene, trans-2-butene, and tra j-4-octene, the average value of ri x /-z at zero time was measured as 4.05 0.05 (66). 

Consider now what the product r, x /-z would be if the conventional mechanism were correct, but the olefin displacement reaction were rate determining. The kinetics of a mechanistic scheme much like that in Scheme 4 shows (66) that ri x at zero time is a function of two ratios, that of the concentrations of butene and octene and that of their reactivities. However, for all reasonable values of these ratios, rj x rz is never greater than 2.94. Accordingly, since in the double cross experiment the product was determined to be about 4 at zero time, the conventional mechanism is excluded no matter which step is ratedetermining. The implication is that the carbene chain mechanism is correct. 

Dipolar recoupling may also be accomplished using continuous rf irradiation as demonstrated in the heteronuclear and homonuclear case by the CP at MAS conditions (or for low-y heteronuclear spins called double-cross-polarization, DCP [103]) and HORROR (homonuclear rotary resonance) [26] experiments, respectively. These experiments may easily be described by transforming the description into the interaction frame of the rf irradiation using (14a) exploiting... 

The cross-polarization (CP), i.e. the transfer of I-spin polarization to the dilute spins (S), is a double resonance experiment in which the I and S spins are coupled by a certain interaction, determined by the cross relaxation time tb. From the dynamics of the CP process, usually described with the spin temperature concept, the following equation for the time dependence of S-spin polarization could be derived ... 

FIGURE 4.8 Experiments and numerical modeling of sample introduction using the double cross-injection system during (a) loading steps and (b) dispensing steps. Reprinted with permission from the American Chemical Society [109]. 

First, Katz13 conducted an experiment similar to that of Herisson and Chauvin (equation 11.8), which he termed the double cross metathesis. If Mechanism 2 were operative, the product ratios [8]/[7] and [8]/[9] should be zero when concentrations were extrapolated back to the very beginning of the reaction (t0), because 8—the double cross product would have to form after the symmetrical products 7 and 9. 

Usually, creep deformation of ice single crystals is associated to a steady-state creep regime, with a stress exponent equal to 2 when basal glide is activated . In the torsion experiments performed, the steady-state creep was not reached, but one would expect it to be achieved for larger strain when the immobilisation of the basal dislocations in the pile-ups is balanced by the dislocation multiplication induced by the double cross-slip mechanism. 

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