Back-flip

The Cossee-Arlman mechanism as originally proposed has a weakness—the back-flip is required to explain isoselective placement since the two active (coordination) sites are assumed to be enantiotopic. However, the structure of the traditional Ziegler-Natta heterogeneous initiators is not sufficently understood to either support or reject the assumption of enantiotopic sites. Further, even if the sites are enantiotopic, there is no overwhelming reason why the polymer chain is more stable at one site than the other—which is the rationale for the back-flip. The mechanism of isoselectivity with various metallocene initiators is much better understood since these are initiators whose molecular structures are well-established [Busico and Cipullo, 2001 Busico et al., 1997, 1999 Cavallo et al., 1998 Ewen, 1999 Rappe et al., 2000 Resconi et al., 2000], Considerable advancements in understanding heterogeneous Ziegler-Natta initiators occur if one assumes that the active sites in these initiators mimic those in metallocene initiators. Two types of metallocene initiators offer possible models... 

C -symmetric initiators have a pair of diastereotopic (nonequivalent) sites one site is sterically crowded and enantioselective, and the other site is less crowded and nonselective. The propagating polymer chain always prefers the less crowded site, but monomer coordination and migratory insertion occur at the more crowded enantioselective site. The polymer chain then back-flips to the less crowded site. This model offers a rationale for the back-flip of the polymer chain—the polymer chain is less stable at the more crowded site. 

Lobsters are most active at night. During the day they typically hide in burrows or cavities in rock piles, which they enter by backing in. Lobsters can crawl in all directions, but when they are foraging they mostly proceed in a forward direction. Smaller lobsters can swim jerkily, by rapidly back-flipping their tail fan. 

Peel back flip over coat with Ti/Au laminate crystal... 

For either experiment we can consider that irradiated protons to flip back and forth between their two spin-states so rapidly that they no longer couple with other protons in the same molecule. An alternative rationale can be couched in terms of the decoupling field equalizing the populations of the two energy levels of the irradiated protons, which is qualitatively equivalent to saturating that resonance. (Although neither of these two models is strictly correct, they do at least provide a simple rationale for the N.M.D.R. experiment.)... 

Reaction Mechanisms In the first edition of this book, I introduced an innovative format for explaining reaction mechanisms in which the reaction steps are printed vertically, with the changes taking place in each step described next to the reaction arrow. This format allows a reader to see easily what is occurring at each step without having to flip back and forth between structures and text. Each successive edition has seen an increase in the number and quality of these vertical mechanisms, which are still as fresh and useful as ever. 

The compound above has two important resonance structures. Notice that we separate resonance structures with a straight, two-headed arrow, and we place brackets around the structures. The arrow and brackets indicate that they are resonance structures of one molecule. The molecule is not flipping back and forth between the different resonance structures. 

There is one more important feature to recognize. Let s go back to the example above with the chlorine. We said that the chair flip moves the chlorine from an equatorial position into an axial position. But what about the up/down terminology Let s see ... 

Let s now go back and review, because it is important that you understand the following points. When we are given a hexagon-style drawing, the drawing shows us which positions are up and which positions are down. No matter which chair we draw, up will always be up, and down wiU always be down. There are two chair conformations for this compound, and the molecule is flipping back and forth between these two conformations. With each flip, axial positions become equatorial positions and vice versa. Let s see an example. 

Below is a diagram that shows the key reactions in this chapter. If yon have completed aU of the problems in this chapter up to this point, you should be able to flU in the reagents necessary for every transformation shown. Try doing that now. If you have trouble remembering the reagents for a particular reaction, then just flip back to the appropriate section, and find the reagents ... 

It is essential to realize that electrons In the nitrate anion do not flip back and forth among the three bonds, as implied by separate structures. The true character of the anion is a blend of the three, In which all three nitrogen-oxygen bonds are equivalent. The need to show several equivalent structures for such species reflects the fact that Lewis structures are approximate representations. They reveal much about how electrons are distributed in a molecule or ion, but they are imperfect instruments that cannot describe the entire story of chemical bonding, hi Chapter 10, we show how to interpret these structures from a more detailed bonding perspective. 

Kuboniwa H, Grezesiek S, Delaglio F, Bax A. Measurement of Hn-H J couplings in calcium-free calmodulin using new 2D and 3D water-flip-back methods. J Biomol NMR 1994 4 871-878. 

You should attempt to solve a problem only after you have studied the appropriate sections in the textbook. If you try to circumvent this process by attempting to solve the problems without looking in the text, you will find yourself constantly flipping through the pages in the chapter to find the concepts you need to approach the problem. This search will, of course, be quite inefficient because you will not be familiar with the material in the chapter. Worse yet, you might simply look back for a sample question that is similar to the one you are working on. This latter technique does not help you learn how to problem solve it simply teaches you how to reproduce someone else s solution. 

Fig. 3. HNCA (a) and two implementations of HNCA-TROSY (b-c) experiments for recording intraresidual HN(/), 15N(/), 13C"(i) and sequential 1 HN(7), l5N(/), 13Ca(i — 1) correlations in 13C/15N/2H labelled proteins. Narrow and wide bars correspond to 90° and 180° flip angles, respectively, applied with phase x unless otherwise indicated. Half-ellipse denotes water selective 90° pulse to obtain water-flip-back.88,89 All 90°...

For rats and mice, the animal is either grasped by its tail and flipped in the air or held upside down and allowed to drop (2 ft above the cart surface) so that it turns head over heels. The normal animal should land squarely on its feet. If it lands on its side, score 1 point if on its back score 2 points. Repeat 4 times and record its total score. For a rabbit, when placed on its side on the cart, does the animal regain its feet without noticeable difficulty ... 

Again, let us emphasize that the actual structure of the nitrate ion is not any of the three shown. It is not flipping back and forth among the three. It is an average of all three. All the bonds are the same, and are intermediate between single bonds and double bonds in strength and length. 

Fig. 9.1 Basic HNN COSY pulse sequence. Narrow and wide pulses correspond to flip angles of 90° and 180°, respectively, whereas low-power (water flip-back) 90° l-l pulses are illustrated as smaller narrow pulses. Delays 8 = 2.25 ms 7=15 ms (can be shorter or longer) (a = 2.5 ms fb = 0.25 ms fc= 2.25 ms fd = 0.5 ms. Unless indicated, the phase of all pulses are applied along...

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