FLIP unit

Since one unit of 59 contains one molecule of 58 and two molecules of 2a, at least two of these units should take part in the photodimerization of 58. In other words, four molecules of 2a are concerned in each photodimerization of 58. It is not clear at present whether this has something to do with the relatively high enantioselectivity and efficiency of the photodimerization or not. In future, an X-ray crystal structural study of 59 is required to clarify this question. As in the case of 37, 58 exists as an equilibrium mixture of two flipping optical conformers. We may also learn from an X-ray analysis which optical conformer is included in 59. 

In the example discussed above, the transition X- X sta s simply for a single spin flip at a randomly chosen lattice site, and W(X - ) = 1 if 5< 0 while W(X- X ) = Q — 3 lkgT) for >0 should be interpreted as transition probability per unit time. Note that other choices for W would also be possible provided they satisfy the principle of detailed balance ... 

Benzylic compounds have—compared to the corresponding methyl derivatives—a higher thermodynamic acidity by 10 to 15 pATa units . Mesomeric stabilization requires a considerable flattening of the carbanionic centre towards sp hybridization (the sum of bond angles is 360° for sp and 328° for sp ). However, we should be aware that even if the carbanionic framework would be completely planar, the ion pair 209 is a planar-chiral species. For epimerization, the cation has to migrate from one face to the other one (equation 48). Due to a more facile flipping of the carbanionic centre and an easier formation of solvent-separated ion pairs, most of chiral benzyUithium compounds 208/ewi-208 racemize with great ease. 

The traditional approach for structure solution follows a close analogy to the analysis of single-crystal XRD data, in that the intensities 1(H) of individual reflections are extracted directly from the powder XRD pattern and are then used in the types of structure solution calculation (e.g. direct methods, Patterson methods or the recently developed charge-flipping methodology [32-34]) that are used for single-crystal XRD data. As discussed above, however, peak overlap in the powder XRD pattern can limit the reliability of the extracted intensities, and uncertainties in the intensities can lead to difficulties in subsequent attempts to solve the structure. As noted above, such problems may be particularly severe in cases of large unit cells and low symmetry, as encountered for most molecular solids. In spite of these intrinsic difficulties, however, there have been several reported successes in the application of traditional techniques for structure solution of molecular solids from powder XRD data. 

The most common particles that you use with the mole are atoms and molecules. When encountering a problem that deals with a specific unit, like molecules, you just replace particles with the correct unit, as in the following example. Like all conversion factors, you can invert this fraction to move in the other direction, from moles to particles. (Flip to Chapter 2 for an introduction to conversion factors.)... 

Let the symbol N-f represent the number per unit area of the flipped dipoles with the hydrogen end toward the solution, and N be the number per unit area of the flop-down position. The subtraction of these numbers should give the net number of dipoles oriented in the up (T) direction, i.e., N = N Nj. Thus, Eq. (6.149) can be written as... 

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