Homonuclear DRAWS

We note that DRAMA has also formed the basis for refined variants such as the chemical-shift-compensated MELODRAMA [85], windowless DRAWS [29], and the DRAMA-XY4 [86] experiments, finding important applications for example for measurement of 13C-13C distances involving carbonyl spins. We should also note the existence of many other important pulsed homonuclear recoupling experiments such as the widely used Radio Frequency Driven Dipolar Recoupling (RFDR) [24] and Back-to-Back (BaBa) [87] experiments. 

Fig. 11.6 Pulse sequence for two-dimensional homonuclear chemical-shift correlation experiments. The gray box indicates the mixing sequence, with some examples shown in more detail a RFDR, b RIL, c C7, d DRAWS, e DREAM.

There are substantial numbers of small molecular species that are important species at high temperatures that are not normally discussed in standard inorganic courses. For example, the vapor over solid NaCl contains diatomic NaCl molecules as well as NaCl dimers. Does the stoichiometry NaCl fit with the simple ideas of valence derived from molecules like SiCU Explain. Construct a qualitative MO diagram with orbital drawings for the NaCl diatomic molecule and compare it with that for a homonuclear, isoelectronic diatomic molecule. Does this help in answering the first question ... 

Exercise 4.2. What hydrocarbon cluster is analogous to Co4(CO)i2 Write the main-group and metal isolobal fragments. Write out the compositions and draw the structures of all the mixed Co/C clusters possible between these two homonuclear Co and C clusters. 

Even though the one-electron H2+ Hamiltonian has been used to derive all 10 molecular orbitals (only one of which is even half-filled in H2+), a MO diagram built from the expanded basis will enable us to draw quite accurate boundary surfaces and relative energy separations for all the second row homonuclear diatomics. (This has been verified by the more rigorous approaches that are detailed later.) The figures sketched in Figure 9 show the MO diagrams that result from the 10 X 10 secular determinant. 

A schematic drawing of different sorts of triple bonds is shown in Figure 2.11, centrosymmetric linear molecules (C2H2) and homonuclear diatomics (02, N2) being given on the left, noncentrosymmetric linear molecules (HCN) and heteronuclear diatomics (NO, CO) on the right. 

For the excitation of DQC under MAS, the interference with the sample rotation must be considered, as was the case for the CRAMPS experiment discussed in the previous section. In particular, if a sequence designed for a static MQ experiment is used without modification, the excitation (and reconversion) time is limited to ttr/2, since the rotor modulation causes the action of the pulse sequence in the second half of the rotor period to be the time reversal of that which occurred in the first half of the rotor period. Starting with the suggestion of Meier and Earl,89,90 who simply proposed the phase switching of the static sequences used by Baum et al.,47,48 every half rotor period to prevent the process of self-time-reversal, many different approaches have been presented that allow excitation (and reconversion) times of one or more rotor periods. Such pulse sequences that counteract the effect of MAS are referred to as recoupling methods,1112 examples that have been used in homonuclear DQ MAS NMR spectroscopy include BABA,91 C7,92 DRAMA,93 DRAWS,94 and HORROR.95 We note that Levitt and co-workers have recently introduced a very helpful classification system, based on symmetry principles, which covers such sequences.96,97... 

A series of articles by Brandolini, Dybowski and co-workers [25-29] reported the use of NMR for static samples of PTFE to obtain information on orientation distribution of crystallites following deformation by tensile stress. The spectra were obtained using the MREV8 pulse sequence to remove homonuclear dipolar coupling so as to observe chemical shift dispersion as a function both of the draw ratio and of the angle between the draw direction and applied magnetic field (Fig. 18.5). The moments of the spectra were used [25-28] to determine the order parameters of the samples up to (Pg(cos 0)). The orientation distribution functions were then calculated... 

Figure 2 Dissociation potential vertically, and Z and Z2 from 2 to 36 horizontally, in almost isometric projection. Transition-metal and rare-earth molecules have been cut out, and the resulting pieces of the surface have been shd and joined to pieces at smaller Z. Homonuclear molecules are on the left-right diagonal, and the terrain is symmetrical with respect to a vertical plane through that diagonal. Any series of isoelectronic molecules has addresses going horizontally and normal to that plane. The scale is established by the value for N2 (9.79 eV), which lies between the peaks for CO and OC. The figure is constructed from stick graphs by drawing lines of least descent from each peak, by draping a surface from those lines down to the 0 eV valleys of rare-gas molecules. In addition, an attempt is made to indicate craters in the bottoms of which are found the alkaline-earth pairs.

Understand how MOs arise from AOs describe the shapes of MOs, and draw MO diagrams, with electron configurations and bond orders and explain properties of homonuclear diatomic species from Periods 1 and 2 ( 11.3) (SPs 11.3, 11.4) (EPs 11.25-11.36)... 

Figure 4.11 Electrostatic forces experienced by nuclei in a homonuclear molecule due to presence of an electron in region between the nuclei (top) and outside this region (middle). The electron-nuclear attraction draws the nuclear together In the former case, and pulls them apart in the latter case. The region in which the electron s presence tends to stabilize the molecule is shaded in the diagram at bottom.

Draw only the left-hand side of the correlation diagram for homonuclear diatomics corresponding to the = 5 united atom atomic orbitals, identifying the appropriate MO representations. 

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