Isochronous hydrogens

Figure 2. Changes of hydrogen concentrations in U-ZrH 60, U-ZrH 90, ZrHi 60 and ZrH o after isochronal annealing at several temperatures for 10 min.

Here is the most important thing to remember. Atoms (i.e., nuclei) that are related by virtue of a symmetry operation are said to be symmetry (or chemically) equivalent. As such, they are indistinguishable in all respects, and they are isochronous that is, they precess at exactly the same frequency. Thus, symmetry-equivalent nuclei cannot be distinguished by NMR. Returning to the case of chloromethane, the three hydrogens are symmetry equivalent by virtue of both the C3 axis and the symmetry planes and therefore give rise to only one signal in the H NMR spectrum of the compound. 

Therefore, these two hydrogens are symmetry equivalent (and isochronous) since they are related by reflection in the mirror plane. But because they are not related by any axis of symmetry, they are not homotopic. They are instead referred to as enantiotopic hydrogens, ones related only by a symmetry plane. 

In order to study the annealing behavior of the hydrogen-related defects, infrared absorbance spectra were recorded at 10 K after each step in a series of isochronal heat treatments (annealings) at temperatures in the range 400-1200 °C. The annealings were performed in a furnace purged with argon gas and the duration of each treatment was 30 min. 

Figure 2 shows the results of isochronal annealing experiments on ZnO samples treated with a hydrogen plasma at 350 °C. It follows from the figure that the H-I defect anneals out at 500 °C, whereas H-II is more stable and disappears from the spectra at 600 °C. Parallel to the annealing of H-I and H-II, another line at 3191.6 cm grows in the spectra. It represents a stretch LVM of the well-known Cu-H defect. It reaches its maximum intensity at 600 °C, when both H-I and H-II are already gone from the spectra. 

Name of family Members Sets of isochronous nuclei carbon hydrogen ... 

Diastereotopic groups are not equivalent and are not isochronous they have different chemical shifts in the NMR spectrum. When the diastereotopic groups are hydrogens, they frequently split each other with a geminal couphng constant J. Figure 5.58c shows the substitution procedure for a molecule with two diastereotopic hydrogens. In this molecule, the replacement of first Ha. and then Hb yields a pair of diastereomers. Diastereomers are produced when substituent Y already has an adjacent stereocenter. Section 5.15 will cover this situation in detail. 

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