Spin flipping

B) SINGLET-TRIPLET SPIN-FLIP CROSS SECTION... 

Figure 7 Relative change of electrical resistivity during isothermal aging condition with falling and rising temperatures obtained by PPM calculations [25, 33] without (a) and with (b) incorporating thermal activation process in the spin flip probability 6. The assumed temperature dependency of 6 is indicated in figure c.

It is recalled that the elementary atomic migration by breaking bondings with surrounding atoms is also driven by thermal activation process. This is modeled through the incorporation of the activation barrier, AG, in the spin flipping event via the following equation. 

In our opinion the spin-flip (SF) phetse transition occurring in AF Cr may be connected with the electron spectrum anisotropy. This transition, observed in... 

From this equation we see that a single spin flip a —) change AH equal to... 

If the oriented nuclei are now irradiated with electromagnetic radiation of the proper frequency, energy absorption occurs and the lower-energy state "spin-flips" to the higher-energy state. When this spin-flip occurs, the magnetic nuclei are said to be in resonance with the applied radiation—hence the name nuclear magnetic resonance. 

Problem 13.2 Calculate the amount of energy required to spin-flip a proton in a spectrometer oper-... 

Figure 13.3 shows both the H and the l3C NMR spectra of methyl acetate, CH3CO2CH3. The horizontal axis shows the effective field strength felt by the nuclei, and the vertical axis indicates the intensity of absorption of rf energy. Each peak in the NMR spectrum corresponds to a chemically distinct 1H or 13C nucleus in the molecule. (Note that NMR spectra are formatted with the zero absorption line at the bottom, whereas IR spectra are formatted with the zero absorption line at the top Section 12.5.) Note also that 1H and 13C spectra can t be observed simultaneously on the same spectrometer because different amounts of energy are required to spin-flip the different kinds of nuclei. The two spectra must be recorded separately. 

It can also be seen from Fig. 6 that if the T+x or T x states mixed with S, this would involve concomitant electron and nuclear spin flipping in order that the total spin angular momentum be conserved, and this would ultimately produce the same polarization in c- and e-products. This point will be discussed further in Section IV. 

Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

Figure 2.3 Spin-echo experiment. The behavior of nucleus X in an AX spin system is shown. (A) Application of the second 180° pulse to nucleus X in the AX hetero-nuclear system results in a spin-flip of the two X vectors across the x -axis. But the direction of rotation of the two X vectors does not change, and the two vectors therefore refocus along the —y axis. The spin-echo at the end of the t period along the -y axis results in a negative signal. (B) When the 180° pulse is applied to nucleus A in the AX heteronuclear system, the spin-flip of the X vectors...

Hence it is clear that if the two delay periods before and after the 180° pulses are kept identical, then refocusing will occur only when a selective 180° pulse is applied. This can happen only in a heteronuclear spin system, since a 180° pulse applied at the Larmor frequency of protons, for instance, will not cause a spin flip of the C magnetization vectors. 

In the spin-flip method, 180° pulses are applied simultaneously to both H and C nuclei at the midpoint of the evolution period so that the C... 

Figure 5.9 Pulse sequence for spin-flip heteronuclear /-resolved spectroscopy.

Figure 5.10 (A) Selective spin-flip pulse sequence for recording heteronuclear 2D / resolved spectra. (B) Its effect on magnetization vectors. The selective 180° pulse in the middle of the evolution period eliminates the large one-bond coupling constants, /< ...

Figure 5.11 (A) Heteronuclear 2D /-resolved spectrum (selective spin-flip method)...

---