Scalar shifts

Next scalar grabbed new C in array Scalars shifted along... 

Figure B2.4.1 shows the lineshape for intennediate chemical exchange between two equally populated sites without scalar coupling. For more complicated spin systems, the lineshapes are more complicated as well, since a spin may retain its coupling infonnation even though its chemical shift changes in the exchange.

Three parameters thus need to be estimated, namely the scalar factor a, the compression factor c, and the shift d. Parameter b was dropped for two reasons (1) the effect of this exponent is to be explored, so it must remain fixed during a parameter-fitting calculation, and (2) the parameter estimation decreases in efficiency for every additional parameter. Therefore the model takes on the form... 

The number of NMR parameters available for measurement is rather small, consisting of the chemical shift, relaxation rates (/1 and lo), scalar (J) couplings, dipolar (D) couplings, cross-relaxation rates (the NOE), and hydrogen exchange rates. All of these have been quantified for many of the amide protons of A131 A, and most of the data suggest the presence of little persistent structure. 

Scalar (J) interactions between nuclei are transmitted indirectly through the electrons of the bonds. In the solid state, the effects of J couplings are usually obscured by the dipolar, chemical shift, and quadrupolar contributions to the linewidth. Nevertheless, the measurement and utilization of J couplings are possible even if... 

In summary, NMR techniques based upon chemical shifts and dipolar or scalar couplings of spin-1/2 nuclei can provide structural information about bonding environments in semiconductor alloys, and more specifically the extent to which substitutions are completely random, partially or fully-ordered, or even bimodal. Semiconductor alloys containing magnetic ions, typically transition metal ions, have also been studied by spin-1/2 NMR here the often-large frequency shifts are due to the electron hyperfine interaction, and so examples of such studies will be discussed in Sect. 3.5. For alloys containing only quadrupolar nuclei as NMR probes, such as many of the III-V compounds, the nuclear quadrupole interaction will play an important and often dominant role, and can be used to investigate alloy disorder (Sect. 3.8). 

Shortest-range interactions (over molecular distances of up to a few bonds away) examined by magnetic resonance are those that cause effects such as chemical shift, spin-spin coupling (also called scalar or J ... 

In addition to the chemical shift information, an NMR spectrum may also contain coupling information. The types of couplings frequently present in NMR experiments include scalar (J) couplings between high-abundance nuclei such as protons, dipolar couplings that are important for cross-relaxation processes and the determination of nuclear Over-hauser effect (NOE) (described later in this chapter), and quadrupolar coupling associated with quadrupolar nuclei (/> 1/2). 

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