Nuclear magnetic resonance local bonding

In this chapter, three methods for measuring the frequencies of the vibrations of chemical bonds between atoms in solids are discussed. Two of them, Fourier Transform Infrared Spectroscopy, FTIR, and Raman Spectroscopy, use infrared (IR) radiation as the probe. The third, High-Resolution Electron Enetgy-Loss Spectroscopy, HREELS, uses electron impact. The fourth technique. Nuclear Magnetic Resonance, NMR, is physically unrelated to the other three, involving transitions between different spin states of the atomic nucleus instead of bond vibrational states, but is included here because it provides somewhat similar information on the local bonding arrangement around an atom. 

Besides local geometries, the intelligent sketchpad can contain any local properties, including bond types and strengths, chromophore optical spectra, and nuclear magnetic resonance and infrared spectra characteristic of a local chemical environment. 

A variety of spectroscopic techniques, however, are of value to determine the local bonding and, occasionally, oxidation states of various ions. Frequently, they can perform satisfactory quantitative analysis or estimates as well. Adsorption, emission, and Raman spectroscopy operating from the UV through the IR region of the spectrum can provide such information. These optical spectroscopies can be performed in either a transmission or surface-scattering mode based on the thickness and absorption properties of the specific sample. Nuclear magnetic resonance (NMR), Mossbauer spectroscopy, and electron spin resonance techniques are some other forms of spectroscopy frequently used to determine local bonding and oxidation states of specific species, primarily in the bulk rather than on the surface. These methods are limited to particular atoms or ions and are not universally applicable. 

Nuclear magnetic resonance (NMR), in particular, deuterium NMR, has proven to be a valuable technique for determining the nature of molecular organization in liquid crystals. The utility of the NMR technique derives from the fact that the relevant NMR interactions are entirely intramolecular, i.e. the dominant interaction is that between the nuclear quadrupole moment of the deuteron and the local electric-field gradient (EFG) at the deuterium nucleus. The EFG tensor is a traceless, axially symmetric, second-rank tensor with its principal component along the C—D bond. In a nematic fluid rapid anisotropic reorientation incompletely averages the quadrupolar interaction tensor q, resulting in a nonzero projection similar to the result in Eq. (5.6) ... 

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