Angular information, obtaining

RDCs are easy to measure and the interpretation with respect to distance and angular information is readily available. Other anisotropic parameters contain similar information, but are either difficult to obtain experimentally or are more complex in terms of interpretation. In the following, a brief introduction into RQCs, RCSA and PCSs is given. 

As described in Section 9.2.4.5, CCR can be used to obtain dihedral angular information. Since CCR processes depend linearly on the correlation time, the requirements for exchange-transferred spectroscopy are fulfilled similarly to the etNOE. 

The applications of polarized x-ray absorption spectroscopy (PXAS) for structure determination in inorganic and bioinorganic systems are discussed. PXAS studies of oriented samples add angular detail to the information obtained from x-ray absorption edges and from EXAFS. In some cases, PXAS can be used to determine molecular orientation. In other cases, PXAS can be used to infer the details of electronic structure or of chemical bonding. Some of the potential future applications of PXAS are discussed. 

Angular information is notably absent from the list of structural parameters normally obtained from XAS. One approach to obtaining angular detail is to make use of multiple scattering effects (17). Unfortunately, this technique is only useful for outer shells (non-nearest neighbor atoms) where there are atoms intervening between the absorber and the scatterer. This technique suffers from complications if the shells of interest overlap in distance with other shells of atoms. 

An alternate method for obtaining angular information is to make use of the plane polarized nature of synchrotron radiation. It has long been known that XAS should exhibit a polarization dependence for anisotropic samples (18) however it is only recently that attempts have been made to exploit this effect. Early attempts to observe anisotropic XAS suffered from the low intensity and incomplete polarization of conventional x-ray sources. This work has been reviewed by Azaroff (19). 

EXAFS data from frozen solutions and polycrystalline samples can provide information about the number, type, and distances of atoms coordinated to the molybdenum atom, but not angular information that defines stereochemistry. Mo L-edge spectroscopy has been used to investigate the 2p 4d electronic transitions of the molybdenum atom in sulfite oxidase and model compounds in order to obtain information about the symmetry and electronic structure of the molybdenum center (70). 

Therefore, XAS—especially with respect to EXAFS—has many advantages as a probe of transition metal centers in biological materials. Beyond the absence of a requirement for crystalline materials, the major attractions are the specificity, and sensitivity of the technique and the provision of interatomic distances with an accuracy of 0.02 A within (say) 4 A of the primary absorber. However, it should be noted that (1) no angular information is usually obtained (2) rarely does the structural information extend beyond 4 A, (3) the spectrum sums data for all atoms of a particular element and, if the element of interest is present in more than one chemical form, an average environment is obtained (4) the possibility of radiation damage must be anticipated and the integrity of samples should be monitored after, and if possible... 

At the same time, we know that the photons carry information obtained from the atom in the process of generation. This information is transmitted through the conservation laws. In particular, the photon carries the angular momentum of the excited state because... 

Obviously, WAXD is the technique of choice for studying the polymorphism of polymers because different crystalline phases display different diffractometric patterns. When more than one phase coexist in the same material, the intensity of the areas of the characteristic peaks of each of them yields a measure of their relative abundances [93,94]. Further information obtainable by WAXD is the size of crystallites, which can be obtained from the width of the peak applying the Scherrer equation [95], and the characteristic dimensions of the crystalline cell, which can be assessed on the basis of the angular position of the peaks. 

The application of Mossbauer spectroscopy in diverse fields of qualitative and quantitative analysis is based on the ease with which hyperfine interactions can be observed. The information obtained from Mossbauer spectroscopy may be correlated with other methods by which HI can be examined such as NMR, EPR, ENDOR, PAC (perturbed angular correlations), nuclear orientation and neutron scattering. However, Mossbauer spectroscopy often proves to be experimentally simpler, more illustrative and an efficient method for studying applied problems. Mossbauer nuclei are ideal spies supplying information on both the microscopic and macroscopic properties of solids. 

---