Another noteworthy example is x-ray absorption fine structure (EXAFS). EXAFS data contain information on such parameters as coordination number, bond distances, and mean-square displacements for atoms that comprise the first few coordination spheres surrounding an absorbing element of interest. This information is extracted from the EXAFS oscillations, previously isolated from the background and atomic portion of the absorption, using nonlinear least-square fit procedures. It is important in such analyses to compare metrical parameters obtained from experiments on model or reference compounds to those for samples of unknown structure, in order to avoid ambiguity in the interpretation of results and to establish error limits.

The absorption spectra in the x-ray absorption near edge structure (XANES) region contain information concerning coordination geometry and metal ion valence. EXAFS and XANES data can be obtained on samples in different physical states or in solution and can be made element selective by tuning the wavelength used for the study. A drawback of these absorption spectroscopies is the need to bring the sample to a large dedicated facility.

Nuclei also have bound energy levels that can be accessed with gamma-ray sources. Of great chemical interest is Mössbauer spectroscopy, which takes advantage of the recoil-free emission of gamma radiation from a solid radioactive material. Because the gamma emission is recoil-free, it can be resonantly absorbed by stationary nuclei in a solid. Typically, the gamma ray source is mechanically vibrated back and forth to Doppler shift the energy of the emitted gamma radiation. A detector records the frequencies of gamma radiation that are absorbed by the sample as the energy of the gamma radiation is scanned by Doppler shifting.

The nuclear transitions are very sensitive to the local environment of the atom, and Mössbauer spectroscopy is a sensitive probe of the different environments an atom occupies in a solid material. By analyzing the chemical shifts and quadrupole splitting in Mössbauer spectra of samples containing Mössbauer-active nuclei, information on the state of oxidation and the local structure can be obtained. Only a few nuclei can be used for this purpose, so this method has limited but powerful applications.

X-ray, neutron, and electron diffraction techniques are used to determine crystal structures and can thus be used for molecular structure determinations. Because of its high resolution and applicability to small and often weakly diffracting samples, x-ray crystallography and powder diffraction are by far the methods of choice for most structure determinations on crystalline compounds,

Front Matter (R1-R14)

Executive Summary (1-10)

1 Introduction (11-15)

2 The Structures and Cultures of the Disciplines: The Common Chemical Bond (16-21)

3 Synthesis and Manufacturing: Creating and Exploiting New Substances and New Transformations (22-40)

4 Chemical and Physical Transformations (41-54)

5 Isolating, Identifying, Imaging, and Measuring Substances and Structures (55-70)

6 Chemical Theory and Computer Modeling: From Computational Chemistry to Process Systems Engineering (71-94)

7 The Interface with Biology and Medicine (95-122)

8 Materials by Design (123-147)

9 Atmospheric and Environmental Chemistry (148-159)

10 Energy: Providing for the Future (160-170)

11 National and Personal Security (171-179)

12 How To Achieve These Goals (180-194)

Appendix A: Biographical Sketches of Steering Committee Members (195-201)

Appendix B: Statement of Task (202-202)

Appendix C: Contributors (203-208)

Index (209-224)