except from single-crystal measurements utilizing polarized X-rays34). Furthermore, EXAFS diminishes rapidly beyond the first and second coordination shells (typically r ;$ 4 Á) except in cases where atoms are nearly collinear. In such cases, EXAFS from atoms as far as 6 A can be observed due to amplitude enhancement called focusing effect. In fact, both amplitude and phase of the EXAFS of a more distant neighbor are significantly affected by the intervening atom(s) for bond angle > 120°. For these systems, one must therefore take into account multiple scattering processes involving the intervening atom(s). Recently, a new multiple scattering formalism has been developed35 which enables bond angle determinations with an accuracy of 5% or ca. 5°. Nevertheless, the structural content of EXAFS is unparalleled by other spectroscopic techniques when (35) B. K. Teo, submitted for publication. one considers that the few most important bonds in a complex system can be probed within minutes. The future of EXAFS spectroscopy is as bright as the future synchrotron radiation sources. Dedicated synchrotron radiations with energy ranging from UV to hard X-rays are now available.36 These highly intense light sources will undoubtedly open up a new era in exciting chemical, biological, and material research.

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We report that the reaction of singlet oxygen with simple trisubstituted olefins shows a general preference for hydrogen abstraction on the more highly congested side of double bonds.

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