X-ray Fingerprinting of Chemical Intermediates in Solution

摘要

In the early 1800s, the English scientist John Dalton postulated that matter is made up of indestructible atoms whose identities are unchanged by chemical reactions. These atoms have a definite size and mass, are countable, and through chemical reactions can combine to produce molecules as simple as diatomic oxygen (O_2), the vital component in the air we breathe, or macromolecules far more complex than myoglobin (C_(822)H_(1035)FeN_(222)O_(220)S_3), an oxygen-storage protein found in our muscle tissue. It is the controlled structural rearrangement of atoms and molecules that adds value to industrial chemical feedstock, and gives life to biological organisms. The selectivity of the transformation from reac-tant to product depends on the reaction mechanism, and the structures of intermediates along the reaction pathway are often hotly debated. Because the atoms are so small (～10~(-10) m), and the time it takes for them to slip past their neighbors is so short (～10~(-13) s), the direct observationof these intermediates has proven quite elusive. Recently, researchers working at the European Synchrotron Radiation Facility (ESRF) rose to this experimental challenge and pursued structural studies of photo-chemically generated, short-lived (<10~(-6) s), iodo radicals. Davidson et al. studied diiodomethane (7), whereas Ihee and co-workers studied diiodoethane (as reported on page 1223 in this issue) (2). In particular, Ihee et al. identified the structure of a radical that purportedly plays a crucialrole in certain stereoselective chemical reactions.