Science/Technology I ranosyl-D-glucose [/3(1^6)] linkage I of a gentiobiose derivative. As in the oxidative coupling meth2 8 od, there was a glucal double bond in the front ring. In that case, oligosaccharide building could go on and on with successive epoxidations of front rings followed by reactions with new glycals. The oxidative coupling technique yielded alpha configurations at C-l, whereas epoxide opening furnished
/3-anomers. In the examples given here, it is a glucal that adds either to another glucal in oxidative coupling or to an epoxyglucose. In practice, any glycals can be used. And at the end of the desired oligosaccharide sequence, a sugar derivative other than a glycal also serves well. The work on both oligosaccaride methods was supported by the National Institutes of Health. Stephen Stinson
New spectrometry method metha for metals research A new form of spectrometry to study tered ions but also neutral atoms metal surfaces and species adsorbed resulting from bombarding ions at those surfaces has been devel- picking up electrons. Thus the unit oped at the University of Houston. is more sensitive than those that Called time-of-flight scattering and detect scattered ions only. Also recoiling spectrometry (TOF-SARS), detected are such low-mass adsorbed the method may find use in charac- species as hydrogen or oxygen atoms Danishef sky: glycals control traffic terizing surfaces, studying catalysts, or molecules that have been knocked for further addition to give a trisac- and investigating embrittlement and off surfaces. Scattered ions and fast neutral charide. The "rear" ring had iodine stress cracking of metals by hydroatoms of several hundred electron at C-2. The iodine offers the possi- gen. bility of removal by hydride reducTOF-SARS yields interatomic dis- volts are counted by a channel election or closure of an epoxide by a tances between metal atoms and be- tron multiplier detector from Galileo hydroxy 1 group at C-3. Danishef sky tween metal atoms and adsorbates Electro-Optic Corp., Sturbridge, has opened just such an epoxide to [Phys. Rev. Letters, 63,51 (1989)]. Thus Mass. Each collision of a particle convert the rear ring to a true it could be complementary to low- with the lead-silica glass semiconglycosyl. energy electron diffraction (LEED), ductor inside causes the glass to emit 7 8 Both organic chemistry professors which gives symmetry patterns of 10 to 10 secondary electrons, which are amplified for counting purposes. Raymond U. Lemieux of the Uni- adsorbates. The time-of-flight mode of detecversity of Alberta, Edmonton, and The ion-bombardment technique Joachim Thiem of Westphalia Wil- of TOF-SARS detects not only scat- tion is more efficient as it collects helm University, Munster, West Germany, had earlier used iodonium perchlorate and N-iodosuccinimide as reagents to add sugars to double bonds of glycals. The importance of the Yale work is the use of functional groups on two glycal molecules to "control the traffic," as Danishefsky puts it. The second oligosaccaride method, perfected, according to Danishefsky, by Halcomb, began with the epoxidation of 3,4,6-tri-O-acetylglucal with dimethyldioxirane. This represents the first direct epoxidation of a glycal. Halcomb reacted the epoxide with 3,4-di-O-benzylglucal, which had an unprotected hydroxyl group at C-6. The configuration of the epoxide at C-l was alpha. The C-6 hydroxyl opened the epoxide with inversion of configuration. Thus the configuration of the rear ring was now beta, and the coupling had produced the 6-0-/3-D-glucopySpectrometry apparatus surrounds Rabalais, fills University of Houston lab 26
August 28, 1989 C&EN
ions and neutrals of all energies. And the machine design measures all angles of incidence, scattering, recoil of adsorbed species, and sample orientation in one 20-second experiment. The instrument was designed by physical chemistry professor J. Wayne Rabalais at Houston, working with graduate students Ming Shi and Haowen Bu and with chemistry professor Oscar Grizzi, on leave from the National University of the Comahue, Bariloche, Argentina. Their work was supported by the National Science Foundation and the State of Texas Advanced Research Program. Chemist Robert R. Rye of Sandia National Laboratory, Albuquerque, N.M., supplied a specially prepared tungsten sample to demonstrate the technique and helped the Houston team with advice about the status of hydrogen and oxygen atoms adsorbed at tungsten surfaces. Theoretical chemistry professor Petr Hochmann of the University of Texas, San Antonio, furnished computer programs to simulate the process. And physics professor Peter Nordlander of Rutgers University did effective medium theory calculations to analyze tungsten-hydrogen interactions, predicting t h e sites where hydrogen atoms might lie. To perform TOF-SARS, the user rotates the sample to different values of the azimuthal angle so as to bombard it from different directions. The 2- to 5-keV helium, neon, or argon ions strike the surface at different angles of incidence. Ions that strike surface metal atoms head-on are scattered backward at different angles. Those that graze surface metal atoms are scattered forward at various angles. If ions strike low-mass adsorbed atoms, they may knock those atoms in a forward direction in a process called direct recoil. Or the ions may drive t h e adsorbed atoms d o w n toward the surface from which they rebound in indirect recoil. Additional information comes from surface metal atoms that shield atoms b e h i n d them from being struck. This is called shadowing. The situation is analogous to the sun rising over a landscape of moun-
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