In Brief from the

149th ACS National Meeting The stereochemistry of sugars found in erythromycin and carbomycin has been determined. Nuclear mag­netic resonance spectra show a D-gluco configuration for mycaminose, an amino sugar found in carbomycin, Dr. Werner Hofheinz (Massachusetts Institute of Technology) and Dr. Hans Grisebach (University of Freiburg, West Germany) told the Symposium on Carbo­hydrates from Antibiotics. The symposium was spon­sored jointly by the Divisions of Medicinal and Carbo­hydrate Chemistry. Mycaminose is 3,6-dideoxy-3-dimethylamino-D-glucose. Mycarose, a branched deoxy sugar found in carbomycin, is L-ribo- and has an erythro configuration at C-4 and C-5. Thus, mycarose is 2,6-dideoxy-3-C-methyl-L-ribohexose. Cladinose, the branched deoxy sugar found in erythromycin, is the methyl ether of mycarose. Erythromycin also con­tains desosamine, an amino sugar which NMR spectra show has a D-gluco configuration. Desosamine is 3,4,6-trideoxy-3-dimethylamino-D-xylohexose. Erythro­mycin C, however, contains mycarose instead of cladinose. Tracer studies with radioisotopes show that all four sugars are derived biosynthetically from D-glucose without cleavage of the carbon chain.

A method for determining perchlorate by amperometric titration using tetraphenylstibonium sulfate as the t i ­trant is free of interferences inherent in earlier titration methods for perchlorate. Chloride, chlorate, nitrate, phosphate, and sulfate do not interfere with the am­perometric titration, Dr. Michael D. Morris of Pennsyl­vania State University (University Park) told the Divi­sion of Analytical Chemistry. The titration takes about 15 minutes. It can be carried out in perchlorate solu­tions (between 0.004M and 0.020M) and is accurate to within about 1.5%. Perchlorate can be titrated by tetraphenylstibonium in acid or neutral solutions. Strongly alkaline solutions can't be used because of the low solubility of tetraphenylstibonium hydroxide. A dropping mercury indicator electrode is used. The diffusion current for tetraphenylstibonium reduction is —0.88 volt (vs. saturated calomel electrode).

A complete LCAO-MO self-consistent-field calculation has been made on the seven-center, 86-electron system of the FeFe"* ion, assuming only the Fe—F distance to be 2.1 A. Dr. James W. Richardson of Purdue Univer­sity (Lafayette, Ind.) constructed the molecular orbitals (MO's) as linear combinations of previously determined orthogonalized atomic orbitals (AO's) of the fluoride and iron (through the 4d) with effective nuclear charges (orbital exponents) fixed at their optimum free-ion values. To simplify the calculation, he makes three main approximations: MO's containing "core" elec­trons are considered to remain pure AO's, although correct coulombic and exchange potential energies are computed from them; AO's on different ligands are considered to be nonpenetrating; and three-center, one-hand two-electron integrals are evaluated by a slight

modification of the Mulliken approximation. All re­maining kinetic energy nuclear attraction and inter-electronic repulsion integrals are evaluated exactly by digital computer programs. Other programs then prop­erly assemble these integrals (about 16,000) for the subsequent calculation of energy and MO wave func­tions, from which various spectral parameters and bonding analyses are drawn. Because the entire cal­culation is almost completely automated, considerable variation and "experimentation" is possible.

Films and fabrics made from aromatic copolyamides are inert to most solvents and retain their strength, electrical insulating, and other useful properties above 300° C. The polymers were prepared by condensing diacid chlorides with symmetrical diamines containing aromatic amide groups in an ordered arrangement, Dr. J. Preston of Chemstrand Research Center (Durham, N.C.) told the Division of Polymer Chemistry. Low-temperature solution polymerizations (at about —20° C.) give higher molecular weight polymers than do interfacial polymerization methods. Each polymer contains a repeating ordered arrangement of units de­rived from aminobenzoic acids, arylene diamines, and arylene diacids. With m- and p-phenylene groups in these units, eight polymers are possible; all were pre­pared. These polymers have melting points that vary from 410° C. for the all-meta polymer to 555° C. for the all-para polymer. Tough, pliable films were made from all these polymers. Three of the polymers were spun into fibers. The fibers retain 7 0 % of their strength after being held at 300° C. in air for 17 hours and over half of their strength after a week.

Polystyrene has exceptional ability to trap negative and positive charges (electrons and holes), usually with an excess of negative charges. Dr. V. J. Caldecourt of Dow Chemical (Midland, Mich.) says that the mobility of these charges increases with voltage and tempera­ture. At low voltages (less than 300 volt per mil) and below —100° C , the equilibrium conduction currents become infinitesimal. The charges are trapped in the bulk of the plastic, not just on the surface. However, the detailed distribution of the trapped charges in the sample and the identity of the trapping sites and trapped species are still unknown, Dr. Caldecourt told the Division of Organic Coatings and Plastics Chemis­try. These phenomena may be a function of a polymer's composition, molecular structure, and physi­cal state. Other experiments at Dow, he adds, indicate that a combination of hole formation, caused by elec­tron emission from the bottom of the sample, and elec­tron migration through the irradiated (by impinging 2-m.e.v. electrons on a solid plastic sample) volume of the sample combine to produce a counter charge. An understanding of space charge phenomena in plastics will be useful in formulating and designing plastic components for use where they will be subject to high-energy particle bombardment (as in space).

44 C & E N A P R I L 12, 196 5

R E S E A R C H