After about an hour's exposure to the neutron beam, the bombs are returned to the laboratory, hooked up to the gas chromatograph, and their contents swept through for analysis. "Our results show that 18 F-for-F substitution always results in the deposition of at least 2 e.v. energy in the product molecules, and usually much more. Pressure must be kept high to prevent most of the resulting activated molecules from fragmenting virtually instantaneously," says Dr. Root, whose work is funded by the Atomic Energy Commission and by the Air Force Office of Scientific Research. Dr. McKnight and he find that the halfpressure for stabilization of CH 3 CF 2 18 F—the pressure needed to prevent 507c of the molecules from decomposing—is about 280 atm. The implication of this figure is that the average lifetime of the excited trifluoroethane molecules is one picosecond. In contrast, the corresponding value for CH 2 T C F 3 (made by a T-for-H reaction when trifluoroethane undergoes recoil tritium bombardment in a TRIGA reactor) is 0.13 atm., and its halfpressure ratio (inverse of its lifetime ratio) is an enormous 2200 to 1. Agreement. "Our T-for-H findings agree well with those of Dr. Rowland's in other systems. These put the intrinsic energy value of T-for-Hsubsituted molecules in the 4- to 5e.v. range," the Davis workers observe. "On the basis of unimolecular reaction kinetics, we further estimate that CH 3 CF 2 1 8 F has an intrinsic average energy between 11 and 12 e.v." They checked their unimolecular kinetics calculations by way of an additional experimental route. This involves synthesis of a third chemically activated trifluoroethane species by UV photolysis of a mixture of 1 4 CH 3 COCH 3 and CF 3 COCF 3 . A major product from the photolysis is 1 4 CH 3 CF 3 , which is monoenergetic with an energy content of 4.3 e.v. "The unimolecular kinetic parameters that emerge from the photolysis experiment lead to a mean residual energy value of 4.7 e.v. for the CH 2 T C F 3 molecule, thus bolstering our confidence in the parameters," Dr. Root explains. "They also conform closely to known parameters for other structurally related compounds. Finally, by applying the parameters to the recoil fluorine data, we arrive at 11.5 ± 2 e.v. as the average intrinsic energy of CH 3 CF 2 1 8 F." As a final clincher to the argument, Dr. Root notes that on a thermodynamic basis, the observed formation of difluorocarbene from CF 3 1 8 F requires an energy of at least 8 e.v. "It is obvious that the average energy of the decomposing CF 3 1 8 F molecules must be greater than 8 e.v.," he adds.
CHARACTERIZATION. Dr. Donald C. DeLong (left), Dr. Robert Williams, and Dr. Koert Gerzon inspect chromatograph of pyrazomycin, a potential antiviral compound which they have isolated and purified
Potential antiviral chemical is isolated MICROBIAL WAQ&^KL^
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When workers at Eli Lilly noticed that the fermentation broth from a strain of Streptomyces candidus inhibits virus growth in tissue cultures, they little thought that they were observing the effect of an unusual chemical. The active compound which they now have isolated and purified turns out to be a ribonucleoside in which the heterocyclic aglycone and sugar moieties are joined together through a carbon-carbon link. Pyrazomycin, the name given to the chemical, belongs to a class of compounds called C-nucleosides, only a few of which have been identified to date, observes Dr. Robert H. Williams, head of Lilly's fermentation products research. It seems to be a potent antiviral agent. In vitro tests being carried out by Dr. Donald C. DeLong show that it prevents proliferation of rhinovirus, measles, Herpes simplex, and
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