Isotopic medium

Boron trifluoride is also employed in nuclear technology by uti1i2ing several nuclear characteristics of the boron atom. Of the two isotopes, B and B, only B has a significant absorption cross section for thermal neutrons. It is used in " BF as a neutron-absorbing medium in proportional neutron counters and for controlling nuclear reactors (qv). Some of the complexes of trifluoroborane have been used for the separation of the boron isotopes and the enrichment of B as (84). 

Even at the lowest temperatures, a substantial pressure is required to soHdify helium, and then the soHd formed is one of the softest, most compressible known. The fluid—soHd phase diagrams for both helium-3 and helium-4 are shown in Eigure 1 (53). Both isotopes have three allotropic soHd forms an fee stmeture at high pressures, an hep stmeture at medium and low pressures, and a bcc stmeture over a narrow, low pressure range for helium-4 and over a somewhat larger range for helium-3. The melting pressure of helium-4 has been measured up to 24°C, where it is 11.5 GPa (115 kbar) (54). 

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

Hartshorn and Ridd48 showed that there is a negligible solvent isotope effect on nitrating anilinium ions in sulphuric acid and deuterated sulphuric acid (cf. an earlier less accurate determination by Brickman and Ridd48). The absence of a solvent isotope effect also argues against reaction on the free base because the free base concentration would be lower by a factor of about four in the deuterium-containing medium. Consequently, the differences in the rate coefficients in Table 6... 

VARIATION OF SOLVENT ISOTOPE EFFECT WITH MEDIUM COMPOSITION IN REACTION... 

The kinetic isotope effect kD kT for hydrogen exchange of ferrocene in both trifluoroacetic acid-acetic acid and trifluoroacetic acid-dichloromethane has been measured558. In the former medium (1 1 molar ratio at 25 °C) kD kT was 1.2—1.3, which was less than that obtained for the 2 position of thiophene (D) = 1,200 xlO-7, ki(T) = 660xl0-7, kD kT = 1.9. It is also lower than that obtained for pentamethylbenzene fcx(D) = 3,300 x 10-7, Art(T) =... 

Since the rate was independent of acidity even over the range where H0 and pH differ, and the concentration of free amine is inversely proportional to the acidity function it follows that the rate of substitution is proportional to h0. If the substitution rate was proportional to [H30+] then a decrease in rate by a factor of 17 should be observed on changing [H+] from 0.05 to 6.0. This was not observed and the discrepancy is not a salt effect since chloride ion had no effect. Thus the rate of proton transfer from the medium depends on the acidity function, yet the mechanism of the reaction (confirmed by the isotope effect studies) is A-SE2, so that again correlation of rate with acidity function is not a satisfactory criterion of the A-l mechanism. 

Thiamine was biosynthesized by resting cells of S. typhimurium strain thilO/T-ath-383, which can synthesize thiamine from exogenous glucose, AIRs, and thiazole.54 Derepression was achieved by conventional means. The organism was cultivated in the presence of a suboptimal amount of thiamine (20 nM), the washed cells were resuspended in a minimal medium containing glucose (10 mM), thiazole (1-2 mM), and labeled AIRs (10 p,M). During the incubation (1.5 hours 37°C), the level of thiamine diphosphate in the cells had risen from about 0.04 to 0.5 nmol/mg. In work with molecules labeled with stable isotopes, thiamine was extracted and cleaved by ethanethiol to 4-amino-5-(ethyl-... 

Table 9.4. C N molar ratios (calculated and measured), total C and N content and stable carbon and nitrogen isotope data from bacteria, their growth medium (nutrient broth), and from collagen (infected and non-infected marten bone). The bacteria for inoculation were raised on nutrient broth (nb), with/without additives.

The alcohol 3) , 28-diacetoxy-6 -hydroxy-18i -12-oleanene was oxidised in an aqueous acetic acid medium Deuteration at the six position had no effect on the rate in solvents of high (> 80 %) acetic acid content, but the isotope effect reached 2 in 60 % acetic acid. Increasing the acetic acid content of the medium produced a much larger effect on the oxidation rate of cyclohexanol than of the polycyclic alcohol. 

---