Partial exchange

Tetrafluoropyrimidine was converted to the antiaeoplastic 5-fluorouracil (5-FU) by a novel process based on the sequence partial exchange chlorination (61% yield), selective hydrogenolysis ia triethylamine (71% yield) and hydrolysis (85—93% yield) (464). 

The compounds were also run with D20 leaking into the ion source from a stainless steel reservoir. The resulting mass spectra show partial exchange of the hydroxyl protons with deuterium. From the shifts of the peaks in the mass spectra of the deuterated analogs, the number of exchangeable hydrogens retained in each fragment can be determined. 

Weak acidic cation (WAC) Provides only a partial exchange capacity (only those cations associated with alkalinity). WACs have a much higher (but fixed) capacity than SACs. High operating effi-... 

In the case of zeolites (Table 4), a very broad reduction peak is observed on the sample partially exchanged, with a poorly defined maximum. On the other hand, two reduction peaks appear for Cu exchanges higher than 100%. In both cases, however, the consumption of hydrogen corresponds to the reduction of Cu + to metallic Cu. A similar situation was previously reported for Cu-MOR [6]. By... 

Upon UV irradiation of Nation partially exchanged with Fe(II) and Fe(III), weak ESR signals from perfluorinated radicals were detected at 77 K even in the... 

After the partial exchange of the Na counter cations for Gd " ", the specific relaxivity (here defined as the measured water relaxation rate per gram of material) was measured at Larmor frequencies ranging from 0.01 to 30 MHz. The NMRD profiles obtained were compared with that obtained after complexing the encapsulated Gd(III) with DTPA. After correction for the differences in the Gd(III) content of the samples, the NMRD curves appeared to be superimposable. Based on this result, a... 

Thus, the overall exchange current density /q is composed of different partial exchange current densities ... 

Silica gel and silica-alumina protons will exchange with deuterium in D2O at 150° or Da gas at 350°, as is evidenced by disappearance of the resonance signal. With partial exchange, the absorption line decreases in... 

Fig. 1.6 C02-graphite partial-exchange experiments in a Northrop and Clayton plot at 700, 800, 1,000 and 1,200°C, The connecting line in experiment at 1,200°C has a plane slope and defines the intercept more precisely than the experiment at 700°C (after Scheele and Hoefs 1992)...

Since half-reactions are present in this reaction mechanism (actually, third-reactions ), partial exchange experiments are also available for investigation. See... 

COMPETITIVE INHIBITION partial derivatives (Basic Properties) Partial exchange reactions,... 

OXIC. Z PACEMAKER REACTION PARTIAL EXCHANGE REACTIONS... 

Even in the first meter of sediments it is apparent that important changes take place in the silicate-fluid equilibria. One of the most significant effects is the partial exchange of sodium and calcium for... 

The influence of exchangeable monovalent cations on the framework vibrations for the hydrated zeolites Linde A and X has been investigated. An approximately linear relationship is found between the frequency of some absorption bands and the inverse of the sum of the cation and framework oxygen ionic radii. It is proposed that the shift in framework vibrations is largely caused by those cations which are strongly interacting with the zeolite framework. Thus the linear relationship indicates that these monovalent cations are all similarly sited in the zeolite lattice. This is consistent with the presently available x-ray analyses on some of these zeolites. Since Rb + and Cs + are only partially exchangeable in both Linde A and Linde X, these cations deviate from this linear relationship. 

Transalkylation reactions are observed in Y zeolites partially exchanged with ethyl-, diethyl-, and triethylammonium cations (EA, DEA, and TEA, respectively) heated above 150° C in air or under vacuum, in the presence of residual water molecules. The main reactions may be depicted schematically as follows (EA) Y — (DEA)Y > (NHt+)Y, (DEA)Y - (EA)Y > (TEA)Y, and (TEA)Y - (DEA)Y > (EA)Y, iAc main constituent in the gas phase being CJh. They are similar to those observed in montmorillonite in the presence of a water vapor pressure of a few torr. It is proposed that in both cases the transalkylation processes are acid catalyzed, the residual water molecules and the surface oxygen being the active spots recycling the protons in montmorillonite and zeolite, respectively. 

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