Deuterium solvent

Two of the most important functions in the application of neutron scattering are the use of deuterium labelling for the study of molecular confomiation in the bulk state and the use of deuterium solvent in polymer solutions. In the following, we will consider several different applications of die general fomuda to deuteration. 

If one limits the consideration to only that limited number of reactions which clearly belong to the category of nucleophilic aromatic substitutions presently under discussion, only a few experimental observations are pertinent. Bunnett and Bernasconi30 and Hart and Bourns40 have studied the deuterium solvent isotope effect and its dependence on hydroxide ion concentration for the reaction of 2,4-dinitrophenyl phenyl ether with piperidine in dioxan-water. In both studies it was found that the solvent isotope effect decreased with increasing concentration of hydroxide ion, and Hart and Bourns were able to estimate that fc 1/ for conversion of intermediate to product was approximately 1.8. Also, Pietra and Vitali41 have reported that in the reaction of piperidine with cyclohexyl 2,4-dinitrophenyl ether in benzene, the reaction becomes 1.5 times slower on substitution of the N-deuteriated amine at the highest amine concentration studied. 

White et al.1A have obtained similar kinetic results for the acid-catalysed rearrangement of N-nitro-N-methylaniline, i.e. a first-order dependence on the nitroamine with a linear H0 plot of slope 1.19 for phosphoric acid, and a deuterium solvent isotope effect of about three, although the results have only been presented in preliminary form. Further, an excellent Hammett a+ correlation was claimed for thirteen para substituted nitroamines which gave a p value of —3.9. Since it is expected that the rate coefficients would correlate with a (rather than different basicities of the amines, the a+ correlation implies that the amino nitrogen is electron-deficient in the transition state,... 

The general acid catalysis, the deuterium solvent isotope effects, and the lack of deuterium incorporation upon partial hydration in D2 0 are particularly convincing evidence for a rate-determining protonation and the discrete intermediacy of a vinyl cation such as 6. 

Selected entries from Methods in Enzymology [vol, page(s)] Anisotropy effects, 261, 427-430 determination by dynamic laser light scattering (quasi-elastic light scattering), 261, 432-433 determination for nucleic acids by NMR [accuracy, 261, 432-433 algorithms, 261, 11-13, 425, 430 carbon-13 relaxation, 261, 11-12, 422-426, 431, 434-435 cross-relaxation rates, 261,419-422, 435 error sources, 261, 430-432 phosphorus-31 relaxation, 261, 426-427, 431 proton relaxation, 261,51,418-422 relaxation matrix calculations, 261,12] deuterium solvent viscosity effects, 261,433 effect... 

A ratio defining the isotopic distribution of two isotopes equilibrated between two different chemical species. If X, followed by a subscript, represents the mole fraction of an isotope (denoted by that same subscript), then the fractionation factor, often symbolized by , with respect to chemical species A and B is (Xi/X2)a/(-X i/-X 2)b- Fractionation factors can also refer to different sites, A and B, within the same chemical species. As an example, the deuterium solvent fractionation factor, used in studying solvent isotope effects, is = (AD/A"H)soiute/(- o/... 

In neutral medium the deuterium solvent isotope effect, (A h2o/ D2o) in the hydrolysis of diaryldiacyloxyspirosulphuranes483, 404-407 has been found to be 1.66. In acidic medium the ratio of catalytic rate constants has been found to be 0.56. Heterocondensed imidazoles 408, are produced in the reaction of N-benzylamides, 409, of nitrogen heterocyclic carboxylic acids with phosphorus pentachloride. Deuterium labelling experiments have been carried out to understand the mechanism of this reaction involving a nitrile ylide species484. 

Deuterium solvent IE in the antibody catalysed hydrolysis of enol ethers... 

Relaxation phenomena are equally evident at the donor side of the reaction center. The well studied, fast (< 1 //s) electron transfer reactions from bound c-type cytochromes of RCs from such species as Chromatium and Rps. viridis f requentlv show a progressive shutoff of electron transfer from the high potential heme, which is closest to P (Gao et al., 1990). It is suggested that this is due to a large (> 100 mV) increase in the midpoint potential of the heme, associated with the freezing out of solvent or hydration-related relaxation processes (Kaminskaya et al., 1990). This is consistent with a deuterium solvent isotope effect for this reaction, as reported by Kihara and McCray (1973). 

Figure 26 Cyclopropane ring formation by CmaC in coronamic acid biosynthesis, (a) Characterization of 7-CI loss by ESI-FTMS. (b) Characterization of a-H exchange by deuterium solvent exchange and PEA. CmaC promotes deuterium incorporation at a-C position, (c) Proposed cyclopropane ring formation mechanism.55...

Specific acid catalysis is also restricted to biologically noncompatible conditions because amides are protonated in oxygen rather than nitrogen with pKa values in the range of <0 [63,64]. The rarity of intermolecular acid/base effects on CTI was affirmed by the kinetic deuterium solvent isotope effects (KSIE) of about 1.0, indicating that the mechanism of spontaneous isomerization does not involve a proton-in-flight [29,42,65]. 

In terms of this scheme, the value of the hysubstituent changed from p-amino to p-nitro if 3/fc4 decreased. This change is in the expected direction since an electronegative substituent in the benzene ring would affect step k3 which concerns a bond nearer the reaction centre more than it would affect step kt. An interesting experiment which Bender suggested in this connection would be the determination of the deuterium solvent isotope effects associated with proton transfer in the reactions of the intermediate. 

Zn > Co > Cd. Additionally, DHO activity is dependent on a single ionization for the dihydroorotate (favorable deprotonation) and carbamoyl aspartate (favorable protonation) substrates with apparent pA), values of 6 and 8.2, respectively. The pi), values change upon substitution with Co, suggesting that this reflects a metal-dependent ionization. There is a significant deuterium solvent isotope effect observed in /feat with the dihydroorotate ( cat / cat = 2.5 (ifcat/ M) /( cat/ M) = 1-1) and thio-dihydroorotate = 2.3 ... 

The proportionality of the rate of rearrangement to the acidity function deuterium solvent isotope effect, and the positive... 

The hydrogen-deuterium solvent isotope effect for the base-catalyzed... 

An examination of the deuterium solvent kinetic isotope effects (SKIEs) for the reactions with (l)Feffl(X)2 and (2)Fe (X)2 show that the SKIEs values are comparable for alkyl hydroperoxides and hydrogenperoxide. Moreover, the SKIEs for the decomposition of IH2 (k lk P) display values that are >2 and <3, while the values for the decomposition of IIH ( k4 lk4 ) are - l.O. This establishes that the decomposition of IIH2 at low pH represents general-base catalysis with water solvent as the catalyst. In contrast, the reactions at intermediate pH (i.e., IIH decomposition) are not subject to such catalysis. Recall that 4-substituted 2,6-dimethylpyridines act as general base catalysts in the decomposition of IIH2 . 

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