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Deuterium substitution determination

Combinations of non-selective and/or single-selective relaxation-rates, or both, with n.0.e. values may conveniently be performed with reliable results, especially when other methods seem impractical. However, these experiments are time-consuming, as they entail the determination of a rather large number of experimental values. Moreover, the n.O.e. parameters carry their own systematic and random errors, which are magnified in the calculation of interproton distances. The deuterium-substitution method requires specific deuteration at a strategic position, which, in many cases, may be inconvenient or impractical. Also, this technique is valid only when the relaxation rates obtained after deuterium substitution are at least 5% enhanced, relative to the relaxation rates of the unsubstituted compound, and it requires that, for a meaningful experiment, the following condition " be satisfied. [Pg.164]

The structure of [TpBut]ZnH has been determined by x-ray diffraction, although the hydride ligand was not located (Fig. 38). However, definitive evidence for the presence of the hydride ligand is provided by NMR and IR spectroscopies. Specifically, the hydride resonance is observed at 8 5.36 ppm in the H NMR spectrum, and p(Zn-H) is observed as a strong absorption at 1770 cm-1 in the IR spectrum, which shifts to 1270 cm 1 (vhIpd = 1.39) upon deuterium substitution (Fig. 39). [Pg.347]

Data from microwave spectra on the centrifugal effect of rotational transitions of selenophene and its deuterium-substituted derivatives have been determined experimentally and compared with the calculated theoretical values of the centrifugal stretching constants by means of the force constants determined from the solution of the inverse vibrational problem.26 The two sets of values show good agreement, indicating that the system of force constants obtained for selenophene correctly reflects the characteristic features of the force field of the molecule. [Pg.132]

Pohl L, Gillette J. 1984-1985. Determination of toxic pathways of metabolism by deuterium substitution. Drug Metab Rev 15 1335-1351. [Pg.128]

Sedimentation. The sedimentation experiments are tabulated in Tables I and II. In Table I typical sedimentation coefficients determined in H20 and D20 are in close agreement here and with previously reported values determined for both protio and deuterio phycocyanin from F. calothricoides (15,16). Each of the tabulated coefficients is for a single experiment at an approximate protein concentration of 15 mg. per ml. Lyophilizing a phycocyanin preparation twice had little effect on the observed sedimentation coefficients. In calculating the S values the same partial specific volume of the protein was used for both D20 and H20. This practice is consistent with the recent results of Edelstein and Schach-man (7). Small increases in sedimentation coefficients from H20 to D20 are to be expected because of deuterium substitution on exchangeable positions. The slope of an S vs. concentration plot for phycocyanin in H20 and D20 would also probably differ. Consequently, small changes in S from H20 to D20 would be expected at a constant protein concentration. [Pg.30]

C-0-C-1H couplings have been determined in methyl /i-cellobioside after deuterium substitution of all protons on carbon atoms with a free hydroxy group. Assuming a Karplus type relationship between 3JC H and the dihedral angle in 13C-0-C-1H groups, the torsional angles 0 and / of methyl / -cellobioside were determined to be 25 30" [734],... [Pg.394]

J. F. Duncan and K. R. Lynn, J. Chem. Soc., 3512, 3519 (1956) J. B. Ley and C. A. Vernon, Chem. Ini. (London), 146 (1956).] That the rate-determining step can be the migration when the first-formed carbocation is particularly stable has been shown by Schubert and LeFevre [note 18(b)]. These workers subjected 1,1-diphenyl-2-methyl- 1,2-propanediol to the pinacol rearrangement and found that deuterium substitution in the migrating methyls caused the reaction to slow down. [Pg.275]

Another series of closely related reactions for which transition-state calculations have greatly helped in providing an understanding of the observed trends is the addition to deuterium-substituted alkenes. Szwarc and co workers (Feld et al., 1962) have determined secondary deuterium isotope effects for methyl and trifluoromethyl radicals by comparing the rate of addition to a terminal alkene with the rate for the deuterium-substituted alkene (25). Isotope effects for cyclopropyl radical addition have been measured by Stefani and coworkers (1970). For these three radicals a small inverse isotope effect (kJkK)... [Pg.76]

Deuterium substitution may give rise to important effects on the reaction kinetics. The maximum kinetic isotope effect is obtained when the bond is broken in the transition state (primary isotope effect). In reactions involving several stages, isotope effects can naturally be observed only if the bond to the isotope is broken in the rate-determining step . In this case, deuterium substitution would be expected to depress the reaction rate. We give below two examples relative to acetylenes, showing how the effect, kulkn, may specify the reaction process or, on the contrary, allow the rejection of a possible mechanism. [Pg.454]

This technique can be used extremely well to determine the geometry of the oxiranes. As long ago as the 1940s it was utilized in combination with dipole measurement to establish the bond lengths and bond angles in various substituted oxiranes. Similar data are found in the literature on deuterium-substituted oxirane. ... [Pg.8]

Is C-H bond breaking in the slow step The fact that a C-H bond is just slightly easier to break than a C-D bond can be used to probe reaction mechanisms that have the C-H bond breaking in their rate-determining step. In this case the rate of the deuterium-substituted reactant will be 2 to 7 times slower. [Pg.318]

Anti-cancer drugs such as cyclophosphamide (15), aniline mustard, and nitrosoureas are transformed to reactive metabolites which are the toxic species required for their anti-cancer activity. Experiments with selectively deuterated analogs of these drugs has distinguished which pathway, among several alternative pathways of metabolism, is responsible for antitumor activity. For example, a deuterium isotope effect was observed for the formation of 4-ketocyclophosphamide (16), formed by the oxidation of the carbon alpha to the phosphoramide nitrogen, but there was no Isotope effect on the anti-tumor activity. However, there was a marked effect on the subsequent -elimination reaction and consequent decrease in anti-tumor activity by deuterium substitution at C-5. Thus, the formation of acrolein and phosphoramide mustard is rate determining for the anti-tumor activity of cyclophosphamide. [Pg.326]

Alcohols 19 and 21 are very unique chiral compounds, the chirality of which is generated by the substitution of isotopes in the case of 19, H vs. D in the case of 21, C vs. C, so it is very difficult to recognize such an extremely small chirahty directly. To synthesize enantiopure alcohols 19 and 21, and to determine their absolute configurations, the indirect chemical conversion method was employed as follows. For example, deuterium-substituted/4-Br alcohol 20 was similarly enantioresolved as in the case of compound 14 (entry 9). The enantiopure alcohol (S)-(-)-20 obtained was reduced to remove the Br atom yielding (S)-(-)-19, which exhibits a negative CD Cotton effect at 270.4 nm. In a similar way, C-substituted... [Pg.291]


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See also in sourсe #XX -- [ Pg.45 , Pg.137 ]




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