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Deuterium substitution limitations

Secondary Deuterium Kinetic Isotope Effects. Deuterium substitution has been employed to probe for bridging in the transition state of 2-norbornyl brosy-late solvolyses. The secondary a-, (3-, and 7-deuterium kinetic isotope effects for exo-and endo-norbornyl brosylate are shown in formulas 721) and (722), respectively. The overall pattern for the mfo-compounds (722), with an a-effect close to the limiting value (1.22, cf. Section 7.2.3)506), with nil effect of C(1)-DS07 a modest... [Pg.267]

An experimental kinetic study has shown that the influence of the a-cyano group on the rate of solvolysis of 2-propyl sulphonates, namely the k(HCMe20Ts)//c(NCCMe20Ts) ratio (extrapolated from 80-170 °C temperature interval to 25 °C) in 100% 2,2,2-trifluoroethanol buffered with 2,6-lutidine, is only 3.5 x 103. This is considerably smaller than the retardation factor expected on the basis of the Taft polar substituent constant. To evaluate the possibility that the investigated methacrylonitrile formation reaction proceed by a rate-limiting concerted E2 elimination, the effect of a-deuterium substitution on the rate of... [Pg.659]

As DEHP metabolism in human blood is accelerated by blood enzymes, such as esterase, its metabolites are frequently measured. In the study where the metabolites of deuterium-substituted DEHP in urine and in blood serum were compared after oral administration of DEHP, MEHP was found to be the main metabolite in blood serum, existing in greater quantities than MEHHP and MEOHP detected in urine. Therefore, we decided to measure DEHP and MEHP in blood serum. Four healthy volunteers were asked to consume food in containers that were made in part of plastic, and DEHP and MEHP concentrations in blood sera were measured. The concentration of DEHP was equal to or less than the quantitative lower limit (trace level) in all the volunteers. The concentration of MEHP was trace in three of the four volunteers and was lower than the quantitative lower limit in one volunteer. In addition, we measured DEHP and MEHP in human blood plasma samples using LC-MC with the column-switching system as pretreatment. DEHP and MEHP concentrations in blood plasma sampled from six healthy volunteers were equal to or less than the quantitative limit (DEHP <25 ng/ml, MEHP <5 ng/ml). " The results prove that DEHP is rapidly metabolized so that the blood concentration is low and quantification is difficult at normal exposure levels. Urine samples are more suitable for the evaluation of the exposure index of PEs than blood samples. [Pg.1136]

From Fig. 5.7, we see that the ratio of the effective lifetime ra for deuterated and nondeuterated materials approaches the ratio of the corresponding optical nonlinearity at the lowest temperature. This is in agreement with the predictions of (5.1) if we may also assume that Ue, and the ratio Dg/De are approximately unaffected by the substitution. We verified that Ug is indeed unaffected by measuring the linear dichroism of the dye-Uquid crystal solutions and finding it unchanged in the deuterated species (see [21] for the relationship between Ug and dichroism). It is hence reasonable to assume that also Ue is unaffected. Moreover, the ratio Dg/D is also not much affected by deuterium substitution as reported in Sect. 5.5. We should also mention that (5.1) is actually exact only in the limit of small liquid crystal order parameter. Nevertheless, a more precise numerical calculation confirms the agreement [23]. [Pg.176]

Electron Spin Echo Envelope Modulation, or ESEEM, is widely used to investigate structure of the immediate surroundings of paramagnetic species. In combination with site-specific spin labeling and selective deuterium substitution, it becomes a helpful tool to study location of proteins and small peptides within membranes, interaction of small molecules with proteins, supramolecular assemblies, water accessibility to specific regions of proteins and water penetration into membranes, protein folding and secondary structures. In this review, experimental approaches and limitations, theoretical background of the method, and recent applications are discussed. [Pg.102]

Recently, it was shown that spin-label ESEEM of partly deuterated systems is helpful for studying different aspects of intermolecular interactions, molecular assemblies and conformation in complex biological media. A short survey has been published by one of us, covering structural ESEEM studies of this kind in biological membranes that appeared before 2012. In this review, we discuss recent applications of spin-label ESEEM with deuterium substituted molecules, covering also other experimental fields. Also, special attention is paid here to discussion of experimental approaches and limitations, and the theoretical background of the method. [Pg.103]

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. [Pg.420]

Apart from a few studies (ref. 7), the use of deuterium kinetic isotope effects (kie s) appears to have had limited use in mechanistic studies of electrophilic bromination of olefins. Secondary alpha D-kie s have been reported for two cases, trans-stilbene fi and p-substituted a-d-styrenes 2, these giving relatively small inverse kie s of... [Pg.117]

This reaction sequence was definitively shown by use of temperature programmed reaction spectroscopy ( 7) The key to the success of this method was that reaction (4) was the rate-limiting step, allowing positive identification of the CH30(a) intermediate by TPRS. Isotopic substitution with b0 and deuterium was used to identify steps (2) and (3). [Pg.62]

In 2-substituted adamantanes 25 both types of 8-positioned carbon atoms (8syn and 8 ) exist within one molecule (Scheme 37). Early measurements with limited spectral resolution (176) did not differentiate between their signals. Later (124,244), differences of up to 0.7 ppm were detected, and application of various independent methods, including addition of lanthanide shift reagents (245), determination of longitudinal relaxation times T, (246), and evaluation of deuterium... [Pg.262]

These reactions proceed through symmetrical transition states [H H H] and with rate constants kn,HH and kH,DH, respectively. The ratio of rate constants, kH,HH/kH,DH> defines a primary hydrogen kinetic isotope effect. More precisely it should be regarded as a primary deuterium kinetic isotope effect because for hydrogen there is also the possibility of a tritium isotope effect. The term primary indicates that bonds at the site of isotopic substitution the isotopic atom are being made or broken in the course of reaction. Within the limits of TST such isotope effects are typically in the range of 4 to 8 (i.e. 4 < kH,HH/kH,DH < 8). [Pg.314]

This reaction proceeds via the transition state illustrated in Fig. 10.2. An Sn2 reaction (second order nucleophilic substitution) in the rate limiting step involves the attack of the nucleophilic reagent on the rear of the (usually carbon) atom to which the leaving group is attached. The rate is thus proportional to both the concentration of nucleophile and substrate and is therefore second order. On the other hand, in an SnI reaction the rate limiting step ordinarily involves the first order formation of an active intermediate (a carbonium ion or partial carbonium ion, for example,) followed by a much more rapid conversion to product. A sampling of a and 3 2° deuterium isotope effects on some SnI and Sn2 solvolysis reactions (i.e. a reaction between the substrate and the solvent medium) is shown in Table 10.2. The... [Pg.320]

In reactions 14.32 and 14.33 the hydrogen atoms are not involved in any bonds that are being made or being broken in the reaction. The isotope effect is therefore referred to as a secondary a-deuterium isotope effect since the position of isotopic substitution is a to the bond being broken in the rate limiting step (see Chapter 10 for discussion of secondary isotope effects). [Pg.437]

The ONSH reaction of the carbanion of 2-phenylpropionitrile (45 c) with nitrobenzene in liquid ammonia at -70 °C involves rate-limiting Carom—H bond breaking, as evidenced by the 9.8 times faster rate than for reaction of the analogous substitution of deuterium in 4-<7-nitrobenzene and perdeuterionitrobenzene. Reactions of the carbanion derived from (45c) with 4-chloro-3-trifluoromethylnitrobenzene and 4-chloronitrobenzene in toluene under phase transfer catalysis has also been studied." ... [Pg.361]

SECONDARY ISOTOPE EFFECTS. Changes in reaction may also result from isotopic substitutions at positions that are immediately adjacent to the reaction center (/.e., the bond broken/made in the chemical reaction under investigation). We deal here only with so-called secondary ce-isotope effects, and we will limit the scope further by considering only a deuterium and a tritium isotope effects on carbon. Isotopic substitution by heavier nuclides will also give rise to a isotope effects, but they are quite small. The magnitudes of the a isotope effects for C—compared to C— as well as for C—compared to C—are also relatively small, frequently necessitating the use of special techniques. [Pg.402]


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See also in sourсe #XX -- [ Pg.45 , Pg.165 ]




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Deuterium substitution

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