Deuterium exchange experiments

Protonated methanes and their homologues and derivatives are experimentally indicated in superacidic chemistry by hydrogen-deuterium exchange experiments, as well as by core electron (ESCA) spectroscopy of their frozen matrixes. Some of their derivatives could even be isolated as crystalline compounds. In recent years, Schmidbaur has pre-... 

Comparison of UV data for 3-aminoisothiazoles with those of reference compounds confirms that they exist in the 3-amino form. A more recent investigation of 4-aminoisothiazole (76MI41701) using deuterium exchange experiments of the type described in Section 4.01.5.2, and analysis of the symmetric and antisymmetric NH2 stretching frequencies in its IR spectrum, show that this compound also exists in the 4-amino form. 

The finding that thiamine, and even simple thiazolium ring derivatives, can perform many reactions in the absence of the host apoenzyme has allowed detailed analyses of its chemistry [33, 34]. In 1958 Breslow first proposed a mechanism for thiamine catalysis to this day, this mechanism remains as the generally accepted model [35]. NMR deuterium exchange experiments were enlisted to show that the thiazolium C2-proton of thiamine was exchangeable, suggesting that a carbanion zwitterion could be formed at that center. This nucleophilic carbanion was proposed to interact with sites in the substrates. The thiazolium thus acts as an electron sink to stabilize a carbonyl carbanion generated by deprotonation of an aldehydic carbon or decarboxylation of an a-keto acid. The nucleophilic carbonyl equivalent could then react with other electro-... 

Ullmann and Buncel attempted to deprotonate A -carene-2,5-dione (4) and the benzannelated derivative 5 in the hope of generating the possibly aromatic dianions 6 and 7. However, the dianions (and the monoanions) could not be isolated. On the grounds of deuterium exchange experiments, the strain energy of benzocyclopropene (1) was estimated to at least 45.5 kcal/mol higher than that of cyclopropane. 

It has been concluded from deuterium exchange experiments, using ethylene and heavy water, that the addition of an adsorbed proton to adsorbed ethylene is the actual rate-determining step. It can be seen that the two schemes differ, mainly in that the latter includes dissociative adsorption of water on the surface of the catalyst and does not specify the adsorption of ethylene, but they are consistent in that they assume the formation of a carbonium ion as the rate-determining step. 

Results of semiempirical calculations and deuterium exchange experiments indicate that the succinyl transfer from aryl enol succinates to the enolates of aryl ketones occurs by an addition-elimination mechanism and does not involve a retro-ene process.38... 

We have examined the X-ray results from various oriented curdlan gels and report our findings and calculations on a number of models. In addition we have investigated the behaviour of curdlan gels in a series of deuterium exchange experiments monitored with infra-red spectroscopy. 

This section is not complete without mentioning the hydrogen-deuterium exchange experiment by Doehring and Hoffmann (12), which indicated an enhanced acidity of the hydrogens in a tetramethylstibonium salt. In the corresponding equilibrium, stibonium ylides play the role of strong bases ... 

Overall, from experimental and calculated isotope effects combined with deuterium exchange experiment was concluded that the key selectivitydetermining step is a nucleophilic displacement on a if-benzyl complex. 

Hydrogen-deuterium exchange experiments have been used extensively to study both structural parameters and dynamic and mechanistic aspects. They are utilized in conjunction with both NMR and IR studies. Mass spectrometry, with its high sensitivity and resolution, is ideally suited for isotopic exchange studies. 

Figure 20 Example of hydrogen-deuterium exchange experiment monitored by HSQC. A portion of the HSQC spectrum of...

Recent work [log] casts some doubt on the generality of this interpretation. Evidence from deuterium-exchange experiments on butan-2-one indicates that the transition state for enolisation has little if any carbanion character that weak bases, like acids, favour formation of the more substituted enol, and that even the fairly strong base DO" caused deute-... 

Prolonged treatment with a strong base e.g. potassium tert-butoxide in tert-butanol) has the effect of permitting equilibration of the end anions, so that the A > -enolate becomes the main product under thermodynamic control. This is apparent in the methylation of the enolate anion at C(4) (see p. 168). Under acidic conditions equilibration proceeds much more rapidly, and acid catalysed substitution via enols occurs almost entirely at C(6) the transient intervention of a small proportion of the A -enol under acidic conditions was detected only by short-term deuterium-exchange experiments [no]. 

A final piece of evidence comes from deuterium-exchange experiments. If an aldehyde or ketone is treated with DgO instead of HsO", the acidic a hydrogens are replaced by deuterium. For a given ketone, the rate of deuterium exchange is identical to the rate of halogenation, indicating that the same intermediate is involved in both processes. That common intermediate can only be an enol (see the reaction mechanism at the top of p. 909). 

Figure 3.17 Differentiation of two isobaric compounds with the same aglycone moiety by deuterium exchange experiment positive-ion ESI mass spectrum of Mv-3-0(6-0-caffeoyl)monoglucoside (above) and of Mv-3,5-0-diglucoside (below) dissolved in D20. (Reprinted from American Journal of Enology and Viticulture 51, Favretto and Flamini, Copyright 2000)...

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