Protons fast exchange

Tile tautomeric equilibrium of pyrimido[4,5-h][4, 5 -e]thiazine 157 was studied in DMSO-dg by NMR spectroscopy (92CHE1219). Based on and NMR chemical shifts, fast proton exchange was concluded to occur between 157b and 157c. Monoprotonation of 157 has been assumed to form... 

Two cryptands [6] show special behaviour [2.1.1] and [l.l.lj. In contrast to the larger cryptands where a fast proton exchange takes place between... 

The pH dependence of the 13C spectra for the rhenium(V) system was studied in the pH range 0-8, and showed only one signal, resulting from the fast proton exchange between the dioxo, hydroxo oxo and aqua oxo species. Thus 13C NMR provides an excellent means for... 

Initiation consists of the reaction of the A -acyllactam with activated monomer followed by fast proton exchange with monomer... 

For die same reason it is generally not feasible to cany out a crossed-Claisen reaction between die enolate of one ester and a second ester which has a protons. This is due to the fact diat if nucleophilic addition to die carbonyl group is not fast, proton exchange can occur, giving a mixture of etiolates and thus a mixture... 

According to a recent ab initio molecular dynamic simulation study639 formation of formyl cation 328 is optimally favored in the 1 1 HF-SbF5 solution. No evidence, however, was found for the formation of the isoformyl cation 329 and diprotonated CO. The fast proton exchange observed earlier was suggested to occur between HCO+ and HF(HF) or SbF6. ... 

The first direct NMR spectroscopic evidence for the existence of primary alkylox-onium ions (protonated alcohols) in superacid solutions was found in 1961 by MacLean and Mackor.50 The NMR spectrum of ethanol in HF-BF3 solution at —70°C gave a well-resolved triplet at about 81 H 9.90 for the protons on oxygen coupled to the methylene protons. In HS03F this fine structure is not observed, even at 95°C, due to the fast proton exchange.51... 

It has been reported in a follow-up study,324 that the product 3-arylindenones underwent double protonation in strong superacids (HSO3F, CF3SO3H) and the stability of formed species allowed their observation by NMR spectroscopy at room temperature. Although fast proton exchange prevented to observe (9-protonation, H and 13C chemical shifts unequivocally showed the presence of dication 93. 

There is one special experimental indication of this mechanism. If the reaction is carried out in a deuterated solvent (D20 instead of H20) the rate of the reaction increases. This is a solvent isotope effect rather than a kinetic isotope effect and needs some explanation. If you examine the three examples of SAC in the previous pages you will see that they share these characteristics a fast proton exchange is followed by a rate-determining step that does not involve the making or breaking of any bonds to hydrogen. In general terms ... 

Deuterium incorporation in the aromatic hydrocarbon/amine systems is usually considered as involving the radical anion of the aromatic hydrocarbon. CIDNP studies (Gardini and Bargon, 1980) have drawn attention to the fact that trialkylaminium ions undergo fast proton exchange and hence... 

Chemical shifts of protons in secondary oxonium ions differ substantially ftom chemical shifts of protons in the primary hydroxy groups. One can expect a fast proton exchange between these two spwies. However, if the individual chemical shifts are known, then the observed chemical drift (due to exchange) permits the determination of the actual proportions of the condary oxonium ion IV.l and tertiary oxonium ion IV.4. 

At the very beginning of polymerization, the linear monomer units are incorporated into the polymer in reaction (24) which is followed by the fast proton exchange (25) regenerating lactam anions. The proton exchange between pyrrolidone and its anion... 

IP Gragerov, VK Pogorelyj, IF Franchuk, Hydrogen Bond and Fast Proton Exchange. Kiev Naukova Dumka, 1978, pp 6-8,... 

The polymerization occurs by the activated monomer mechanism, which supposes a two-step mechanism involving the acylation of the lactam anion (NaL) by the AT-acyllactam end-group followed by a fast proton-exchange with the monomer. In bulk polymerization, the preformed AT-acyllactams or their precursors (so-called CIs) are introduced into the system in order to avoid the slow initiation step due to the absence of AT-acyllactam groups at the beginning of polymerization ... 

Platinum complexes (78) have been obtained by transmetallation of K[Ph2P(E)NP(E )PPh2] with cw-PtCl2(HL)2. P NMR shows the two P centres to be identical, pointing to a fast proton exchange between the nitrogen... 

In 2003, Watanabe reported the development of proton-conducting Brpnsted acid-base ILs [32], With the help of the PSE pulse sequence they could demonstrate fast proton-exchange processes between protonated imidazolium cations and imidazole. Recently, Judeinstein also published two papers on the investigation of proton-conducting ILs, here based on amines and perfluorinated acids [33,34], With a combination of diffusion NMR and varios NMR correlation methods, interionic spatial correlations have been obtained. Additionally, three different types of ion association were found for three different ILs (a) mostly associated ion-pairs, (b) dissociated ions and (c) fully dissociated protons (Fig. 4). 

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