Line broadening exchangeable protons

The NMR investigation is presented in two parts the first covers i3C 99rpc ancj i5N studies where protonations of the dioxo complexes induce chemical shift changes but where line-broadening effects due to exchange are not observed. The second part covers 170 studies which produce more complicated spectra due to protonation and simultaneous exchange with bulk water. 

Shortening of the life time due to reaction leads to an increase in line width. The increase in line width will not continue indefinitely. As the rate of exchange of protons rises, two protons in two different environments do not behave as independent and, therefore, the lines broaden and finally merge. If average time spent by a given proton is 0 and if two lines are separated by Avab then at the point where the lines coalesce... 

The splitting patterns due to ligand protons and nuclei differ in the free ligand and the complexes. There is rapid intramolecular exchange of between two possible sites (shown in 13) and this leads to line broadening which can be used to measure the exchange rate constants (5 x 10 s to 2 x 10 s ). See Table 7.4. 

Proton exchange between nitrogen atoms in the same spin state does not affect line shape and since there are three spin states for nitrogen, only 2/3 of the proton transfers in reaction (c) contribute to line broadening. 

Planar-tetrahedral equilibria of nickel(II) complexes were the first spin-equilibria for which dynamics were measured in solution. It had been known that such complexes were in relatively rapid equilibrium in solution at room temperature, for their proton NMR spectra were exchange averaged, rather than a superposition of the spectra of the diamagnetic and paramagnetic species. At low temperatures, however, for certain dihalodiphosphine complexes, it is possible to slow the exchange and observe separate resonances for the two species. On warming the lines broaden and coalesce and kinetics parameters can be obtained. Two research groups reported such results almost simultaneously in 1970 (99,129). Their results and others reported subsequently are summarized in Table V. 

This means that the electron goes from naphthalene A with a particular set of +V2, —V2 proton nuclei to naphthalene B with a different set. The result is that the lines broaden and, if the exchange is very fast, the splitting vanishes. Because the splittings are about 5 gauss (14 MHz), the mean lifetime before exchange has to be about 10-8 sec or less to obscure the splitting (see Sections 27-1 and 27-2). 

The slow keto enol proton transfer means separate signals in the NMR spectrum for the tautomers. The second exchange (ii) is responsible for the line broadening and loss of multiplet structure of the NMR signal of the enol proton. The third type of proton motion, (iii), is not resolvable by NMR so that ways around this have been sought in order to obtain a time-averaged analysis of the proton s location in the cis enol. 

The rates of proton-exchange involving phosphate ions in aqueous solutions enriched with oxygen-17 have been examined by an nmr line-broadening technique . The process appears to be second order in phosphate species and is attributed to the reaction... 

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