Water/proton exchange rate

Fig. 1. A Gd(III) complex with one inner sphere water molecule, surrounded by bulk water. Inner sphere proton relaxivity is due to interactions between the Gd electron spin and the water protons on the inner sphere water. Outer sphere relaxivity arises from interactions between the Gd electron spin and bulk water protons. rR stands for the rotational correlation time of the molecule, kex for the water/proton exchange rate and 1/T, 2c for the electron spin relaxation rates of the Gd(III)...

Proton exchange rates in aqueous solutions are enhanced by small amounts (0.5% V/V) of hydrophobic substances (e.g., methanol, dioxane) because of a consequent increase in H-bonded water structure in the hydration shells through which the proton transfer is mediated (9). 

The four examples given above illustrate the effect of water exchange or more exactly proton exchange rate and of the number of water molecules in the first coordination sphere of the paramagnetic metal ion on the water proton relaxation rate. Nonetheless, the relaxivity of a contrast agent can also be strongly... 

The spectrum of an aqueous solution of pymvic acid consists of a total of three bands for the species involved in the equilibrium given by Eq. (1). The methyl protons of pymvic acid give a band A with a chemical shift = 2.6 ppm and the methyl proton band for dihydroxypropanoic acid, denoted band B, occurs at Sg = 1.75 ppm. A third band at a larger 8 value represents the resonances of the carboxyl, hydroxyl, and water protons. This band is a singlet since the proton exchange rate between —COOH, —OH, and H2O environments is very rapid. The positions of all three bands will vary somewhat with the composition of the solution. 

The NMR spectrum of an aqueous A1(C104)3 solution in [Dgjacetone shows nicely the two different signals of bulk water and hydration water in the AP inner shell, even at room temperature [245]. The addition of acetone slows down the proton exchange rate. A primary hydration number of six for Al has been obtained in this way [245]. 

Porphyrin inner proton exchange. Ribo and co-workersreported a study of the effect of steric factors due to bridge substitution on the modulation of proton exchange rates in a water soluble porphyrin, 161. The authors utilized both HSQC and... 

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