Residence times from NMR and ultrasonic relaxation

The residence time found from NMR data is the average lifetime of a water molecule in the primary solvation shell. Estimates of mean lifetimes can be found from the line widths of the signals obtained for both fast and slow exchange. The resonance signal which is required is 

The signals obtained in the NMR study of hydration can be either from proton resonances or from resonances. The latter is preferable for cations since the O is the atom which is coordinated to the cation. Observation of the line width of the signal is easy to detect for paramagnetic ions, not so for diamagnetic ions. Consequently studies are limited to paramagnetic ions and is used for diamagnetic cations. 

If anions are smdied it is the H end of the O-H dipole which is attracted to the anion. Proton resonance will thus correspond to exchange of the water molecule. 

If the resonances are studied over a range of temperatures, then activation parameters can be found for the exchange of water between the solvation shell and the rest of the water. 

A wide variety of lifetimes are found ranging from 25000 ps for a water molecule near Mn to 10 ps for Na+, 5 ps for Cl and 4 ps for 1 . These can be compared with 3 ps for the average lifetime of a water molecule at a given site in the pure liquid. 

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