Counterion hydrogen-bonding nature

Nmr methods have unrivalled potential to explore interfaces, as this account has striven to show. We have been able to determine the mobility of hydrated sodium cations at the interface of the Ecca Gum BP montmorillonite, as 8.2 ns. We have been able to measure the translational mobility of water molecules at the interface, their diffusion coefficient is 1.6 10 15 m2.s. We have been able to determine also the rotational mobility of these water adsorbate molecules, it is associated to a reorientational correlation time of 1.6 ns. Furthermore, we could show the switch in preferred reorientation with the nature of the interlayer counterions, these water molecules at the interface tumbling about either the hydrogen bond to the anionic surface or around the electrostatic bond to the metallic cation they bear on their back. And we have been able to achieve the orientation of the Ecca Gum BP tactoids in the strong magnetic field of the nmr spectometer. 

There is a wealth of information on gas phase ion thermodynamics because of the power of mass spectrometry and ion cyclotron resonance techniques. Before we discuss carboca-tion, and subsequently carbanion, stabilities, keep in mind that ionic structures are much more sensitive to environmental influences than radicals. The polarity, nucleophilicity, and hydrogen bonding ability of the solvent are important influences, as are the nature of the counterion. As such, thermodynamic information is a less reliable predictor of reactivity for carbocations and carbanions than it is for radicals. Nevertheless, gas phase thermodynamics is an excellent starting point, defining the intrinsic stabilities of ions. Any deviation in trends between gas phase and solution studies is likely a consequence of solvation effects, a theme we will visit many times throughout this book. 

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