Hydration in the Gas Phase

Now that some methods for investigating the structure of the ion-solvent complex in solution have been described, it is time to learn systematically what is known about it. One can start by considering systems that avoid the complexity of liquid water. By varying the partial pressure of water vapor while keeping it low (0.1-lOkPa), it is possible to find the equilibrium constant between water vapor and the entities represented by a number of ion-solvent aggregates, M(H20), in the gas phase (Kebarle and GodMe, 1968). 

if the equilibrium constant for one of these equilibria is known, AG°can be derived from the well-known thermodynamic relation K=exp -AG°/RT). If K (and hence AG°) is known as a function of T, AH° can be obtained from the slope of an In AT - l/Tplot and AS° from the intercept. 

The seminal work in this field was carried out by Kebarle and it is surprising to note the gap of 30 years between the foundation paper hy Bernal and Fowler on solvation in solution and the first examination of the simpler process of hydration in the gas phase. A series of plots showing the concentrations of various hydrate complexes for NafHjO) as a function of the total pressure of water vapor is given in Fig. 2.29.  

an interesting thing can be done with the AH values obtained as indicated earlier. One takes the best estimate available for the primary hydration number in solution (see, e.g.. Tables 2.7 and 2.11). One then calculates the corresponding heat of hydration in the gas phase for this number and compares it with the corresponding individual heat of hydration of the ion in solution. The difference should give the residual amount of interaction heat outside the first layer (because in the gas phase there is no outside the fu-st laya ). 

The hydration energy for the outer shell turns out to be 15% of the whole for cations and about 30% for anions. Thus, in hydration of the alkali and halide ions. 

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Janin,1995). The procedure involves two steps. The first step assembles the complexes in the gas phase in the second step, the system is transferred into aqueous solution and hydrated. In the gas phase, van der Waals and electrostatic interactions are formed. Their energy, evaluated... 

Hydration and etherification. The direct hydration of ethylene has been discussed (section 11.5). Propylene can also be hydrated in the gas phase over supported phosphoric acid (c. 180°C), or with an ion-exchange resin catalyst at about 140°C, with liquid water and gaseous propylene. The use of an ion-exchange resin as a catalyst has also been commercialized for the hydration of n-butenes, though the sulphuric acid two-stage process still predominates. The use of very weak acid systems at much higher temperatures (> 250 "C) has also been studied. 

The relative acidities in the gas phase can be detennined from ab initio or molecular orbital calculations while differences in the free energies of hydration of the acids and the cations are obtained from FEP sunulations in which FIA and A are mutated into FIB and B A respectively. 

Pure HCIO4 is a colourless mobile, shock-sensitive, liquid d(25°) 1.761 gem" . At least 6 hydrates are known (Table 17.23). The structure of HCIO4. as determined by electron diffraction in the gas phase, is as shown in Pig, 17,20. This... 

These special features are explained by an interaction between the proton and one of the water molecules, which is not merely electrostatic but also covalent. This yields a new chemical species, the hydroxonium ion, HjO. The existence of such ions was demonstrated in the gas phase by mass spectrometry and in the solid phase by X-ray diffraction and nuclear magnetic resonance. The H -H20 bond has an energy of 712kJ/mol, which is almost two-thirds of the total proton hydration energy. 

The hydrated sulfate S04(H20)2 could be produced by electrospray in the gas phase,80 but neither the triply charged orthophosphate P04 nor the doubly charged HOPOj" were observed as the naked ion or the hydrate.81 CID of the hydrated sulfate led to simple desolvation down to n = r = 4. The decomposition of the r = 4 hydrate led to charge reduction by intracluster proton transfer ... 

The gas-phase lifetime of N20- is 10-3 s in alkaline solutions, it is still >10-8 s. Under suitable conditions, N20- may react with solutes, including N20. The hydrated electron reacts very quickly with NO (see Table 6.6). The rate is about three times that of diffusion control, suggesting some faster process such as tunneling. NO has an electron affinity in the gas phase enhanced upon solvation. The free energy change of the reaction NO + eh (NO-)aq is estimated to be --50 Kcal/mole. Both N02- and N03- react with eh at a nearly diffusion-controlled rate. The intermediate product in the first reaction, N02-, generates NO and... 

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