Solvation in the Gas Phase

Solvent Molecule Interactions and Heterolytic Organic Reactions 

To a worker in the field of ionic solutions, studies of isolated ion-solvent molecule complexes may have seemed impossible, yet the mass spectrometric investigations of ion cluster reactions and ionic equilibria in the gas phase represent exactly such studies. The mass spectrometric method allows studies of the interactions of inorganic ions such as Na, Li, Cl , NO3 , etc. with a variety of solvent molecules H2O, NH3, CH3OH, etc. However, interactions of a variety of stable organic ions with many organic and inorganic solvents are also possible. The subsequent parts of this section illustrate the application of mass spectrometric ion equilibria studies to ionic solvation. 

Before turning to the task, it would be appropriate to illustrate with one more general example the use to which available gas phase data may be put. The energy change in the gas phase dissociation reaction. 

The energy changes for such gas-phase reactions can be obtained from the heats of formation of the reactants. For example, if one is to calculate the enthalpy changes for the dissociation reactions of water and methanol 

It is found that reaction (27) is more favorable for small n, but that the preferential solvation of the proton by methanol decreases as n increases. At n 9 water and methanol solvate equally well and for n 9 water becomes the better solvent. The changes of energetics in the stepwise solvation of the corresponding negative ions, 

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It has been shown (by running reactions on ions that are solvated in the gas phase) that solvation by even one molecule of solvent can substantially affect the order of basicities. ... 

The extension of analytical mass spectrometry from electron ionization (El) to chemical ionization (Cl) and then to the ion desorption (probably more correctly ion desolvation ) techniques terminating with ES, represents not only an increase of analytical capabilities, but also a broadening of the chemical horizon for the analytical mass spectrometrist. While Cl introduced the necessity for understanding ion—molecule reactions, such as proton transfer and acidities and basicities, the desolvation techniques bring the mass spectrometrist in touch with ions in solution, ion-ligand complexes, and intermediate states of ion solvation in the gas phase. Gas-phase ion chemistry can play a key role in this new interdisciplinary integration. 

You may remember that at the beginning of Chapter 8 we said that the cleavage of H-Cl into H+ and Cl- is possible in solution only because the ions that are formed are solvated in the gas phase, the reaction is endothermic with AG = +1347 kj mol-1, a value so vast that even if the whole universe were made of gaseous HC1 at 0... 

Not only reaetion rates but also the kind of products obtained ean be changed in going from the gas phase to solution. For instance, the reaetion of the stepwise solvated hydroxide ion with aeetonitrile follows three distinct pathways bare and singly-solvated (in the gas phase) as well as bulk-solvated hydroxide ions reaet with aeetonitrile to give three different sets of produets as result of displacement, proton-transfer, and hydrolysis reactions [488] ... 

Hiraoka et al. have also discussed how the results of their measurements on solvation in the gas phase are related to the more usually discussed liquid phase solvation. The first water molecules go onto the ion and are structure-forming. 

Supermolecule model. By a "supermolecule" we imply a model consisting of the solute molecule surrounded by a certain number of solvent molecules. Pair complexes solute-solvent and solvent-solvent may be considered the simplest supermolecules. Since the cost of the supermolecule approach becomes prohibitive as the number of solvent molecules is increased, in most treatments only the first solvation shell is assumed. Such small clusters cannot of course provide a realistic model of a liquid but rather they give us a theoretical picture of what is referred as to "the solvation in the gas phase". As with the approach dealt with in the last paragraph, the ab initio calculation on the supermolecule should be followed by a statistical thermodynamic treatment. The use of the standard statistical thermodynamic is straightforward, in which case the supermolecule approach becomes e-quivalent to treatment of common chemical equilibria dealt with In Section 5.F. The calculations presented in Table 5.17 are just of this... 

Figure 4.2 Schematic representation of solute-solvent clustering in an SCF, eom-pared with liquid-phase solvation and lack of solvation in the gas phase.

Hiraoka, and solvation in the gas phase, 97 -Histefflaffl - shewing-distribtttion of O-Na-O... 

Finally, we consider the relevance of these solvated-ion studies in the gas phase to the corresponding reactions in solution. In the gas phase, the products are predominantly unsolvated in solution, they are completely solvated. In the gas phase, reaction 4 is apparently quenched by solvating the reactant with more than two solvate molecules in solution, the reaction proceeds when the reactants are infinitely solvated. This highlights the importance of the bulk solvent in solution. All pervasive, the bulk solvent can always enable the concerted desolvation of the nucleophile and solvation of the leaving group. In contrast, in the gas phase, the same solvate molecules that are released in the desolvation must be used in the solvation and if the solvate does not transfer, solvation must stop the reaction. In the gas... 

Competitive Solvation in the Gas Phase. by Water and Methanol and by Water and Ammonia Molecules... 

Stace AJ (2002) Metal ion solvation in the gas phase the quest for higher oxidation states. J Phys Chem A 106 7993-6005... 

The differences in the mechanisms in the gas and solution phases necessarily arise from differences in solvation. In the gas phase the nucleophile and electrophile solvate one another prior to reaction, and achieving the correct orientation between the nucleophile and electrophile for backside attack leads to a substantial entropy barrier. The rate constants therefore have little correlation with the enthalpy of the reaction. In solution, however, the electrophile and nucleophile are solvated prior to the reaction, the transition state is solvated during the reaction, and the product is solvated after the reaction. This yields rate constants that correlate with the heat of the reaction. 

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