Gas-Phase Solvation

Most of the methods for estimating reaction enthalpies are applicable only to the gas phase. Solvation enthalpy data are thus particularly important because they allow gas-phase estimates to be extended to reactions in solution—which is the most common medium for reactions of practical interest. However, solvation enthalpies are not very abundant and must often be estimated. Unfortunately, this can be a difficult exercise, especially when A is a solid, because sublimation enthalpies are scarce and hard to estimate. Thus, ASU, H°(A) is usually the unknown term in equation 2.44. The solution enthalpy term, Asi 7/°(A), is generally small and can often be predicted—or determined with a calorimeter. 

The important and stimulating contributions of Kebarle and co-workers 119 14 > provide most of the data on gas-phase solvation. Several kinds of high pressure mass spectrometers have been constructed, using a-particles 121>, proton- 123>, and electron beams 144> or thermionic sources 128> as primary high-pressure ion sources. Once the solute A has been produced in the reaction chamber in the presence of solvent vapor (in the torr region), it starts to react with the solvent molecules to yield clusters of different sizes. The equilibrium concentrations of the clusters are reached within a short time, depending on the kinetic data for the... 

Tandem mass spectrometry H6,i46) both stationary 116> and flowing afterglow-methods 118,147) and drift tube techniques U6> have also been applied to some of the clustering reactions. Results for the gas-phase solvation of H+ by H2O and NH3 generally agree well with the values obtained by high pressure mass spectro-metric observations 148). 

Systems which might be of interest for molecular solvation, and which have been investigated by various techniques are indicated in Table 7. Gas-phase hydration enthalpies for the ions Pb+ X58) and Bi+ 159> are also given in Table 7. In their studies, Tang and Castleman iss.is ) used an apparatus similar to that used by Kebarle and co-workers. Tantalizing as the existence of data for these ions might be for quantum chemists, who would prefer to know more about small ions like Be++, it allows nevertheless the optimistic conclusion that a remarkable increase of activities in the field of gas-phase solvation can be expected in the near future. One should bear in mind that the experimental techniques were introduced only a few years ago. Probably very soon theoreticians will have at their disposal experimented reference data for a lot of interesting systems. 

We shall now turn our attention to the results obtained for the hydration of alkali- 128,130,isi, 158) and halide ions 135,131,133,139 which have been the focus of all kinds of theoretical calculations. Some features are clearly evident from the gas-phase solvation data (Table 8). 

Table 7. Survey of experimental literature pertinent to thermodynamical data for gas-phase solvation...

Table 9. Comparison of experimental gas-phase solvation enthalpies for the ions H+, F, and Cl- by different solvents.

Table 10. Thermodynamic data for individual clustering steps in derived by high pressure mass spectrometry gas-phase solvation of H+, ...

Gas-phase solvation has so far given only very indirect evidence concerning the structure and details of molecular interactions in solvation complexes. Complex geometries and force constants, which are frequently subjects of theoretical calculations, must therefore be compared with solution properties, however, the relevant results are obscured by influences arising from changes in the bulk liquid or by the dynamic nature of the solvation shells. With few exceptions, structural information from solutions cannot be adequately resolved to yield more than a semiquantitative picture of individual molecular interactions. The concepts used to convert the complex experimental results to information for structural models are often those of solvation numbers 33>, and of structure-making or structure-... 

Fig. 8. Comparison of total single-ion solvation enthalpies and gas-phase solvation...

These results of Narten et al. 175> are an excellent basis for a recapitulation of the preceding short account on the status of experiments. We shall first derive a picture of ionic solutions and the connection lines to gas-phase solvation and to molecular theories in a speculative manner, which must not be taken really literally, but should be considered to be an artificially optimistic stimulus for a concerted effort of theory and experiment. 

Obviously this picture might be supported and supplemented by according data from different experimental investigations, or it might be modified to fit these data. Interactions within the basic hydrated structures, as well as their energetics, are obtainable from gas-phase solvation experiments or from accurate MO calculations. For the simulation of real solutions, dynamic calculations will be inevitable. There is, however, a demand for acceptable effective potentials to be used in molecular dynamics, or in Monte Carlo calculations. 

In conclusion of this short account on experiments, which is clearly far from complete, detailed structural data for solutions will be available in the near future. They may serve well to support theoretical calculations of solvation processes and to present challenges for theoretical considerations, which will in any case have to be dynamic ones. Data which may be compared quantitatively with molecular calculations will, however, have to come from gas-phase solvation experiments. There already exists a great variety of according data and their number will certainly increase further. 

In Cl plasma, all the ions are liable to associate with polar molecules to form adducts, a kind of gas-phase solvation. The process is favoured by the possible formation of hydrogen bonds. For the adduct to be stable, the excess energy must be eliminated, a process which requires a collision with a third partner. The reaction rate equation observed in the formation of these adducts is indeed third order. Ions resulting from the association of a reagent gas molecule G with a protonated molecular ion MH+ or with a fragment ion F+, of aprotonated molecular ion MH+ with a neutral molecule, and so on, are often found in Cl spectra. Every ion in the plasma may become associated with either a sample molecule or a reagent gas molecule. Some of these ions are useful in the confirmation of the molecular mass, such as... 

Moreover, ion-molecule adduct formation is observed in the case of polar molecules, a type of gas-phase solvation, for example... 

In conclusion, these gas-phase measurements provide new elues to the role of solvation in ion-moleeule reaetions. For the first time, it is possible to study intrinsie reactivities and the extent to which the properties of gas-phase ion-moleeule reaetions relate to those of the eorresponding reactions in solution. It is clear, however, that gas-phase solvated-ion/moleeule reaetions in which solvent moleeules are transferred into the intermediate elusters by the nucleophile cannot be exaet duplieates of solvated-ion/ molecule reactions in solution in which solvated reactants exchange solvent molecules with the surrounding bulk solvent [743]. For a selection of more recent experimental [772] and theoretical studies of Sn2 reactions in gas phase and solution by classical trajectory simulations [773], molecular dynamics simulations [774, 775], ab initio molecular orbital calculations [776, 777], and density functional theory calculations [778, 779], see the references given. For studies of reactions other than Sn2 ion-molecule processes in the gas phase and in solution, see reviews [780, 781]. 

Takashima, K. Riveros, J. M. Gas-phase solvated negative ions, Mass Spectrom. Rev. 1998,17,409-430. 

Riveros, J. M. Ingemann, S. Nibbering, N. M. M. Formation of gas phase solvated bromine and iodine anions in ion/molecule reactions of halobenzenes. Revised heat of... 

Obtaining the individual properties of ions with solvation numbers from measurements of ionic vibration potentials and partial molar volumes is not necessary in the study of gas phase solvation (Section 2.13), where the individual heats of certain hydrated entities can be obtained from mass spectroscopy measurements. One injects a spray of the solution under study into a mass spectrometer and investigates the time of flight, thus leading to a determination of the total mass of individual ions and adherent water molecules. 

Generally, the competitive gas-phase solvation of the proton seems to favor H2O in the outer shell of the cluster. This is probably due to the strong hydrogen bonding of H2O, which is also responsible for the large heat of vaporization of bulk water. 

Nmr and gas-phase solvation techniques are especially useful in elucidating interaction phenomena. 

In addition, ion-molecule adducts of polar molecules are frequently observed in the Cl mass spectra. This type of reaction can be regarded as gas-phase solvation ... 

Actually, the reaction may be influenced by environmental effects which are comparable with gas phase solvation by further infiltrated molecules. Furthermore, a direct (field effects) and/or indirect (mediated by the associates and other adsorbates) interaction of the adsorbat with other surface centres will occur. This can theoretically be treated and interpreted on the basis of an extended supermolecule approach. 

For alkali metal cations—with the one exception of the Li" ion, whose solvation is a mix between electrostatic and covalent interactions—classical electrostatic calculations can account quite well for the observed enthalpies of gas-phase solvation. For instance, hydration can be split up into energy contributions from ion-dipole induced dipole dispersion (E g), and repulsive (E - p) contributions (18) (Table 8). Clearly, the dominant term is the Coulombic attraction between the cation and the water dipoles, which decreases in magnitude as the ion gets bigger. 

Halide ions in water and acetonitrile Gas phase solvation 179... 

Davidson, W.R. and Kebarle, P. (1976) Ionic solvation by aprotic solvents. Gas phase solvation of the alkali ions by acetonitrile. J. Am. Chem. Soc., 98, 6125-6133. 

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