Ion-molecule clustering

We were also curious to extend the Marcus relation a bit further to see if we could somehow eliminate the need to calculate two very expensive ion-molecule clusters for each crossreaction required to obtain AE. As Professor Brauman pointed out, the energy difference between the transition state and the separated reactants can be used as a measure of the overall efficiency of the reaction for structurally similar reactions. 

Fig. 5-4. Schematic one-dimensional enthalpy diagram for the exothermic bimolecular Finkelstein reaction Cl -I- CFI3—Br Cl—CH3 -I- Br in the gas phase and in aqueous solution [469, 474, 476]. Ordinate standard molar enthalpies oi (a) the reactants, (b, d) loose ion-molecule clusters held together by ion-dipole and ion-induced dipole forces, (c) the activated complex, and (e) the products. Abscissa not defined, expresses only the sequence of (a). ..(e) as they occur in the chemical reaction.

Blades AT, Jayaweera P, Ikonomou MG and Kebarle P (1990) Ion molecule clusters involving doubly charged metal ions. Int J Mass Spectrom Ion Processes 102 251-258. 

Good A. Third-order ion-molecule clustering reactions. Chem Rev. 1975 75 561-83. 

Figure 5.56 An ion mobility spectrometer and mobility spectrum. Ion molecule clusters move toward the detector and separate on the basis of size and charge. Ion drift time is recorded in milliseconds (ms). Image courtesy of Dr. G. A. Eiceman, New Mexico State University. 

Ion mobility spectrometry Gas phase separation of ion/molecule clusters at atmospheric pressure generi-cally gas phase electrophoresis. 

The SPC/E model approximates many-body effects m liquid water and corresponds to a molecular dipole moment of 2.35 Debye (D) compared to the actual dipole moment of 1.85 D for an isolated water molecule. The model reproduces the diflfiision coefficient and themiodynamics properties at ambient temperatures to within a few per cent, and the critical parameters (see below) are predicted to within 15%. The same model potential has been extended to include the interactions between ions and water by fitting the parameters to the hydration energies of small ion-water clusters. The parameters for the ion-water and water-water interactions in the SPC/E model are given in table A2.3.2. 

Collision-induced dissociation mass spectrum of tire proton-bound dimer of isopropanol [(CH2)2CHOH]2H. The mJz 121 ions were first isolated in the trap, followed by resonant excitation of their trajectories to produce CID. Fragment ions include water loss mJz 103), loss of isopropanol mJz 61) and loss of 42 anui mJz 79). (b) Ion-molecule reactions in an ion trap. In this example the mJz 103 ion was first isolated and then resonantly excited in the trap. Endothennic reaction with water inside the trap produces the proton-bound cluster at mJz 121, while CID produces the fragment with mJz 61. 

The third step is solvation of the ions by solvent molecules. Water molecules cluster around each ion, oriented to give attractive ion-dipole interactions. This step releases energy. Although each individual ion-dipole interaction is weak, each ion forms from four to eight such interactions, depending on the size of the ion and the concentration and temperature of the solution. Taken together, the vast number of ion-dipole interactions results in a substantial release of energy. 

The different hydration numbers can have important effects on the solution behaviour of ions. For example, the sodium ion in ionic crystals has a mean radius of 0 095 nm, whereas the potassium ion has a mean radius of 0133 nm. In aqueous solution, these relative sizes are reversed, since the three water molecules clustered around the Na ion give it a radius of 0-24 nm, while the two water molecules around give it a radius of only 017 nm (Moore, 1972). The presence of ions dissolved in water alters the translational freedom of certain molecules and has the effect of considerably modifying both the properties and structure of water in these solutions (Robinson Stokes, 1955). 

A second major class of ion-molecule reactions that is relatively poorly studied consists of systems involving very unsaturated hydrocarbon neutrals, especially radicals. The unsaturated nature of the organic chemistry in interstellar clouds leads to sizeable abundances of very unsaturated hydrocarbons such as the polyacetylenes HC H, the carbenes H2C , the radicals C H, and the clusters Cn. Although some work has been done on the chemistry of such species, much of the relevant ion-molecule chemistry involving ions such as C+, CH3, and even C2H2 must be guessed at from generalizations based on a small number of studied systems. 

A much more detailed and time-dependent study of complex hydrocarbon and carbon cluster formation has been prepared by Bettens and Herbst,83 84 who considered the detailed growth of unsaturated hydrocarbons and clusters via ion-molecule and neutral-neutral processes under the conditions of both dense and diffuse interstellar clouds. In order to include molecules up to 64 carbon atoms in size, these authors increased the size of their gas-phase model to include approximately 10,000reactions. The products of many of the unstudied reactions have been estimated via simplified statistical (RRKM) calculations coupled with ab initio and semiempirical energy calculations. The simplified RRKM approach posits a transition state between complex and products even when no obvious potential barrier... 

Based on the AID mechanism, the neutral clusters are ionized through multiphoton ionization and the protonated clusters are formed through the intracluster ion-molecule reaction between NH3 and the neighboring NH3 species as expressed in the following ... 

An example of the usefulness of the reflectron technique discussed earlier in this chapter is evident for the case of ammonia clusters in Figures 3b and c. Upon ionization, ammonia undergoes an internal ion-molecule reaction leading to protonated cluster ions, and concomitant evaporative unimolecular dissociation. This can be viewed in the context of equations 7-9 and the following ... 

---