Complex formation in condensed phases

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

Observations of alkali-metal ion adducts of the type [M+Li]+ [M+Na]+ etc. are common in the desorption ionization (DI) mass spectra of a variety of polar molecules. In fact, alkali-metal ion association reactions are observed with FD ionization, FAB ionization, Cf plasma desorption (PD), secondary ion mass spectrometry (SIMS), MALDI, and ESI. Ion yields can be greatly enhanced by addition of alkali-metal salts to the sample. Particularly for the MALDI analysis of synthetic polymers, metal cations are often intentionally added to enhance signals. A qualitative description of the current understanding of formation mechanism of alkali-metal ion complexes from the condensed phase was presented [75]. Knowledge of the ionization mechanisms is important and helpful from the perspective of increasing the analytical utility of the method. 

The present chapter is concerned primarily with measured molecular structural effects on reactions 2 and 4 in the gas phase. These have been obtained only very recently from direct equilibrium-constant determinations. Work in this area is still in a very active state, so that this chapter serves as a preliminary progress report. Useful comparison can now be made of structural effects on equation 2 with the following related topics (1) proton-transfer equilibria in condensed phases (2) other Lewis acid-base equilibria in the gas phase (3) theoretical calculations of proton-transfer energetics (4) hydrogen-atom transfer equilibria between cation radicals and saturated cations (5) hydrogen-bonded complex formation, in hydrocarbon solvents and (6) gas-phase equilibria for attachment of neutral molecules to cations and anions. Each of these topics is considered at least briefly. 

A reaction mechanism is a series of simple molecular processes, such as the Zeldovich mechanism, that lead to the formation of the product. As with the empirical rate law, the reaction mechanism must be determined experimentally. The process of assembling individual molecular steps to describe complex reactions has probably enjoyed its greatest success for gas phase reactions in the atmosphere. In the condensed phase, molecules spend a substantial fraction of the time in association with other molecules and it has proved difficult to characterize these associations. Once the mecharrism is known, however, the rate law can be determined directly from the chemical equations for the individual molecular steps. Several examples are given below. 

Condensed-phase flame retardant mechanisms, 44 484—485 Condensed phosphates, 18 841-852 colloidal properties of, 48 851 complex ion formation in,... 

The same is true for the chiral polysiloxanes described here. Their use as stationary phases in gas chromatography allows the calculation of the differences in enthalpy and entropy for the formation of the diaste-reomeric association complexes between chiral receptor and two enantiomers from relative retention time over a wide temperature range. Only the minute amounts of the polysiloxanes required for coating of a glas capillary are necessary for such determinations. From these numbers further conclusions are drawn on the stereochemical and environmental properties required for designing systems of high enantio-selectivity in condensed liquid systems. 

Table 8.16 Standard molal Gibbs free energies of formation from the elements for aqueous ions and complexes and condensed phases, partly adopted in constructing the Eh-pH diagrams in figure 8.21. Data in kcal/mole. Values in parentheses Shock and Helgeson s (1988) tabulation. Sources of data (1) Wagman et al. (1982) (2) Garrels and Christ (1965) (3) Pourbaix (1966) (4) Berner (1971)...

Table 8.17 Main predominance limits of aqueous complexes and saturation limits between solutes and condensed phases in iron-bearing aqueous solutions (see figure 8.22). Standard state Gibbs free energies of formation of species are listed in table 8.18. (c) = crystalline ... 

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