Solution Complexes with Neutral Molecules

The chemical reactivity of crown-ether complexes with neutral molecules has received little attention. Nakabayashi et al. (1976) have reported crown-ether catalysis in the reaction of thiols with l-chloro-2,4-dinitrobenzene. The catalytic activity was attributed to deprotonation of thiols by dicyclohexyl-18-crown-6 in acetonitrile solution. Blackmer et al. (1978) found that the rate of aquation of the cobalt(III) complex [333] increases on addition of... 

Solution State Complexes with Neutral Molecules... 

The derivation of the Debye-Hiickel equations is not included here, but only the end results. A complete discussion is given by Bull (1964), Chapter 3. There are, however, several basic phenomena that we need to examine. There are three mechanisms which produce ions in water solutions. These are (1) solution of an ionic crystal, (2) oxidation of a metal or reduction of a nonmetal, and (3) ionization of a neutral molecule. Most metals when they ionize give up electrons to an electronegative element so that both acquire the electronic structure of a rare gas. Exceptions of interest in electrode work are iron, copper, silver, mercury, and zinc. In the ionized state, these metals do not acquire the completed outer shell structure of a rare gas and do have residual valences. They are then somewhat unstable and complex with various molecules more easily than do the stable ionized metals with completed shells. This accounts for the poisoning of silver-silver chloride electrodes and p02-measuring electrodes when used in high-protein environments such as blood. 

Metal cations in aqueous solution often form chemical bonds to anions or neutral molecules that have lone pairs of electrons. A silver cation, for example, can associate with two ammonia molecules to form a silver-ammonia complex ... 

There are a number of interferences that can occur in atomic absorption and other flame spectroscopic methods. Anything that decreases the number of neutral atoms in the flame will decrease the absorption signal. Chemical interference is the most commonly encountered example of depression of the absorption signal. Here, the element of interest reacts with an anion in solution or with a gas in the flame to produce a stable compound in the flame. For example, calcium, in the presence of phosphate, will form the stable pyrophosphate molecule. Refractory elements will combine with 0 or OH radicals in the flame to produce stable monoxides and hydroxides. Fortunately, most of these chemical interferences can be avoided by adding an appropriate reagent or by using a hotter flame. The phosphate interferences, for example, can be eliminated by adding 1 % strontium chloride or lanthanum chloride to the solution. The strontium or lanthanum preferentially combines with the phosphate to prevent its reaction with the calcium. Or, EDTA can be added to complex the calcium and prevent its combination with the phosphate. 

It should be pointed out that one cannot expect quantitatively correct data from such calculations. Clearly, the complexes considered do not appropriately represent real solutions. Most of the results obtained could have been guessed equally well by chemical experience and intuition anyway we expect ions to be more strongly hydrated than neutral molecules. In the actual calculations, the method employed is known to overemphasize the expected effects. The merits of attempts like the ones mentioned axe therefore not to be found in the realization of quantitative results, but verify that our expectations are definitely reproducable in terms of quantum chemical data, and they demonstrate how such calculations could be made. There have also been attempts to describe reactions of solvated molecules by an MO theoretical treatment for the two reaction partners, with inclusion of the solvent by representing it as point dipoles. As a first step, Yamabe et al. 186> performed ab initio calculations on the complex NH3.HF, solvating each of the partners by just one point dipole. A study of MO s of the interacting complex with and without dipoles shows that the latter has a favorable effect on the proceeding of the reaction. 

As illustrated in the previous sections, an array of MS methodologies are nowadays available to enantiodiscriminate chiral molecules by complexation with chiral selectors and to investigate the intrinsic factors affecting their stability in the gas phase. The advantages connected with studying enantioselectivity in simple ionic and neutral complexes in the gas phase instead of in complicated associations in solution come from the possibility to make precise statements upon the nature of... 

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