Structural Data in Solution

The salts of carbonyl metalates arc generally soluble only in polar solvents, in which absorptions are broader due to the dipole interactions of the solvent. The spectra are also often considerably dependent on the particular cation and solvent, due to the formation of ion pairs. The largest differences have been observed on going from alkali salts to salts of the large tetra-substituted ammonium and phosphonium cations the latter generally provide simpler spectra that indicate minor formation of ion pairs. A similar trend is observed in going from less polar (e.g., THF) to more polar solvents (e.g., CH3CN). 

In the carbonylmetalates the main absorption of terminal carbonyl groups is fairly well related to the ratio of the number of metal atoms to 

Introduction of a carbide atom results in a lowering of the stretching absorptions of the terminal groups (e.g., [Co6(CO)15]2- 1980 cm-1 and [Co6(CO)i3C]2- 1975 cm-1) in agreement with a positive character for this central heteroatom 10). Sometimes, however, the opposite shift is observed (compare [Rh6(CO)ir,]2 and f Rh6(CO)i5C]2- in Fig. 12). 

Generally absorptions at about 180 to 210 ppm [low field from tetra-methylsilane (TMS) ] are characteristic of terminal carbonyls, whereas those at about 220 to 260 ppm are typical of bridging carbonyl groups. 

The main limitation of the method derives from the fluxionality of the carbonyl groups, which, on the other hand, is itself a potential source of basic information. 

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Many additional refinements have been made, primarily to take into account more aspects of the microscopic solvent structure, within the framework of diffiision models of bimolecular chemical reactions that encompass also many-body and dynamic effects, such as, for example, treatments based on kinetic theory [35]. One should keep in mind, however, that in many cases die practical value of these advanced theoretical models for a quantitative analysis or prediction of reaction rate data in solution may be limited. 

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. 

The carbonyl bases constitute another important class of weak bases that present interesting possibilities for investigation of structural effects. In solution, experiments with these compounds are subject to severe difficulties. The result is a serious lack of agreement among different investigators about pAa. Arnett and co-workers point out that pKa values reported for acetophenone cover a range of over four units ( — 3.65 to —7.99), while those for acetone span seven units (-0.2 to -7.2).102 In view of these uncertainties, it is impossible to say whether aldehydes, ketones, or carboxylic acids are the most basic in solution. Gas-phase data are available for some of these substances. 

The fact that, in the case of solution spectra, the spectroscopic data are obtained from the solvated species while the corresponding molecular mechanics structure usually represents the naked species is somewhat unsatisfactory. This is one reason for some observed, albeit small, differences between structural parameters in solution and in the solid state. Nevertheless, the accuracy is often surprisingly high, and this might be explained by the fact that the parameterization of the force field is generally based on crystal structural data, and the calculated structures therefore repre-... 

More than half a century has elapsed since Linus Pauling discussed his ideas about the fundamental nature and properties of the hydrogen bond in his landmark book [1]. Pauling had based these ideas on the information that was available at the time. He made reference to papers going back as far as 1912 [2] in which the presence of such an interaction was inferred from the weakness of a particular base. In 1920, mention was made of the widespread occurrence of H-bonds [3], in connection with anomalous properties of certain liquids. Most of the information that had accumulated up to 1940 was based on the structures found in crystals, infrared spectroscopic data in solution, and various other physical-chemical data. 

The pentacoordinate organotin compounds 416a-c also contain a tridentate dianionic N,N,0-chelate ligand . The structure of 416c was determined by X-ray diffraction which conformed the rigid polycyclic structure proposed on the basis of H, B, C, and Sn NMR data in solution (S Sn —150.4 ppm) . In contrast, the dimethyltin compound 416a displays a fluxional structure in solution. 

It is clear that a good deal of information needs to be obtained in order to interpret the structural data in particular, the question has to be faced as to whether the oxidised crystal structure is that of a resting state or whether the heme iron ligand switching occurs on each catalytic cycle. We do, however, know that the oxidised protein in solution has the same His/His coordination of the c-type heme as in the oxidised state of the crystalline enzyme. This was determined from MCD spectroscopy (Cheesman et al., 1997). Other solution spectroscocpic measurements have shown that the ligation of the d heme is very likely the same as in the crystalline state (Cheesman et al., 1997). These studies also showed that the d heme iron appeared to be in an unusual room temperature high/low spin equilibrium. 

Table 8). This permits the interpretation of experimental data by using the electro-optical properties of flexible-chain polymers in terms of a worm-like chain model However, EB in solutions of polyelectrolytes is of a complex nature. The high value of the observed effect is caused by the polarization of the ionic atmosphere surrounding the ionized macromolecule rather than by the dipolar and dielectric structure of the polymer chain. This polarization induced by the electric field depends on the ionic state of the solution and the ionogenic properties of the polymer chain whereas its dependence on the chain structure and conformation is slight. Hence, the information on the optical, dipolar and conformational properties of macromoiecules obtained by using EB data in solutions of flexible-chain polyelectrolytes is usually only qualitative. Studies of the kinetics of the Kerr effect in polyelectrolytes (arried out by pulsed technique) are more useful since in these... 

Most chemical reactions in the natural surroundings and in the chemical industrial processes take place in solution, and this aggregation state constitutes the main field of interest for the majority of chemists and biochemists. However, in contrast to the large number of detailed crystal structures, the amount of available structural information for species in solution is limited. The reason for this situation is certainly the inherent disorder of the solution state, from which follows the lack of an experimental method as hard as the single-crystal X-ray diffraction technique. Certainly, spectroscopic methods can be used for studies of symmetry and bonding properties, but in order to obtain accurate interatomic distances diffraction techniques (or EXAFS, extended X-ray absorption fine structure) have to be used. These techniques are not always easily accessible and have some weak points however, they are the only ones able to provide the latter type of structural data. In the following, the few reported (and one unpublished) studies of this type of thallium species in aqueous solution will be discussed. 

For thallium(I), only three structural studies in solution are familiar to the author. The first one is an exploratory study on the structure of the hydrated Tl ion in solution, performed by means of the EXAFS and X-ray diffraction techniques on concentrated TIF solutions (0.5 M and 3.0 M, respectively) in water (53). The EXAFS data showed no features which could be assigned to a well-structured hydration sphere around the thallium ion. The X-ray data could be explained by assuming two water molecules at about 2.74 A and four more loosely bound... 

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