Characterization of Solution Species

Crans et aC propose that paramagnetic H NMR provides a simple yet powerful approach to monitoring transition-metal complex stoichiometry and speciation in solution. Their experimental work involves Cr +, Fe, Fe +, Co +, and Ni complexes of 2,6-pyridinedicarboxylic acid. Panyushkin et aC have used H NMR to study compexation in solutions containing carboxylic acids and two cations, Mg + and Tm +. Kiremire et describe quahtative trends of 

Ebihara et have used H contact chemical shifts to estimate electron spin density on the ligands of a benzylidyne-capped tricobalt cluster. 

Yoder et aV have used NMR to measure the rates of electron and H-atom 

---

Good summaries of accepted experimental techniques can be found in the references that are cited for individual radionuclides in the sections below. Nitsche (1991) provides a useful general summary of the principles and techniques of solubility studies. A large number of techniques have been used to characterize the aqueous speciation of radionuclides. These include poten-tiometric, optical absorbance, and vibrational spectroscopy. Silva and Nitsche (1995) summarize the use of conventional optical absorption and laser-based photothermal spectroscopy for detection and characterization of solution species and provide an extensive citation list. A recent review of the uses of Raman and infrared spectroscopy to distinguish various uranyl hydroxy complexes is given by Runde et al. (2002b). 

The formation of polyoxometallates is a characteristic of V, Mo, W (see Section 22.7) and, to a lesser extent, Nb, Ta and Cr. Characterization of solution species is aided by and V NMR spectroscopies, and solid state structures for a range of salts are known. The structural chemistry of V2O5 and vanadates is complicated and only a brief survey... 

This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

Tadjeddine and co-workers have used SFG [Guyot-Sionnest and Tadjeddine, 1990 Eisenthal, 1992 Richmond, 2002 Vidal et al., 2002, 2004, 2005] to study the adsorbed CO produced from a variety of solution species, including methanol [Vidal et al., 2002, 2005]. With BB-SFG, we studied the electrochemical kinetics of methanol chemisorption as surface CO, as shown in Fig. 12.13. We used apolycrystal-line Pt electrode and 0.1 M H2SO4 electrolyte with 0.1 M methanol. Figure 12.13a-d characterize the potential-dependent SFG spectra obtained under the voltammetric... 

Mass spectrometry (MS) in its various forms, and with various procedures for vaporization and ionization, contributes to the identification and characterization of complex species by their isotopomer pattern of the intact ions (usually cation) and by their fragmentation pattern. Upon ionization by the rough electron impact (El) the molecular peak often does not appear, in contrast to the more gentle field desorption (FD) or fast-atom bombardment (FAB) techniques. An even more gentle way is provided by the electrospray (ES) method, which allows all ionic species (optionally cationic or anionic) present in solution to be detected. Descriptions of ESMS and its application to selected problems are published 45-47 also a representative application of this method in a study of phosphine-mercury complexes in solution is reported.48... 

In addition to the decreased polarizability of the heavier metals, their larger radii require higher metal coordination numbers to achieve steric saturation. As a result, extensive aggregation, frequently coinciding with rather limited solubility in non-donor solvents, and occasionally even in donor solvents, complicates the characterization of these species in solution and the solid state. In fact, several structural characterizations of organoalkali species have relied on recent advances in powder diffraction techniques using synchrotron radiation.1 ... 

Characterization of Surface Species by SERS. Before presenting the results obtained with 1, the spectral features which have proven to be useful in identifying surface species will be reviewed. Both in solution and by SERS, pyridines show a ring mode in the Raman spectrum near 1600 cm-. When the ring nitrogen is protonated, this band disappears and is replaced by a band near 1640 cm-1. The... 

Acylium ions can be formed in superacid solutions from carboxylic acids and acyl halides (8). They are among the best characterized carbenium ions, and single-crystal X-ray structures of a number of them have been determined as BFf, SbFg, or TaClfi salts (135-139). Solid-state NMR characterization of these species on AlBr3 and other solid superacids was described earlier in this review. 

While the redox titration method is potentiometric, the spectroelectrochemistry method is potentiostatic [99]. In this method, the protein solution is introduced into an optically transparent thin layer electrochemical cell. The potential of the transparent electrode is held constant until the ratio of the oxidized to reduced forms of the protein attains equilibrium, according to the Nemst equation. The oxidation-reduction state of the protein is determined by directly measuring the spectra through the tranparent electrode. In this method, as in the redox titration method, the spectral characterization of redox species is required. A series of potentials are sequentially potentiostated so that different oxidized/reduced ratios are obtained. The data is then adjusted to the Nemst equation in order to calculate the standard redox potential of the proteic species. Errors in redox potentials estimated with this method may be in the order of 3 mV. 

Up to some years ago, this problem was still unsolved and it was left to theoretical chemistry to find a solution. Recent developments in spectroscopy27,28 (such as rotational spectroscopy of supersonically expanded jets) allows the characterization of associated species (hetero-dimers)26 between a proton donor and a proton acceptor, before the formation of salts, as shown in Scheme 3. 

Wilhelmy et al, (1985) found that a static spectrophotometric procedure was not sufficiently sensitive to detect successive complexation. However, determination of absorbance was an effective method for identifying a series of species in solution. The detection of a ferric diphosphate complex, Fe(H2PO4)2 , demonstrated the value of using a kinetic technique for characterizing the solution species. 

Preference for neutral metallocene dimethyl coordination over anion coordination to form /(-Me di-nuclear cationic complexes (e.g., 43) is observed for those truly weakly coordinating anions such as B(C6F5)4 and MePBB . Stabilization of highly reactive and unstable metallocenium cations by /(-Me coordination allows isolation and characterization of such species in the pure state, yet affords excellent polymerization activity in solution, presumably via dissociation to a more reactive monomeric form, as indicated by NMR studies.For more coordinating anions such as CH3B(C6F5)3, /(-Me bimetallic cationic complexes are not detected, except when an excess of neutral metallocene dimethyl is employed (Scheme 25) 143,315 equilibrium can be utilized to stabilize... 

Stable particulate alumina sols are precursors for many materials such as controlled porosity catalysts, ceramics, and coatings. They are commonly formed by peptization of pseudoboehmites (AIOOHKH2O) by dilute monoprotic acids. They can be prepared either by the process developed hy Yoldas [4] from hydrolyzed aluminum alkoxides or by acidic peptization of the precipitates derived from hydrolyzed aluminum salts [5,48]. As will be seen below, these alumina sols are usually multi-component fluids that also contain soluble aluminum species. Furthermore, colloidal aluminas can also be produced by the polymerization of hydrolyzed aluminum cations. Therefore, a brief review on the characterization of aluminum species in solution by At NMR is presented in Sec. III.B.l. The NMR studies on colloidal aluminas are then discussed in Sec. III.B.2. 

Over the past few years, there has been a rapid increase in the application of on-line API-MS monitoring techniques to the study of organic reaction mechanisms, mainly to intercept for the first time reactive intermediates from these reactions. The ability to isolate ions direct from crude reaction mixtures has a variety of outstanding features and advantages that allow new applications of transient species in mechanistic chemistry. Undoubtedly, API-MS is now an important tool for future studies of labile and sensitive intermediaries in solution, with no need for prior purification or isolation for further characterization of active species, intermediates of reactions, and products due to on-line purifications. Such new MS techniques are also... 

---