PART 2 SPECTROSCOPIC METHODS

The focus of this chapter is photon spectroscopy, using ultraviolet, visible, and infrared radiation. Because these techniques use a common set of optical devices for dispersing and focusing the radiation, they often are identified as optical spectroscopies. For convenience we will usually use the simpler term spectroscopy in place of photon spectroscopy or optical spectroscopy however, it should be understood that we are considering only a limited part of a much broader area of analytical methods. Before we examine specific spectroscopic methods, however, we first review the properties of electromagnetic radiation. 

Much of the experimental work in chemistry deals with predicting or inferring properties of objects from measurements that are only indirectly related to the properties. For example, spectroscopic methods do not provide a measure of molecular stmcture directly, but, rather, indirecdy as a result of the effect of the relative location of atoms on the electronic environment in the molecule. That is, stmctural information is inferred from frequency shifts, band intensities, and fine stmcture. Many other types of properties are also studied by this indirect observation, eg, reactivity, elasticity, and permeabiHty, for which a priori theoretical models are unknown, imperfect, or too compHcated for practical use. Also, it is often desirable to predict a property even though that property is actually measurable. Examples are predicting the performance of a mechanical part by means of nondestmctive testing (qv) methods and predicting the biological activity of a pharmaceutical before it is synthesized. 

Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A Methods of Surface Analysis edited by J.L.G. Fierro... 

Volume I includes the spectroscopic methods for nano interfaces and nanostructure characteristics and dynamics. Volume 2 groups different topics together in 3 parts active surfaces, single crystals and single biocells. 

Volume 227. Metallobiochemistry (Part D Physical and Spectroscopic Methods for Probing Metal Ion Environments in Metalloproteins)... 

This reaction has been reexamined using optical, IR and NMR spectroscopic methods to probe NO reactions with Fe(TPP)(NO) and the more soluble Fe(TmTP)(NO) (92). These studies confirmed the formation of Fe(Por)(NO)2 in toluene-dg at low temperature (Eq. (43)). NMR line shape analysis was used to calculate K43 = 23 M-1 at 253 K (3100 M-1 at 179 K, AH° = —28kJmol 1) (92). The failure of the Fen(Por) complexes to promote NO disproportionation, in contrast to the behavior of the respective Ru(II) and Os(II) analogs, may find its origin partly in the relatively low stability of the dinitrosyl intermediate (K52 estimated to be 2.8 M-1 at 298 K) and unfavorable kinetics of subsequent reaction of this species with NO. 

Electron transfer reactions of metal ion complexes in homogeneous solution are understood in considerable detail, in part because spectroscopic methods and other techniques can be used to monitor reactant, intermediate, and product concentrations. Unfavorable characteristics of oxide/water interfaces often restrict or complicate the application of these techniques as a result, fewer direct measurements have been made at oxide/water interfaces. Available evidence indicates that metal ion complexes and metal oxide surface sites share many chemical characteristics, but differ in several important respects. These similarities and differences are used in the following discussions to construct a molecular description of reductive dissolution reactions. 

Spectroscopic methods, in the infrared region, have been rapidly developed in scope and power since 1949. Excellent reviews of this topic have been given by Eischens and Pliskin 126) and, more recently, by Sheppard 127). In chemisorption, new species are formed and drastic changes take place between, say, the frequencies of a CO molecule in the gas phase and those of one adsorbed on platinum 128). Extensive work has been done in the physical adsorption field by Terenin and his co-workers (reviewed elsewhere, see 126,127). Most of this work has been concerned with changes which adsorption produces in the surface OH groups of porous glass. These groups may be considered part of the adsorbent spectral studies of the adsorbate as such have been less frequently made. 

The physical chemist of today has a wide variety of methods at his disposal for the experimental investigation of electronic structure and all of them have been used in attempts at obtaining evidence of the participation of outer d-orbitals in bonding. One such group of methods is constituted by the various techniques of radiofrequency spectroscopy, which have the advantage that they yield information about the molecule in its ground state. In this they have a distinct superiority over, say, electronic absorption spectra where it is necessary to consider both ground and excited states. Moreover much of the data derived from radiofrequency spectroscopic methods concerns essentially just one part of the molecule so that attention can be concentrated on those atoms of interest in whatever study happens to be under way. 

In short, the spectroscopic methods appear to be reliable and specific for HCHO. The derivatization methods are generally in reasonable overall agreement with the spectroscopic methods where intercomparisons have been carried out, but there can be very large discrepancies in individual measurements. Part of the reason for these discrepancies may be related to the fact that some of the spectroscopic methods average over long distances whereas the derivatization methods sample at a point. On the other hand, the latter methods involve collecting the sample over a period of time, usually several hours, whereas the spectroscopic methods are real-time measurements. Finally, variations in collection efficiencies and possible interferences must be taken into account for the derivatization methods. 

Let us compare the methods applied by Pedersen for establishing the complex formation with a modern approach. Today tedious solubility studies are carried out almost exclusively with practical applications in mind, but they are not performed to prove the complex formation. For instance, one ofthe main reasons for the use of cyclodextrin complexes in the pharmaceutical industry is their solubilizing effect on drugs [8]. There, and almost only there, solubility studies are a must. As concerns spectroscopic methods, at present the NMR technique is one ofthe main tools enabling one to prove the formation of inclusion complex, carry out structural studies (for instance, making use of the NOE effect [9a]), determine the complex stability [9b, c] and mobility of its constituent parts [9d]. However, at the time when Pedersen performed his work, the NMR method was in the early stage of development, and thus inaccurate, and its results proved inconclusive. UV spectra retained their significance in supramolecular chemistry, whilst at present the IR method is used to prove the complex formation only in very special cases. 

Other spectroscopic methods cannot provide the same overall picture of protein structure or dynamics. However, they can give information about specific atoms or groups in the protein. In order to gain detailed information from these techniques, it is generally necessary to study metal atoms, which in some cases are a natural part of the protein and in other cases may be specifically introduced. Techniques such as UV, visible, Raman, and epr spectroscopies provide information about the metal atom and its environment, which is concerned both with structural features and with energetic features. 

Many important heterogeneous catalytic reactions occur at the interface between a solid catalyst and liquid or liquid-gas reactants. Notwithstanding the importance of solid-catalyzed reactions in the presence of liquid reactants, relatively little attention has been paid to spectroscopic methods that allow researchers to follow the processes occurring at the solid-liquid interface during reaction. This lack can be explained in part by the fact that there are only a few techniques that give access to information about solid-liquid interfaces, the most prominent of them being attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray absorption fine structure (XAFS) spectroscopy. 

Free hydroxyl is an active center which is of great importance for oxidation and combustion processes. In order to study the part played by free hydroxyl in chemical reactions, a spectroscopic method for determining free-hydroxyl concentrations by its absorption—method of linear absorption—was developed at the Institute of Chemical Physics of the Academy of Sciences of U.S.S.R. 

The prime goal of our own investigations has been the mechanistic elucidation of the molecular aspects of the Pr -> Pfr transformation in vitro at physiological temperatures. The analytical requirements were in part such that some, notably time-resolved, spectroscopic methods first had to be improved or newly developed. They are described wherever appropriate. 

---