Vibrational spectra techniques

Table III presents integral excess entropies of formation for some solid and liquid solutions obtained by means of equilibrium techniques. Except for the alloys marked by a letter b, the excess entropy can be taken as a measure of the effect of the change of the vibrational spectrum in the formation of the solution. The entropy change associated with the electrons, although a real effect as shown by Rayne s54 measurements of the electronic specific heat of a-brasses, is too small to be of importance in these numbers. Attention is directed to the very appreciable magnitude of the vibrational entropy contribution in many of these alloys, and to the fact that whether the alloy is solid or liquid is not of primary importance. It is difficult to relate even the sign of the excess entropy to the properties of the individual constituents.

The V (OCO) ion has a structured electronic photodissociation spectrum, which allows us to measure its vibrational spectrum using vibrationally mediated photodissociation (VMP). This technique requires that the absorption spectrum (or, in our case, the photodissociation spectrum) of vibrationally excited molecules differ from that of vibrationally unexcited molecules. The photodissociation spectrum of V (OCO) has an extended progression in the V OCO stretch, indicating that the ground and excited electronic states have different equilibrium V "—OCO bond lengths. Thus, the OCO antisymmetric stretch frequency Vj should be different in the two states, and the... 

Vj = 1 <— v" = 1 transition will be at a different energy than the Vj = 0 <— v" = 0. We use this fact to measure the vibrational spectrum of V (OCO) in a depletion experiment (Fig. 12a). A visible laser is set to the Vj = 0 Vj = 0 transition at 15,801 cm producing fragment ions. A tunable IR laser fires before the visible laser. Absorption of IR photons removes population from the ground state, which is observed as a decrease in the fragment ion signal. This technique is a variation of ion-dip spectroscopy, in which ions produced by 1 + 1 REMPI are monitored as an IR laser is tuned. Ion-dip spectroscopy has been used by several groups to study vibrations of neutral clusters and biomolecules [157-162]. 

The entropy difference A5tot between the HS and the LS states of an iron(II) SCO complex is the driving force for thermally induced spin transition [97], About one quarter of AStot is due to the multiplicity of the HS state, whereas the remaining three quarters are due to a shift of vibrational frequencies upon SCO. The part that arises from the spin multiplicity can easily be calculated. However, the vibrational contribution AS ib is less readily accessible, either experimentally or theoretically, because the vibrational spectrum of a SCO complex, such as [Fe(phen)2(NCS)2] (with 147 normal modes for the free molecule) is rather complex. Therefore, a reasonably complete assignment of modes can be achieved only by a combination of complementary spectroscopic techniques in conjunction with appropriate calculations. 

Principles and Characteristics Vibrational spectroscopic techniques such as IR and Raman are exquisitely sensitive to molecular structure. These techniques yield incisive results in studies of pure compounds or for rather simple mixtures but are less powerful in the analysis of complex systems. The IR spectrum of a material can be different depending on the state of the molecule (i.e. solid, liquid or gas). In relation to polymer/additive analysis it is convenient to separate discussions on the utility of FUR for indirect analysis of extracts from direct in situ analysis. 

The vibrational spectrum of a metal complex is one of the most convenient and unambigious methods of characterization. However, it has not been possible to study the interactions of metal ions and biological polymers in this way since the number of vibrational bands from the polymer obscure the metal spectrum. The use of laser techniques for Raman spectroscopy now make it very likely that the Raman spectra of metals in the presence of large amounts of biological material will be measured (34). The intensity of Raman lines from metal-ligand vibrations can be... 

The vibrational spectrum of cis- and fr s-[Pt(II) (NH3)2C12] in the crystalline phase has also been measured with laser-Raman techniques by Dr. Hoeschele of the Biophysics Department, Michigan State University. This technique shows great promise. 

Another technique of vibrational spectroscopy suited for the characterization of solids is that of Raman spectroscopy. In this methodology, the sample is irradiated with monochromatic laser radiation, and the inelastic scattering of the source energy is used to obtain a vibrational spectrum of the analyte [20]. Since... 

Both infrared and Raman are vibrational spectroscopic techniques, and the Raman scattering spectrum and infrared absorption spectrum for a given species often resemble one another quite closely. There are, however, sufficient differences... 

The dipole and polarization selection rules of microwave and infrared spectroscopy place a restriction on the utility of these techniques in the study of molecular structure. However, there are complementary techniques that can be used to obtain rotational and vibrational spectrum for many other molecules as well. The most useful is Raman spectroscopy. 

The vibrational spectrum of TeF4 has been studied extensively, including matrix-isolation techniques (2). The most dilute matrices reveal absorptions attributable only to the monomeric TeF4 molecule, with C2v symmetry. The more concentrated matrices contain absorptions arising from several dimeric or oligomeric species (2). 

Figure 0.1 Stimulated emission pumping (SEP, Hamilton et al., 1986 Northrup and Sears, 1992) is a new experimental technique for accessing higher-lying vibrational levels of molecules in their ground electronic states. Shown is the SEP vibrational spectrum of S02, where a pair of dips represent one vibrational level. (Adapted from Yamanouchi, Takeuchi, and Tsuchiya, 1990.) The stick spectrum at the bottom represents the position of the vibrational levels given by Equation (0.1) with the constants given in Table 0.1. The bright levels are represented by longer sticks.

Sensitivity and complexity represent challenges for ATR spectroscopy of catalytic solid liquid interfaces. The spectra of the solid liquid interface recorded by ATR can comprise signals from dissolved species, adsorbed species, reactants, reaction intermediates, products, and spectators. It is difficult to discriminate between the various species, and it is therefore often necessary to apply additional specialized techniques. If the system under investigation responds reversibly to a periodic stimulation such as a concentration modulation, then a PSD can be applied, which markedly enhances sensitivity. Furthermore, the method discriminates between species that are affected by the stimulation and those that are not, and it therefore introduces some selectivity. This capability is useful for discrimination between spectator species and those relevant to the catalysis. As with any vibrational spectroscopy, the task of identification of a species on the basis of its vibrational spectrum can be difficult, possibly requiring an assist from quantum chemical calculations. 

Bimolecular photoinduced electron transfer between an electron donor and an electron acceptor in a polar solvent may result in the formation of free ions (FI). Weller and coworkers [1] have invoked several types of intermediates for describing this process (Fig.la) exciplex or contact ion pair (CIP), loose ion pair (LIP), also called solvent separated ion pair. The knowledge of the structures of these intermediates is fundamental for understanding the details of bimolecular reactions in solution. However, up to now, no spectroscopic technique has been able to differentiate them. The UV-Vis absorption spectra of the ion pairs and the free ions are very similar [2], Furthermore, previous time resolved resonant Raman investigations [3] have shown that these species exhibit essentially the same high frequency vibrational spectrum. 

The identification of species adsorbed on surfaces has preoccupied chemists and physicists for many years. Of all the techniques used to determine the structure of molecules, interpretation of the vibrational spectrum probably occupies first place. This is also true for adsorbed molecules, and identification of the vibrational modes of chemisorbed and physisorbed species has contributed greatly to our understanding of both the underlying surface and the adsorbed molecules. The most common method for determining the vibrational modes of a molecule is by direct observation of adsorptions in the infrared region of the spectrum. Surface spectroscopy is no exception and by far the largest number of publications in the literature refer to the infrared spectroscopy of adsorbed molecules. Up to this time, the main approach has been the use of conventional transmission IR and work in this area up to 1967 has been summarized in three books. The first chapter in this volume, by Hair, presents a necessarily brief overview of this work with emphasis upon some of the developments that have occurred since 1967. 

---