Far-IR spectroscopy

The far-IR region ( 650 cm-1) is very useful for inorganic chemists and will be used in both this chapter and in Chapter 4 in the analysis of metal coordination compounds. In this region, stretching and bending vibrations of M-L bonds occur. Some assignments useful for our study are found in Table 3.9. 

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Optical Methods Reflectance and ellipsometry snffer from lack of a theory at the molecnlar level. The same is true for SERS and SHG. The main advances will be in the nse of far-IR spectroscopy and SFG. SFG measurements performed with femtosecond lasers open np new possibilities for time-resolved adsorbate stndies. 

Far IR spectroscopy is a useful technique to study the structural environment and dynamics of water molecules and ions in the interlayer galleries of LDHs, since it directly probes the intermolecular and hydrogen bonding interactions [223]. Spectra, which can be recorded by pressing small amounts of powder onto one side of a piece of Scotch tape, are often difficult to interpret because of the complex network of intermolecular interactions present and molecular dynamics simulations have been employed in order to assist in the interpretation [223]. 

Far IR spectroscopy and molecular dynamics calculations have demonstrated the structural and dynamic similarity of chloride anions in the interlayer galleries of LDHs to those in bulk aqueous solution [223], as discussed... 

Risen et al. investigated cation—anion interactions using far IR spectroscopy (50—300 cm ) to study Nafion sulfonate membranes that were neutralized by cations in the series Na+, K+, Rb+, and Cs+ and the series Mg +, Ca +, Sr +, and Ba +, as well as the acid form." The spectra in this region for hydrated samples show a broad but well-defined band below 300 cm that is not present for the acid form. For both the monovalent alkali and divalent alkaline earth series, the band monotonically shifts to lower frequencies, f, such that foe where Mis the... 

Raman spectroscopy or far-IR spectroscopy can determine the fundamental vibration frequencies of the host. However, these methods give information about the whole glass matrix and do not account for the local nature of electron-phonon interactions. So, the fundamental frequencies are preferably determined by recording the phonon-side bands (PSB) of rare-earth transitions or by studying the temperature-dependence of multiphonon relaxations [42,43]. The phonon energies determined by PSB spectroscopy, which is the most direct method, are usually lower (400 cm-1 in ZBLAN) than those measured by other methods ( 500 cm-1) suggesting that weak M—F bonds are coupled to the rare-earth [43]. 

Ab initio calculations have given vibrational wavenumbers for Cn.H2/D2 complexes,675 and for HCl(NH3)n clusters, where n = 1 - 4.676 High-resolution far-IR spectroscopy for the OC H35C1 heterodimer showed that the band origin of the HC1 libration band, vf, was 201.20464(27) cm-1.677 DFT calculations gave vibrational wavenumbers for the adduct H20.C10.678... 

Recent far IR spectroscopy on liquid water indicates that the basic H-bonded unit is indeed the pentameric unit described above. The clusters are then built up by further H-bonding from this unit. The structure which is relevant to hydration is that of bulk water where what is primarily studied is the relation of the individual molecules with respect to each other throughout the water in bulk. Diffraction and NMR are the major techniques for probing this structure. 

Conformations A and B are of the eclipsed type, whereas C and D are bisected. It has been determined by microwave spectroscopy that the eclipsed conformations are more stable than the bisected ones and that B is about O.lSkcal more stable than A. MO calculations at the HF/6-31G level found relative energies of 0.00, —0.25, 1.75, and 1.74 kcal/mol, respectively, for A, B, C, and More recently, experimental far-IR spectroscopy and MP2/6-31G-I— -(3d/,3pd) computations indicate a difference of about 0.2 kcal (favoring B). ... 

The use of near-IR and far-IR spectroscopies for characterisation of PETP films was investigated. After a brief review of previous work, some examples of transmission spectra of annealed PETP, showing complex changes in the absorption bands, are presented. Near-IR spectroscopy was used to follow the crystallinity (or conformation) of thick PETP films without any sample preparation. 40 refs. 

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