Atom-selective vibrational spectroscopy

The low Ti content (up to 3 wt % in Ti02) makes the extraction of vibrational, energetic, and geometric features specific to Ti04 moieties a difficult task as the experimental data are dominated by the features of the siliceous matrix. This is the reason why the structure of the local environment around Ti(IV) species inside TS-1 was only definitively assessed more than 10 years after the discovery of the material, when the atomic selectivity of X-ray absorption spectroscopies (both XANES and EXAFS) were used [58-60]. 

In traditional vibrational methods snch as Raman or IR spectroscopy (see Vibrational Spectroscopy), only vibrational modes allowed by selection rules will appear in the spectra. To help assign the modes, isotope difference spectra are used. But owing to selection rules, some modes do not appear in the spectrum. Spectral congestion also raises difficulty in resolving modes with close frequencies, particularly for macromolecules containing thonsands of atoms. 

Fortunately, molecular symmetry can help us, for 2.1, 2. Ill and 2.IV have different rotational axes and mirror planes. We will see in due course that this can affect the selection rules for vibrational spectroscopy techniques, which in turn affect how many peaks we would expect to observe in the spectra of the various forms. Symmetry also changes the numbers of equivalent atoms we have in a molecule, and this influences the number of peaks we would observe in, for example, a NMR spectrum. 

Although the above equations imply that a lot of symmetry analyses must be performed, that is not always the case. Equations 14.2 and 14.3 allow for the possibility of broad statements about which transitions will and will not be allowed for particular atomic or molecular systems. Such general statements, ultimately based on quantum mechanics and symmetry, are called selection rules. Selection rules allow us to easily determine which transitions will occur. When one is faced with a spectrum to interpret, knowledge of the selection rules is an indispensable tool in deriving physical information from the spectrum. Rotational and vibrational spectroscopy, in this chapter, are simplified to a large extent thanks to selection rules. 

From an experimental standpoint, information on the dye binding modes at the semiconductor/dye interface, are conventionally accessed by vibrational spectroscopy [Fourier Transform InfraRed (FT-IR) spectroscopy and Surface-Enhanced Raman Spectroscopy (SERS)] [228-237]. These techniques can provide structural details about the adsorption modes as well as information on the relative orientation of the molecules anchored onto the oxide surface. Photoelectron Spectroscopy (PES) has also been successfully employed to characterize the dye/oxide interface for a series of organic dyes [238-242]. The analysis of the PES spectra yields information on the molecular and electronic structures at the interface, along with basic indications of the dye coverage and of the distance of selected atoms from the... 

A much more detailed discussion of the choice of basis for a quantitative description of molecular vibrations is given in the text by Bright Wilson et al. referenced in this chapter s Further Reading section. This covers the use of mass-weighted coordinates and systems of internal coordinates based on bond vectors, bond angles and dihedral angles. Here, we are interested in the application of symmetry to vibrational spectroscopy to understand selection rules, and usually the much simpler basis of a few carefully chosen atom or bond displacements will suffice. 

McKelvie, 2008). Detection methods have included UV/Vis spectroscopy (the largest number of applications for its robustness, versatility, simplicity, and low cost), luminescence and chemiluminescence (CL) (which offer low detection limits and high sensitivity, being therefore especially favored for biological, biochemical, and trace analysis), atomic absorption and emission spectroscopy (which benefit enormously from automated sample pretreatment, used for matrix removal and analyte accumulation), electrochemistry (pH, fluoride ion selective electrodes, stripping voltammetry and conductivity), turbidimetry, vibrational spectroscopy (Fourier transform infrared spectroscopy [FTIR] and Raman) and mass spectrometry. 

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