Vibrational spectroscopy, selection rules

Chemical symmetry has been noted and investigated for centuries in crystallography, which is at the border between chemistry and physics. It was probably more physics when crystal morphology and other properties of the crystal were described, and more chemistry when the inner structure of the crystal and the interactions between the building units were considered. Later, discussion of molecular vibrations, the selection rules, and other basic principles in all kinds of spectroscopy also led to a uniquely important place for the concept of symmetry in chemistry with equally important practical implications. 

The complexity of Raman spectra for polymers is reduced as with infrared spectra because vibrations of the same type superimpose. In addition, as with infrared spectroscopy, selection rules aid in determining which molecular vibrations are active. However, the criterion for Raman aetivity is a change in bond polarizability with molecular vibration or rotation in contrast to the infrared criterion of a change in dipole moment (Figure 6.6). This means that, for molecules such as carbon dioxide that show both a change in dipole moment and a change in polarizability,... 

Not only do the experimental vibrational predissociation lifetimes require interpretation, so do the increasingly sophisticated theoretical calculations whose results often fall out of a web of coupled differential equations or the convoluted algebra of quantum mechanics. In order to offer a qualitative overview of dynamical processes in van der Waals molecules, we shall introduce a selection rule which can provide insight into possible relaxation channels of vibrationally excited molecules. This selection rule concerns the change in a quantum number, Anj., which is to remain small for efficient vibrational predissociation processes. It bears a close analogy to the selection rules of optical spectroscopy which require small changes in quantum numbers Au, AJ, AS, etc. for efficient transitions between molecular states. Let us review the origin of the vibrational predissociation selection rule which has been developed in more detail elsewhere. ... 

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

CAHRS and CSHRS) [145, 146 and 147]. These 6WM spectroscopies depend on (Im for HRS) and obey the tlnee-photon selection rules. Their signals are always to the blue of the incident beam(s), thus avoiding fluorescence problems. The selection ndes allow one to probe, with optical frequencies, the usual IR spectrum (one photon), not the conventional Raman active vibrations (two photon), but also new vibrations that are synnnetry forbidden in both IR and conventional Raman methods. 

Perhaps the best known and most used optical spectroscopy which relies on the use of lasers is Raman spectroscopy. Because Raman spectroscopy is based on the inelastic scattering of photons, the signals are usually weak, and are often masked by fluorescence and/or Rayleigh scattering processes. The interest in usmg Raman for the vibrational characterization of surfaces arises from the fact that the teclmique can be used in situ under non-vacuum enviromnents, and also because it follows selection rules that complement those of IR spectroscopy. 

A second, independent spectroscopic proof of the identity of 4 as rans-[Mo(N2)2(weso-prP4) was provided by vibrational spectroscopy. The comparison of the infrared and Raman spectrum (Fig. 7) shows the existence of two N-N vibrations, a symmetric combination at 2044 cm-1 and an antisymmetric combination at 1964 cm-1, indicating the coordination of two dinitrogen ligands. In the presence of a center of inversion the symmetric combination is Raman-allowed and the antisymmetric combination IR allowed. The intensities of vs and vaK as shown in Fig. 2 clearly reflect these selection rules. Moreover, these findings fully agree with results obtained in studies of other Mo(0) bis(dinitrogen)... 

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. 

As in infrared spectroscopy, not all vibrations are observable. A vibration is Raman active if it changes the polarizability of the molecule. This requires in general that the molecule changes its shape. For example, the vibration of a hypothetical spherical molecule between the extremes of a disk-shaped and a cigar-shaped ellipsoid would be Raman active. We recall that the selection rule for infrared spectroscopy was that a dipole moment must change during the vibration. As a consequence the stretch vibrations of for example H2 (4160.2 cm"1), N2 (2330.7 cm-1) and 02 (1554.7 cm"1) are observed in Raman spectroscopy but not in infrared. The two techniques thus complement each other, in particular for highly symmetrical molecules. 

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

Some characteristics of, and comparisons between, surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRRAS) for examining reactive as well as stable electrochemical adsorbates are illustrated by means of selected recent results from our laboratory. The differences in vibrational selection rules for surface Raman and infrared spectroscopy are discussed for the case of azide adsorbed on silver, and used to distinguish between "flat" and "end-on" surface orientations. Vibrational band intensity-coverage relationships are briefly considered for some other systems that are unlikely to involve coverage-induced reorientation. 

The high sensitivity of tunneling spectroscopy and absence of strong selection rules allows infrared and Raman active modes to be observed for a monolayer or less of adsorbed molecules on metal supported alumina. Because tunneling spectroscopy includes problems with the top metal electrode, cryogenic temperatures and low intensity of some vibrations, model catalysts of evaporated metals have been studied with CO and acetylene as the reactive small molecules. Reactions of these molecules on rhodium and palladium have been studied and illustrate the potential of tunneling spectroscopy for modeling reactions on catalyst surfaces,... 

Information concerning conformation, tacticity and crystallinity may also be obtained (1). Vibrational transitions accesssible to IR spectroscopy are governed by the selection rule that there must be a change in dipole moment during excitation of the polymer vibrations. Thus symmetric vibrations which are detected by Raman spectroscopy are inaccessible to IR absorption measurements. 

An important consequence of the presence of the metal surface is the so-called infrared selection rule. If the metal is a good conductor the electric field parallel to the surface is screened out and hence it is only the p-component (normal to the surface) of the external field that is able to excite vibrational modes. In other words, it is only possible to excite a vibrational mode that has a nonvanishing component of its dynamical dipole moment normal to the surface. This has the important implication that one can obtain information by infrared spectroscopy about the orientation of a molecule and definitely decide if a mode has its dynamical dipole moment parallel with the surface (and hence is undetectable in the infrared spectra) or not. This strong polarization dependence must also be considered if one wishes to use Eq. (1) as an independent way of determining ft. It is necessary to put a polarizer in the incident beam and use optically passive components (which means polycrystalline windows and mirror optics) to avoid serious errors. With these precautions we have obtained pretty good agreement for the value of n determined from Eq. (1) and by independent means as will be discussed in section 3.2. 

The differences in selection rules between Raman and infrared spectroscopy define the ideal situations for each. Raman spectroscopy performs well on compounds with double or triple bonds, different isomers, sulfur-containing and symmetric species. The Raman spectrum of water is extremely weak so direct measurements of aqueous systems are easy to do. Polar solvents also typically have weak Raman spectra, enabling direct measurement of samples in these solvents. Some rough rules to predict the relative strength of Raman intensity from certain vibrations are [7] ... 

Raman and IR spectroscopies are complementary to each other because of their different selection rules. Raman scattering occurs when the electric field of light induces a dipole moment by changing the polarizability of the molecules. In Raman spectroscopy the intensity of a band is linearly related to the concentration of the species. IR spectroscopy, on the other hand, requires an intrinsic dipole moment to exist for charge with molecular vibration. The concentration of the absorbing species is proportional to the logarithm of the ratio of the incident and transmitted intensities in the latter technique. 

Raman spectroscopy Is a form of vibrational spectroscopy which, like Infrared spectroscopy. Is sensitive to transitions between different vibrational energy levels in a molecule (1). It differs from Infrared spectroscopy In that Information Is derived from a light scattering rather than a direct absorption process. Furthermore, different selection rules govern the Intensity of the respective vibrational modes. Infrared absorptions are observed for vibrational modes which change the permanent dipole moment of the... 

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