Raman Spectra of Adsorbed Molecules

There are, at present, two overriding reasons an experimentalist would choose to employ laser Raman spectroscopy as a means of studying adsorbed molecules on oxide surfaces. Firstly, the weakness of the typical oxide spectrum permits the adsorbate spectrum to be obtained over the complete fundamental vibrational region (200 to 4000 cm-1). Secondly, the technique of laser Raman spectroscopy is an inherently sensitive method for studying the vibrations of symmetrical molecules. In the following sections, we will discuss spectra of pyridine on silica and other surfaces to illustrate an application of the first type and spectra of various symmetrical adsorbate molecules to illustrate the second. 

It will be noted that as the hydrogen bond strength increases, Py (H2O) Py(CH2Cl2) Py(CHCl3), the relative intensity ratio of the two ring modes increases and they may be used to assess the strength of the hydro- 

Characteristic Raman Lines and Their Relative Intensities for Various Pyridine (Py) 

Bands at 1000 and 1035 cm-1 have been assigned (43) to a spectrum approximating to structure (I), a very weak hydrogen bond, and bands at 1010 cm-1 and 1035 cm-1 to a spectrum approximating to structure (II) in which the proton has become completely attached to the nitrogen atom. 

Earlier in this review, the relationship between the Raman and infrared spectra of molecules possessing high or low symmetry was considered. It was indicated that for molecules possessing a center of symmetry, no vibration is active in both the Raman and infrared spectra. Several adsorbates in this category and one of intermediate symmetry have been studied by laser Raman spectroscopy (Table IX), and most of these spectra are considered in this section. 

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All of these facilities have special relevance in recording Raman spectra of adsorbed molecules. 

For the most part, the UV Raman spectra of adsorbed molecules are similar to those of their free-molecule counterparts. A significant exception, reported here, is that of benzene adsorbed in silicalite (the all-silica form of zeolite MFI) (56). 

It is desirable that the oxide chosen for an adsorption study has a high surface area. This would potentially allow a greater number of adsorbate molecules to be adsorbed and consequently more intense spectra would be obtained. In general, the observed spectra of adsorbed molecules at low coverages are weak. Further, some adsorbates (e.g. H2O) give rise to inherently weak Raman spectra even at high coverage. 

Typically, the UV Raman spectra of various hydrocarbons adsorbed in zeolites have been found to be similar to their spectra in solution, as a pure liquid, or as a pure solid (25). This is an important finding because the UV Raman spectra of free molecules (which are relatively quick and easy to measure) can be used for fingerprint identification of adsorbed species. One minor exception to this rule is the Raman spectrum of naphthalene, which shows some changes in the pattern of peak intensities between solid naphthalene and naphthalene adsorbed in ultrastable Y-zeolite. In this case, the adsorbed naphthalene spectrum more closely resembles that of the molecule in solution with benzene or CCI4, which suggests that interaction with the pore walls of the zeolite was similar to solvent interactions. The smaller pore diameters and pore intersections in zeolite MFI compared to Y-zeolite might be expected to produce more pronounced changes in molecular vibrational spectra as a consequence of steric interactions of the molecules with the pore walls. 

Considerable interest has been shown recently in the chemistry of species adsorbed on zeolite surfaces. For example, a Raman spectroscopic investigation of zeolites and the molecules adsorbed thereon has been undertaken. All the zeolites examined give weak Raman spectra in nearly all cases the samples gave rise to an excessive background, and in order to minimize the problem it was necessary to use high-purity materials. Although these spectra are not as informative as the i.r. framework frequencies, they still show differences between zeolite structures. The Raman spectra of adsorbed... 

Determination of surface functional groups, e.g., —OH, —C - C—, and >C = O, and identificadon of adsorbed molecules comes principally from comparison with vibrational spectra (infixed and Raman) of known molecules and compounds. Quick qualitative analysis is possible, e.g., stretching modes involving H appear for v(C—H) at 3000 cm and for v(0—H) at 3400 cm L In addition, the vibrational energy indicates the chemical state of the atoms involved, e.g., v(C=C) " 1500 cmT and v(C=0) " 1800 cm"L Further details concerning the structure of adsorbates... 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Among the important observations that have been made by studying the Raman spectra of molecules adsorbed on solid surfaces (Table X), the following may be noted. The list is not intended to be exhaustive. 

Raman spectra, 296, 298, 303, 304 of adsorbed molecules, 333-339 of adsorption systems, 320-332 of Cab-O-Sil disk, 320 different from infrared spectra, 302-304 effect of fluorescence on, 321-327 molecular symmetry and, 304, 305 of oxides, 321... 

Since stationary electrodes are employed in most SERS experiments, a relatively small number of adsorbed molecules are continuously irradiated by laser beams. When exciting lines are within strong absorption bands of the adsorbed species, surface-enhancement resonance Raman spectra (SERRS) are obtained. However, this may lead to decomposition of such species due to local heating. Use of a cylindrical rotating electrode can circumvent this problem (57). 

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