Spectra of Physically Adsorbed Molecules

Most of the work discussed in connection with the shift of the surface OH bands concerned systems in which physical adsorption occurred. In this work the spectra of the adsorbed molecules were not emphasized. In 

It is likely that these small shifts will eventually be explained in terms of induced polarization interactions of the type described by the Kirk-wood-Bauer-Magat equation. At this time, however, it is not clear whether these interactions are between adjacent adsorbed molecules or between adsorbed molecules and surface OH groups. All studies of physical adsorption have involved surfaces which have OH groups. Further studies of physical adsorption on surfaces which are free of OH groups appears to be highly worth while. 

Yoshino (51) studied the abundance ratios of the gauche and trans forms of dichloroethane and the keto-enol isomers of acetyl acetone when these compounds were adsorbed (less than monolayer quantities) on silica gel. The gauche-trans ratio, which is unity in chloroform solution and 1.4 in the pure liquid, was found to be 1.9 in the adsorbed state. The 1430-cm.-1 gauche band and the 1450-cm.-1 trans band were not affected by the adsorption. The absorption bands of acetyl acetone were measured for a 1.5 % chloroform solution, the pure liquid, liquid saturated with water, and the adsorbed state. The relative intensity of the 1600-cin. 1 enol band and the 700-cm.-1 keto baud were used to determine the isomer ratio. The 1600- 

Extremely interesting infrared studies of physically adsorbed molecules were carried out by Sheppard and Yates (52). These workers studied the spectra of methane, ethylene, acetylene, and hydrogen on porous glass. They found that the perturbing effects of surface forces made it possible to detect bands which are found in the Raman spectra but are not observed in the normal infrared spectra. This indicates that the degree of symmetry of the adsorbed molecule is less than in the gaseous state because of the one-sided nature of the surface forces. This effect was discovered independently by Karagounis and Peter (52a) in studies 1,3,5-trichlorobenzene physically absorbed on silica. 

The spectrum of physically adsorbed hydrogen is shown in Fig. 26. The band due to physically adsorbed hydrogen at a surface coverage of 0.2 is found at 4131 cm.-1. The corresponding Raman band is at 4160 cm.-1. 

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A large variety of problems related to the nature of the adsorption processes have been studied by infrared spectroscopy. The most extensive and productive application of this method has been in studies of chemisorption on supported-metal samples. Spectra of physically adsorbed molecules have provided important information on the interaction of these molecules with the surface of the adsorbent. Experimental developments have reached a state where it is evident that the infrared techniques are adaptable to practically all types of samples which are of interest to catalytic chemists. Not only are the infrared techniques applicable to studies of chemisorption and physical adsorption systems but they add depth and preciseness to the definitions of these terms. 

Fia. 9. p-Nitrophenol vapor adsorbed in high vacuum on a sublimed BaF, layer. Curve 1 firstly adsorbed molecules (band 413 mp) curve 2 beginning of a second layer of molecules, adsorbed on the first ones by Van der Waals forces curve 3 second layer of physically adsorbed molecules formed (band 320 mp). Transmission spectra. From Custers and de Boer (3). 

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]. 

Comparing gas phase spectra of the free molecules with that of adsorbed ones, two observations are made loss of rotational fine structure, thus broadening of peaks, and a shift in energies on an IP scale. The extent of the energy shift reflects the state of the adsorbed molecule [101, 118]. However these shifts compared to the gas phase values cannot be directly related to chemical properties since they consist of mainly two parts that can be separated in contributions from physical adsorption, called relaxation shifts AErv and chemical adsorption AErond (Eq. 2.21) [97, 118]. 

Sometimes, FI mass spectra show signals due to reactions of the analyte with the emitter surface or between molecules adsorbed to that surface. In case of acetone for example, it was demonstrated that [M+H]" quasimolecular ions are produced mainly by a field-induced proton-transfer reaction in the physically adsorbed layer. [59] The mechanism of this field-induced reaction depends on the existence of tautomeric structures of the neutral molecule. Besides the [M+H] quasimolecular ions, [M-H] radicals are formed ... 

Low-temperature spectra of species formed from the adsorption of alkanes on single-crystal metal surfaces have all indicated the presence of undissociated molecules physically adsorbed flat on the surface. The wave-number lowering of the eCH3/ eCH2 absorptions indicates appreciable adsorptive interactions with surface metal atoms. 

Even when the solid-state physical methods do not indicate that properties of A-in-B are very different from those of A-in-A, it can still be possible that small changes in the electronic structure (a ligand effect on A) can be important enough for chemisorption and catalysis [25]. This should in principle be seen by (i) IR spectra of adsorbed molecules (ii) adsorption calorimetry (iii) changes in the activation energy of a simple catalytic reaction. There is currently experimental information available on all three points. 

The band at 2142 cm" is present in the spectra of all the studied samples and has the same position as that of free CO in gas phase (2143 cm" ). It belongs, evidently, to CO molecules physically adsorbed at the surface. The band disappears immediately after CO removal from the gas phase, and its presence could be used as an indication about saturation of all more or less strong surface sites by CO. 

Low-temperature luminescence spectra of naphthalene (Nph) adsorbed in the alkali and alkaline-earth forms of the X zeolites indicate molecules physically adsorbed and bound in donor-acceptor complexes. Using spectral methods the Nph oxidation reaction has been found to be promoted by water vapor. The oxygen chemisorbed in the cages is considered as an oxidizer of Nph. Nph forms photostable dimer associates on the surface of amorphous aerosil. 

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