Rotation, hindered, adsorbed molecules

Spohr found a significant reduction in the dipole reorientation time for a different model of water (but using the same water/Pt potential). In that paper, the reorientation dynamics are characterized by the spectral densities for rotation around the three principal axes of the water molecule. These calculations demonstrated the hindered rotation of water molecules in the plane parallel to the surface. In addition, a reduction in the frequency of rotation about the molecular dipole for water molecules in the adsorbed... 

The appearance of additional peaks in the monolayer spectrum suggests the existence of surface vibratory modes associated with rotations and translations of the free molecule hindered by adsorption. To identify these modes, it is necessary to perform normal mode calculations of the vibrational spectrum of the adsorbed molecule. These calculations are also of interest because of the sensitivity of the frequency and intensity of the surface vibratory modes to the molecular orientation and the location and strength of its bonds to the substrate. 

Hindered rotations of the hydrogen molecule adsorbed in zeolites represent a sensitive probe of their adsorption sites. Systematic studies of the INS spectra of hydrogen in different zeolites can lead to an... 

Recently, Fenelon and Rubalcava (18) studied the interaction of CO with Na and Ca A and X zeolites at pressures of about 10 torr. Using isotopic CO and by analyzing the absorption band contours of the adsorbed species, gaseous and liquid CO, they concluded that the CO molecules freely rotate in the Na zeolites until they collide with the cage walls. For Ca zeolites, the absorption bands of adsorbed CO indicate strongly hindered rotation. This is plausible since CO adsorbs preferentially on the multivalent ions and is held more strongly than on univalent ions. 

The term spectroscopy implies questioning molecules about their eigenvalues of molecular motion. In the case of free molecules these are usually the eigenvalues of free rotation, vibration or electronic motion, which changes for adsorbed molecules to more or less hindered or perturbed motions, including frustrated translation. 

Considerable changes in the intramolecular vibrations of the adsorbed molecule result from adsorption. The first is the transformation of some of the translation and rotational degrees of freedom into vibrational modes, the so-called frustrated translation and rotational modes. The frustrated translation mode perpendicular to the oxide surface corresponds to the vibration of the new adsorption bond. Depending on the geometry and the interactions in the adsorption complex, this bond is formed between one or more atoms from the adsorbate and the surface. The simplest case is one-end adsorption of the molecule, i.e., only one of the atoms is bonded to a surface atom (see Scheme 4) and the frequency of the frustrated translation is equal to the frequency of the adsorption bond. When two or more atoms are connected in the adsorption bond, it is convenient to consider the frustrated translation as the center of mass oscillation of the whole molecule. The frustrated rotational modes usually have a lower frequency, but they are important in the interactions with neighboring surface atoms, in surface diffusion, and in lateral interactions with other adsorbate molecules. In addition to these new vibrational modes, some of the gas-phase vibrations are also changed due to the hindering of the solid surface. 

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