Rotation, hindered, adsorbed

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... 

A secondary amide group is nearly planar and has a hindered rotation around the C-N bond which exists predominantly in the transoid conformation. This provides a dipole which is accessible for inter- and intramolecular hydrogen donor and acceptor interactions and dipole stacking. The silanol groups (Si-OH) of the adsorbent are ideal partners not only for hydrogen bonding but also, due to their acidity, for electrostatic interactions. 

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. 

Fig. 16. Fundamental and overtone spectra of adsorbed CO at high coverage. Free or hindered rotation That s the question.

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. 

The N2 rotational distribution also showed a sub-thermal distribution (Fig. 27) with a rotational temperature of 450 K, similar to the translational temperature. Despite the cold translational and rotational distributions, the vibrational co-ordinate is excited, with excess population in the high vibrational states. Remarkably, this result had been suggested previously on the basis of threshold ionisation measurements of N2 desorbed from Pd covered field ionisation tips [129]. Unlike the translational energy distributions observed for desorption from Ru(0001) [103], the energy release on Pd(l 1 0) does depend on the vibrational state, (E) decreasing rapidly for excited N2(u) states [127]. A cold translational distribution is indicative of desorption from a bound state, where cooling of the adsorbate hindered... 

Changing the E structure of VI to the less stable Z, VII, produces yet a third deuterium distribution, in which more deuterium is incorporated into a and p-positions of Z than of E. This may result from adsorbed VII (adsorbed at its Si face) adding one deuterium from the surface to the P-carbon (making a half hydrogenated VII shown below), isomerizing by rotation about its newly formed C -C bond to the less sterically hindered transoid conformation, and, thereby, storing deuterium in a p-position relatively inaccessible to the surface. 

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