Atomic interaction guest-host

Fig. 18. Closed H-bond loops (dotted and hatched regions indicating host-guest and host-host interactions, respectively) in alcohol inclusions of l and 26. Packing excerpts show the rings formed in the coordinatoclathrates (a) 1 1-PiOH (2 1)77>, (b) 26 1-BuOH (1 1) 71 (H-atoms of the hosts are shown as sticks only)...

Because the size regime of n=l-6 atoms is of great practical significance to the spectroscopic, chemical and catalytic properties of supported metal clusters in both weakly and strongly interacting environments (28), it is important to study very small metal clusters in various types of substrate as well as in the gas phase. In this way, one can hope to develop a scale of metal cluster-support effects (guest-host interactions) and evaluate the role that they play in diverse technological phenomena. 

One of the objectives of this paper is to evaluate the spectroscopic and photochemical consequences of the occurrence of markedly disparate guest-host interactions in the ground and optically excited states of Cu and Ag atoms, and some of their low nuclearity clusters, in rare gas as well as other supports. Original papers should be consulted for details. 

The observation of large Stokes shifts for the 2P-2S transition of entrapped Ag atoms indicates that the guest-host interactions are markedly different for the 2S and 2P states of this system and can be explained in terms of matrix cage relaxation effects. 

From the above discussion, it should be possible to appreciate how extremely subtle differences in guest-host interactions in the ground and excited states of Cu and Ag atoms and dimers in both non-reactive and reactive supports can lead to dramatically distinct chemical reactivity patterns and dynamical processes. Photochemical and photophysical phenomena of this kind should provide chemists of the 21st century with a rich field for fundamental and applied research, offering considerable scope for experimental challenges and intellectual stimulation. 

Bandyopadhyay and Yashonath (31), in an extension of their work on MD studies of noble gas diffusion, presented MD results for methane diffusion in NaY and NaCaA zeolites. The zeolite models were the same as those used in the noble gas simulations (13, 15, 17, 18, 20, 28, 29) and the zeolite lattice was held rigid. The methane molecule was approximated as a single interaction center and the guest-host potential parameters were calculated from data of Bezus et al. (49) (for the dispersive term) and by setting the force on a pair of atoms equal to zero at the sum of their van der Waals radii (for the repulsive term). Simulations were run for 600 ps with a time step of 10 fs. 

Within the frame of this approach the interaction energy of two molecules is described as the sum of pairwise interactions. It is assumed that each atom of the host interacts with each atom of the guest independently ... 

Ag° atoms isolated in the cubo-octahedral site of rare gas solids. The observation of multiple structure on the 2P 2S absorption and large red spectral shifts for the 2P - 2S emission of site I entrapped Ag° atoms, indicates that the guest-host interactions are markedly different for the 2S and 2P states and can be explained in terms of site I relaxation effects, using a vibronic coupling model similar to that described in detail for Ag° atom rare gas cage complexes (5). 

Rather dramatic alterations in the electronic properties and relaxation dynamics of supported silver atoms and clusters have been traced to extremely subtle differences in ground and excited state guest-host interaction potentials. For Ag° and Ag2+ in faujasite zeolites, pronounced changes in their optical... 

As we noted previously, an isolated small molecule cannot decay non-radiatively and the relaxation is therefore a sensitive function of the guest-host interaction potential. For this reason it is often instructive to compare the relaxation rates in different hosts. A particularly useful set of solvents for such comparison studies are the four rare-gas solids Ne, Ar, Kr, and Xe. While the pairwise interaction energy increases in this series from 60 cm for two Ne atoms to sa400 cm in the case of Xe, this increase is paralleled by the increase in mass, and the Debye frequency remains for all the rare gases relatively constant near 65 cm . 

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