Face-on configuration

Excimer formation has been shown to be a diffusion controlled process 41-43 in which a sandwich or face-on configuration of the two interacting molecules is required.44-48 It has been deduced that intermolecular distance in the excimer state is smaller than for the same configuration with both molecules in their electronic ground states.44-48 Apart from pyrene, excimer-like emission has been observed from a wide range of aromatic compounds including many alkyl derivatives of such hydrocarbons43-47 and vinyl polymers.48-80... 

Due to a splitting ascertained from the Raman spectrum for the vn CHi wagging mode of cyclopropane in zeolite NaX Cooney et al. inferred an edge-on arrangement [126]. As in the case of zeolites A a splitting of the P9 CHi deformation mode was not observed, a face-on configuration was favoured, which was supported by inelastic neutron scattering experiments on zeolites CoNaA and MnNaA [127]. 

Configurations of different spin or spatial symmetry may not mix. For configurations which are related by a single electron transfer the extent of mixing is related to the overlap of the orbitals which differ in one-electron occupancy. For example, DA and D+A for the face-on dimerization of two ethylenes do not mix significantly since the 0 and < orbitals cannot... 

The reaction has been applied to the synthesis of C-2, C-3 branched-chain uridine derivatives of either lyxo 67a or ribo configuration 67b. Interestingly, only one diastereo-isomer 68a is formed when the olefin chain is located on the (3-face of the furanose ring lyxo configuration), whereas a mixture of the two possible diastereoisomers 68b is obtained when the olefin is located on the a-face ribo configuration) [91] (see Scheme 26). 

The discussion about the possible presence of a small contribution of d-n overlap forces at the surface of NiO is of interest because it may occur with Ni2+ interacting with adsorbates with r-acceptor characteristics, such as CO, NO (Section IV.I.2), and O2. IR spectra of O2 adsorbed at 77 K on progressively sintered NiO sampels (274) follow a trend similar to that observed for CO. In particular, on high-surface-area samples, O2 species formed at edge, step, and corner sites are predominant, whereas on progressively more sintered samples neutral species adsorbed in side-on configuration on Ni2+ of the (001) faces become the only species detectable by IR spectroscopy. 

The open-face-sandwich configuration will have considerable side losses when finite dimensions are imposed on its area and it no longer is an infinite plane. This is because there is no confinement to gas expansion in the direction parallel to the plate. Let us look at the example of a cylinder of explosive driving a plate off its end, Figure 27.8. 

Figure 8.27 Cells for liquid samples, loose powders and chips, (a) Two types of disposable polyethylene sample cups for liquids and loose samples, consisting of a cup and snap-ring to hold the polymer film cover. The cells with polymer film in place are shown at the top of the photograph. The disassembled cup and ring pieces are shown at the bottom of the photograph. [Courtesy of SPEX Certiprep, Inc. Metuchen, NJ (www.spexcsp.com).] (b) Liquid and other loose samples in cells such as those shown in the photo, and then inserted into a sample cassette of the type shown in Fig. 8.26(a) for a face-down configuration spectrometer. As shown, dust sampled on impact filters or thin layer samples may be inserted directly into the sample cassette. [Courtesy of Thermo ARL (www.thermoARL.com).]...

Figure 3. Representative post mortem fracture surfaces (face-on) for DR Oeft) and DS (right) Table I. Crack density results for the three configurations...

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