Bond frequency pattern

It provides a kind of bond-frequency pattern. It is also independent of conformational flexibility. In this case, the smoothing parameter B has the unit 1 (one). 

Although binary pattern descriptors exclusively contain information about the presence or absence of distances, frequency pattern descriptors additionally contain the frequency of distances. Frequency pattern descriptors are valuable for direct comparison of structural similarities. For instance, a substructure can be assumed to exist if the frequencies in a substructure pattern occur in the query descriptor. Bond patterns can be used in a similar pattern search approach to determine structural similarities. In this case bond-path RDF descriptors are used. 

RDF descriptors may be used in any combination to fit the required task. For instance, it is possible to calculate a multidimensional descriptor based on bond-path distances and restricted to nonhydrogen atoms in the shape of a frequency pattern. Consequently, more than 1,400 different descriptors are available. A final summary of RDF descriptor types, their properties, and applications is given in Table 5.1. This section summarizes typical applications, some of which are described in detail in the next chapter. 

The hydrogen-bonded liquid amides can be considered as supramolecular entities in this way, the low-frequency pattern described and vibrational coupling to be described may be part of a more general description of the fast dynamics in supramolecules. Two phenomena showing the intermolecular coupling in liquid amides shall be mentioned briefly. 

Figure 8. (a) Search structures used in the inter-nodal bond separation frequency pattern similarity searching (b) Graph showing the frequency curves for the two structures in (a)... 

Table 1-28 lists the mean vibration frequencies characteristic of CH bonds (t/CH, 5CH, yCH) as a function of the substitution pattern. For the v(CH) vibrations, the highest frequency peak disappears in the spectra of 5-substituted derivatives, whereas it is unchanged by substitution at the 2-or 4-positions. This band has been assigned to the v(CH) vibration connected with the CH bond at the 5-position (173). 

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

Figure 4 shows the cleavage pattern of RG-lyase toward the various RGO s. Again no bond-cleavage frequencies could be given since the response factors of the various products were not known. The RG-lyase cleaved the chain four units from the reducing end. When the... 

Figure 5.12 depicts the corresponding adiabatic one-dimensional potential for the covalent rHF proton-transfer coordinate, showing the barrierless switch-over at equilibrium between FH F and F HF bond patterns. The potential well is seen to be extremely flat in the neighborhood of equilibrium, corresponding to the extremely low IR frequency of the proton-transfer mode (1299 cm-1, red-shifted... 

Adsorption of 0.05 monolayers (ML) of CO on this surface gives rise to a peak at 2015 cm-1 corresponding to the internal C-0 stretch frequency of the molecule in the on-top adsorption site and one at 470 cm-1 due to the metal-molecule bond. The latter is not easily observable in infrared spectroscopy. Increasing the CO coverage to 0.33 ML enhances the intensity of the HREELS peaks. In addition, the C-O stretch frequency shifts upward because of dipole-dipole coupling [16, 17]. The LEED pattern corresponds to an ordered (V3xV3)R30° overlayer in Wood s notation (see the Appendix) in accordance with the coverage of 0.33 ML. 

The interpretation of the HREELS spectrum and the structure belonging to the (2x2)-3CO LEED pattern has been the subject of some debate in the literature [57— 59], The CO stretch peak at the lower frequency had previously been assigned to a bridge-bonded CO [57], with obvious consequences for the way CO fills the (2x2) unit cell. A recent structural analysis from the same laboratory on the basis of tensor LEED has confirmed the structures of both the (V3xV3)R30° and the (2x2)-3CO as given in Fig. 8.14, i.e. with CO in linear and threefold positions in the (2x2)-3CO structure [58]. The assignments have also been supported by high-resolution XPS measurements [59],... 

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