Feature counts

At identical variable number, the overlay-based model outperforms its overlay-independent equivalent training RMS = 0.712 (pICso units), training = 0.712, validation RMS = 0.698, training = 0.724. ComPharm field descriptors therefore appear to have a better predictive power than pairwise pharmacophore feature counts alone. 

Calculated properties were restricted to ClogP <3, the number of rotatable bonds <5, the number of hydrogen bond acceptors (HBA) <4 and the number of hydrogen bond donors (HBD) between 1 and 3. The topological polar surface area (TPSA) was set to <70 A2. In addition, a special feature count excludes structures that are too functionalised (feature rich) or dull (absence of pharmacophoric interaction points). This feature count is the sum of HBA, HBD and the number of five-and six-membered aromatic rings and selection was restricted to fragments with a feature count in the range 4-7. 

There are several ways in which molecular descriptors can be classified. The majority of descriptors are atom-based rather than field-based. The bulk of this review is focused on a discussion of atom-based descriptors. As the name implies, atom-based descriptors are based on individual atoms, with the description extending outward to incorporate information about the atom s environment. The descriptors are typically generated by analysis of 2D or 3D connection tables, and can include ID, 2D, or 3D information about the molecule. Atom-based descriptors include individual atoms, feature counts, substructural fragments, topological indices, atomic properties, pharmacophores (see Chapters 17 and 18 of this book), and calculated physicochemical properties. 

Figure Cl.5.2. Fluorescence excitation spectra (cps = counts per second) of pentacene in /i-teriDhenyl at 1.5 K. (A) Broad scan of the inhomogeneously broadened electronic origin. The spikes are repeatable features each due to a different single molecule. The laser detuning is relative to the line centre at 592.321 nm. (B) Expansion of a 2 GHz region of this scan showing several single molecules. (C) Low-power scan of a single molecule at 592.407 nm showing the lifetime-limited width of 7.8 MHz and a Lorentzian fit. Reprinted with pennission from Moemer [198]. Copyright 1994 American Association for the Advancement of Science.

Clearly, the next step is the handling of a molecule as a real object with a spatial extension in 3D space. Quite often this is also a mandatory step, because in most cases the 3D structure of a molecule is closely related to a large variety of physical, chemical, and biological properties. In addition, the fundamental importance of an unambiguous definition of stereochemistry becomes obvious, if the 3D structure of a molecule needs to be derived from its chemical graph. The moleofles of stereoisomeric compounds differ in their spatial features and often exhibit quite different properties. Therefore, stereochemical information should always be taken into ac-count if chiral atom centers are present in a chemical structure. 

Descriptors have to be found representing the structural features which are related to the target property. This is the most important step in QSPR, and the development of powerful descriptors is of central interest in this field. Descriptors can range from simple atom- or functional group counts to quantum chemical descriptors. They can be derived on the basis of the connectivity (topological or... 

This episode has been displayed in some detail because colloid science is a clear instance of a major field of research which has never quite succeeded in gaining recognition as a distinct discipline, in spite of determined attempts by a number of its practitioners. The one feature that most distinguishes colloid science from physical chemistry, polymer science and chemical engineering is that universities have not awarded degrees in colloid science. That is, perhaps, what counts most for fields with ambitions to become fullblown disciplines. 

Reductions of [PtClg] " in an atmosphere of CO provide a series of clusters, [Pt3(CO)6] ( = 1-6,10) consisting of stacks of Pt3 triangles in slightly twisted columns Pt-Pt = 266 pm in triangles, 303-309 pm between triangular planes (Fig. 27.12). A feature of these and other Pt clusters is that they mostly have electron counts lower than predicted by the usual electron counting rules. In the series just mentioned for instance, = 1 and n — 2 have electron counts of 44 and 86 whereas 48 and 90 would... 

For Clean Rooms (rooms of a very high standard) dust count per unit volume will be specified, but other specifications for room cleanliness are usually in terms of filtration performance against a standard test dust. Other important features are resistance to air flow and dustholding capacity, leading to the fan energy required and filter life. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

Mossbauer Spectral Analysis and Analog Measurements. Mossbauer spectra were obtained in the temperature range between 200 and 270 K and in two different energy windows (14.4 and 6.4 keV), which provide depth selective information about a sample [346]. To compensate for low counting statistics due to limited integration time, all available spectra were summed for the integrations on the undisturbed and brushed surface, respectively. In addition to kamacite (a-(Fe,Ni)) ( 85%) and small amounts of ferric oxide (see Fig. 8.38), all spectra exhibit features indicative for an additional mineral phase. Based on analog measurements... 

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