Constitution and conformation

The recognition of differences in molecular structures — the characterization of structural similarity — is a special feature of RDF descriptors. Changes in the constitution of a molecule will generally lead to changes in peak positions. For instance, a typical Cartesian RDF descriptor of a linear alkane shows periodic peaks — essentially the sum of the C-C distances. Small changes in the structure can lead to a series of changes in the descriptor. Some of the typical effects on a Cartesian RDF descriptor are as follows  

Conformational variations have significant effects on the shape of the entire descriptor because of the overall change in interatomic distances. For instance, the RDF descriptor of the chair and boat conformer of cyclohexane can be distinguished by the frequency of distances (i.e., in a first approximation by the intensity of the peaks). 

A typical feature of Cartesian RDF descriptors is a (at least virtual) decrease in characteristic information with increasing distance. The influence of the short distance range (in particular, the bond information) dominates the shape of a Cartesian RDF. In contrast to that, the bond-path descriptor is generally simpler it exhibits 

FIGURE 5.7 Comparison of Cartesian and bond-path RDF descriptor (256 components each) of a cyclohexanedione derivative. The bond-path descriptor exhibits sharper peaks in particular single bond-distance patterns and is generally larger than the corresponding Cartesian descriptor. 

The topological-distance descriptor is a quite rough representation of the shape of a molecule because the original distance information reduces to integers (the number of bonds). However, a normalized topological-distance descriptor represents a probability distribution, which is useful for analysis methods relying on interpolation. 

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The stereochemistry and dynamics were investigated by spectroscopic and structural techniques yielding a consistent picture of the general constitutional and conformational pattern, which is best described as based on the chair- or (less frequently) tub-form1 4 of a folded six-membered ring with two opposite edges extended by the... 

With due consideration of the explanations just presented for the observed, relative stabilities of cyclic acetals derived from polyols, in terms of their constitution and conformation, nearly all of the following observations on the selective hydrolysis of cyclic acetals of alditols and dialkyl dithioacetals may be readily understood. 

H and 13C NMR techniques have widely been used to determine the configuration of new dioxepins and dithiepins and to elucidate the constitution and conformation of new naturally occurring substances. For example, the configuration of oximes 14 was determined by H and 13C correlated spectra, correlation spectroscopy (COSY), nuclear Overhauser enhancement spectroscopy (NOESY), heteronuclear correlation (HETCOR) spectroscopy, and hetero-nuclear multiple bond correlation (E1MBC) spectroscopy <1998CCA557>. 

The photolysis of organic nitrites of appropriate constitution and conformation in solvents such as benzene or acetonitrile transforms them into A-nitroso alcohols via the sequence (1) a homolytic fission of the O-N bond of their nitrosoxy group (2) an intramolecular d-hydrogen abstraction of the resulting alkoxyl radicals to generate a d-carbon radical and (3) formation of d-nitroso alcohols by combining of the d-carbon radical with the generated nitric oxide. The nitroso alcohols are isolated as d-hydroxyimino alcohols as a result of spontaneous thermal isomerization or as nitroso-dimers [1] (Scheme 1). This transformation has been named the Barton reaction [2, 3],... 

Often, these adducts were characterized by vibrational spectroscopy, and Raman spectroscopic investigations were particularly informative and indicative for the occurrence of E E bonds. Some E E stretching vibrations vE E are summarized in Table I. They nicely show the expected dependency of vE E on the atomic masses of the respective elements and on the masses of the substituents. Some compounds were further characterized by nuclear quadrupole resonance spectroscopy, which gave indications for phase transfer processes and for the extent of electron transfer from the ligands to Ga or In atoms.15,16 Most helpful for the discussion of the constitution and conformation of these subhalides are the results of X-ray... 

In this section it will be shown, how special molecular constitution and conformation may force the formation of macrocycles. Furthermore, examples will be discussed in which the photoinduced electron transfer (PET) from electron rich functional groups to the excited keto carbonyl group initiates a macrocyclization. The exceptionally successful PET assisted macrocyclizations of phthalimides will be treated separately in Sec. 3.4.6. 

Changes in constitution and conformation also have significant effects on the statistical evaluation of data sets. The following examples of small molecule data sets... 

The terms similarity and diversity can have quite different meanings in chemical investigations. Describing the diversity of a data collection with a general valid measure is almost impossible. Descriptor flexibility allows the characterization of similarity by means of statistics for different tasks. The statistical evaluation of descriptors shows that it is recommended to interpret correlation coefficients together with the symmetry of distribution. In contrast to correlation coefficients, skewness and kurtosis are sensitive indicators to constitutional and conformational changes in a molecule. This feature allows a more precise evaluation of structural similarity or diversity of molecular data sets. 

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