Atom Center Fragment concept

However, one of the most successfiil approaches to systematically encoding substructures for NMR spectrum prediction was introduced quite some time ago by Bremser [9]. He used the so-called HOSE (Hierarchical Organization of Spherical Environments) code to describe structures. As mentioned above, the chemical shift value of a carbon atom is basically influenced by the chemical environment of the atom. The HOSE code describes the environment of an atom in several virtual spheres - see Figure 10.2-1. It uses spherical layers (or levels) around the atom to define the chemical environment. The first layer is defined by all the atoms that are one bond away from the central atom, the second layer includes the atoms within the two-bond distance, and so on. This idea can be described as an atom center fragment (ACF) concept, which has been addressed by several other authors in different approaches [19-21]. 

The relationships between spectral data (NMR, IR, UV-VIS, MS) and molecular chemical structures cannot be described sufficiently by existing theoretical concepts. Since the start of chemometrics it has always been a challenge to model these hidden relationships at least partially by a multivariate statistical approach. C NMR data exhibit rather strict relationships between chemical shifts and atom-centered molecular fragments. In other fields of spectroscopy, however, the widely used correlation tables are less successful for spectra interpretation and spectra prediction. The correspondence between spectroscopic data (key fragments in MS or band frequencies in IR) and... 

As noted above, it is widely adopted that trigonal aluminum is one of the most important chemisorption and catalytic sites in aluminosilicates. Formation of these centers is usually associated with dehydroxylation. In the preceding section this concept was used to discuss different types of BASs formed as a result of water adsorption on the dehydroxylated surface of a model aluminosilicate fragment. The activity of the trigonal aluminum atoms was particularly manifested in the strong activation of the coordinatively bonded water molecules. In the chemical sense, such a site comprises a typical Lewis acid, which is also confirmed by quantum-chemical calculations. 

The first question posed by this structure concerns the formal charge distribution between the two Gd, B6 and B2 units. If we naively apply the Zintl-Klemm concept we arrive at [Gd3+]2[B6, B2]6. The [B6]2 cluster with six external bonds obeys the cluster electron-counting rules. Consequently, the B2 fragment must have a charge of —4. This corresponds to saturated eight-electron B centers, and requires non-planar (tetrahedral) B centers. This does not agree with the observed planar B centers. But we know the metal does not need to be fully oxidized. Consider sp2-hybridized B atoms which satisfy the octet rule. This would lead to [Gd2+]2[B62 ][B22 ] and suggests a B=B double bond. OK, but planar B is... 

---