Descriptor bond orders

Table 3.33. NBO/NRT descriptors of molecules in Table 3.32, including atomic charges (Q), central atom d-orbital occupancy (c/m )> NRT bond orders, and central-atom valency (with percentage ionic character)...

Table 3.36. Geometries and NBO/NRT descriptors of common oxyanions XOmn (see Fig. 3.91), showing symmetry, bond length Rxo, NRT bond order bxo and central-atom valency Vx, atomic charges Qx and Qo, and d-orbital occupancy dx for representative first- and second-row species...

FIGURE 23.5 Profiles of different local reactivity descriptors (electrophilic attack) along the path of the gas phase SN2 substitution F + CH3—Fb —> Fa—CH3 + Fb. Profiles of energy and bond order are also shown. (Reprinted from Chattaraj, P.K. and Roy, D.R., J. Phys. Chem. A, 110, 11401, 2006. With permission.)... 

Fig. 3.2 Schematic of the CATS descriptor calculation, (a) The hydrogen-depleted two-dimensional molecular graph provides the input, (b) The graph is simplified for the distance matrix computation different bond orders are not considered (unweighted graph) and all element types are disregarded. The algorithm starts at an arbitrary chosen atom and visits all nodes of the graph in a breadth-first manner, thereby building up the distance matrix. The numbers at the vertices are used to reference individual atoms in the distance matrix.

A complete dichotomy between bond order and other functionality. This is in contrast to IUPAC s organic nomenclature, which uses morphemic suffixes to specify both degree of bond unsaturation (ane, -ene, and -yne), and also selected functional groups (-one, -al, etc.). In the proposed system, on the other hand, the individual chemical symbols for atoms alternate with bond descriptors (small integers or selected symbols). In particular, traditional single, double and triple bonds are... 

These are descriptors for molecule bonds proposed with the aim of estimating the bond order defined in quantum-chemical theory or of generally defining bond weights so as to distinguish the bonds in a - molecular graph. 

Molecular descriptors are derived by the additive contributions of the graphical bond orders of all bonds in the molecule as ... 

They are usually called graphical bond order descriptors. 

Other encountered graphical bond order descriptors are the /V/ index, the W AV index, the WW AVW index, the J7J index, the CID7CID index, and the - Z7Z index derived, respectively, from the - Randic connectivity index, the -> Wiener index, the - hyper-Wiener index, the - Balaban distance connectivity index, the - Randic connectivity ID number, and the Hosoya Z index. 

The L index was proposed as the molecular descriptor defined as the simple linear combination of molecular path counts of order one P (the number of bonds), order two (the -> connection number Na), and order three P ... 

Bond multiplicity is taken into account by augmenting the edge distance matrix with a supplementary column and row where the elements are conventional bond orders, therefore obtaining an edge distance matrix for multigraphs [Bonchev, 1983]. All the local vertex invariants and molecular descriptors defined above can also be calculated on this matrix. 

The Z7Z index is among the graphical bond order descriptors and can be obtained from the Hosoya Z matrix only by considering the entries relative to adjacent vertices (i.e. bonds) ... 

Jenkins steric parameter -> steric descriptors Jt index -> Balaban distance connectivity indices J/J index -> bond order indices (O graphical bond order)... 

Merrifield-Simmons bond order -> symmetry descriptors (O Merrifield-Simmons index) Merrifield-Simmons index -> symmetry descriptors mesomeric effect -> electronic substituent constants... 

Randic, M., Mihalic, Z., Nikolic, S. and Trinajstic, N. (1994c). Graphical Bond Orders Novel Structural Descriptors. J.Chem.Inf.Comput.ScL, 34,403-409. 

Fig. 10 Use of pharmacophore descriptors in order to find the correct superposition of dihydrofolate and methotrexate [73] (see legend of Fig. 9 for the pharmacophore types), a) According to the structural formula the molecules are very similar and can be superimposed easily, b) However, visualization of hydrogen bonding acceptor and donor sites turns up major differences in the hydrogen bonding pattern, c) One-by-one mapping of the pharmacophore vectors leads to a different superposition the heterocyclic ring system of methotrexate is rotated to reproduce the pharmacophore of dihydrofolate.

The bonding situation in the VO moiety [and also as in the isoelectronic V(NR) moiety] is sometimes treated in terms of a triple bond (cr - - 2t7 donor bonds). Since the respective bonding orbitals are available, this view may be considered reasonable, and is supported by DFT calculations. In the frame of the valence bond description, the bond order between V and (or NR ) is 2, or less, if the oxo group is involved in additional (weak) bonding interactions. Throughout this book, the VO bond will be referred to in terms of a double bond, and hence the descriptor V=0 will be used rather than V=0. 

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