Aromatic atom

In order for the orbitals to overlap most effectively so as to close a loop, the sp atoms are forced to lie almost vertically above the plane of the aromatic atoms. In 107, Hb is directly above the aromatic sextet and so is shifted far upfield in the NMR. All homoaromatic compounds so far discovered are ions, and it is questionable as to whether homoaromatic character can exist in uncharged systems. Homoaromatic ions of 2 and 10 electrons are also known. 

HA - hydrogen bond acceptor) and having an interatomic distance of about d A. These pharmacophore pair counts are truncated to a maximum value of 10, for example, Ar-Ar6 will equal 10 for all molecules with 10 or more aromatic atom pairs at about 6 A. Reported averages and variances take this truncation into account. 

At methyl-aromatic is the number of methyl groups attached to aromatic atoms. 

In Benson notation, an aromatic carbon, Cb, is presumed to be attached to two other aromatic atoms. Thus, for example, benzene can be entirely described by six Cb-(H) groups. In the present example, the molecule is divided into five Cb-(H) and one Cb—(Cl) groups. The ring strain (in this case a stabilization) is automatically accounted for in the contribution from each group containing a Cb, and no additional groups or contributions are necessary. 

Number of nitrogen atoms Number of aromatic atoms Number of rotatable bonds Number of hydrogen bond acceptors Sum of van der Waal surface areas of basic atoms... 

Consider only one 2pz AO per aromatic atom. (Ignore all hydrogens, core electrons, "sigma bond" electrons, lone pairs, etc.)... 

The structural parameter Ham/Car, which gives the ratio of substitutable edge atoms to total aromatic atoms, is important because it is a measure of the average aromatic ring system which is independent of molecular weight measurements. Figure 2 shows the Ham/Car values for some typical 1-7 cata-condensed aromatic ring systems. When the Ham/... 

R)) for aromatic donor and acceptor in edge-to-edge configuration (52), where and Njj are the number of aromatic atoms in acceptor and donor, respectively, and R the edge-to-edge... 

Nalami(1 is the total number of linkages between amide (-CONH-) and similar (such as urea) groups and non-aromatic atoms. If an amide group is attached to non-aromatic atoms on both sides [as in poly(e-caprolactam)] it contributes 2 to Naianijd. If it is attached to a nonaromatic atom on one side and an aromatic atom on the other side [as in poly(hexamethylene isophthalamide)] it contributes 1. If it is attached to aromatic atoms on both sides [as in poly(p-phenylene terephthalamide)] it does not contribute anything. 

Benzene is typically thought of as a combination of two equivalent resonance structures. These could be written as the SMILES C1=C-C=C-C=C1 and C1-C=C-C=C-C=1. In order to have just one representation for benzene and other aromatic systems, SMILES handles these aromatic systems specially, treating the atoms in an aromatic ring as a special aromatic type and the bonds as a special aromatic type. The lowercase symbol is used to denote an aromatic atom in SMILES and SMARTS. The SMILES for benzene then becomes clcccccl. A bond between aromatic atoms is an aromatic bond, unless otherwise spelled out. For example, biphenyl can be written as clcccccl-clcccccl. 

While the several resonance forms for aromatic systems are neatly solved using aromatic atom types in SMILES, the issue of multiple tautomers cannot be handled as neatly. This is a good thing. After all, it is quite possible to distinguish two different tautomers experimentally and measure different properties that may need to be stored in a database. On the other hand, it is not possible to distinguish two different resonance forms experimentally. Every equivalent resonance form for a structure ought to be considered to be the same structure. Resonance is simply a theoretical concept. 

Aromatic structures are obtained using lowercase letters for aromatic atoms. Benzoic acid is clccccclC(=0)0. 

---