CSLogWS Home

CSLogD Home

CSLogP Home

CSpKa Home

CSBBB Home

CSPB Home

CSGenoTox Home

E-state Descriptors

Atom Level E-state E-state Descriptors

The E-State index (S) for an atom is composed of two parts [3,10,11,13].

The intrinsic state (I) combines in a unified manner both the electronic and topological character of the atom in its valence state in the molecule.  The Kier-Hall valence-state electronegativity characterizes the electronic state; the count of sigma bonds (d) to skeletal neighbors characterizes the local topology of the atom.  In this manner electronic and topological (steric) character is encoded, similar to the way that these effects arise from the electron distribution across the molecule.  Electronic and steric character are not artificially separated in the formalism and then recombined in a linear manner as has often been done. 

The perturbation, which is the influence of each other atom (within a molecule) and is encoded through perturbation terms.  The influence of one atom on another, the perturbation:

is computed as the difference between their intrinsic states, diminished by the square of the topological distance (rij) between pairs of atoms.  The perturbations are summed across the whole molecule.

(See the Methods and Descriptors Reference Page for a listing of  intrinsic state values)

The relation for the intrinsic state atom of atom i is given as:

The E-State index for atom i in a molecule is given as:

E-State values tend to be large for atoms in which electron density accumulates and for which intermolecular access is open, such as terminal atoms: =O, -F, -OH.  Atoms that are buried in the structure and/or that have decreased electron density have low E-State values, such as quaternary carbon and nitrogen atoms and the phosphorous or sulfur in phosphate and sulfate esters.  (See E-State spectrum.)

These atom level indices are powerful structure descriptors for models on data sets of structures with a common core. 

(See  references 3,6,7,9,10-28 on our Methods and Descriptors Reference Page for additional information )

Atom-Type E-State Descriptors

For data sets containing diverse structures, atom-type E-State descriptors have been developed [15,16,23].  Since atoms of the same chemical type tend to have similar intermolecular interactions, those atoms or groups can be considered together.  For example, the atom level E-State values for each aliphatic CH groups are summed across the molecule to create the aliphatic CH atom-type E-State descriptor.  This is shown in the figure below, along with the atom type E-state descriptors for aromatic carbons.

Atom Level E-state Example

Here the electron accessibility at the aromatic S(4) group (··CH··) decreases while the electronegativity of the variable substituent increases as the electron accessibility of the (–OH) group decreases, as paralleled by the S(7) value. E-State index (S) for an atom is composed of two parts [3,10,11,13].

Hydrogen Atom Type E-state Example

Examples of the hydrogen E-state (HE-state) with phenols. Here the 1st OH group effects the HE-states for hydrogen attached to -CH, while the 2nd –OH causes decrease in HE-state for –OH groups themselves as well decreasing HE-state on –CH.

Back to: ChemSilico Descriptors