Steric effects INDEX

Kier, L.B. (1987d). The Substituent Steric Effect Index Based on the Molecular Graph. Quant Struct.-ActRelat, 6,117-122. 

The Topological Steric Effect Index (TSEI) was proposed to describe the steric effects of substituents in terms ofthe relative specific volume ofthe reaction center screened by the atoms... 

Then, the topological steric effect index TSEI for an alkyl substituent, ignoring hydrogen atoms, is defined as... 

The topological steric effect index TSEIx for a substituent containing any heteroatom X is analogously calculated as... 

TSEI = Topological Steric Effect Index steric descriptors (O Taft steric constant)... 

Kier, L.B. (1987d) The substituent steric effect index based on the molecular graph. Quant, Struct, -Act, Relat., 6, 117-122. 

Photo transformations in films of copolymers of a-(2-hydroxy-4-metha-cryloyloxyphenyl)—(2,6-dimethylphenyl)nitrone (HMDN) with methyl methacrylate (MMA) showed that, on radiation at 366 nm, an increase of the steric effect in the A-aryl group leads to a decrease of the refractive index (468, 469). 

Finally, we should establish some index of steric repulsion since in certain instances it may not be immediately obvious which of two isomers is more destabilized by steric effects. The most convenient index is the nuclear repulsion energy, En, which can be calculated readily for any molecular system0. ... 

This order of stability is reflected in the nuclear repulsion energy, which constitutes an index of steric effects , as shown below ... 

An increase of char yield is generally reflected as an improvement in oxygen index. In the styrylpyridine based polyesters and polycarbonate an intermolecular thermally induced Diels-Alder reaction has occurred through the double bond, this increased the char yield and decreased the flammability. The Fries rearrangement, as well as dimerization and isomerization, occurred simultaneously during the UV irradiation of p-VPPB, but no dimerization or isomerization occurred for p,p -BVPDPC, probably due to steric effects. 

Log P and MR are considered thermodynamic descriptors, pR a combined thermodynamic and electronic index, and a an electronic property index, E is designed to account for steric effects. Corrections for non-additivity, based upon the chemical bonding topology, have been suggested and used. These include proximity, bond type, ring, and group shape correction features. (8-10)... 

Kutter and Hansch have analyzed the monoamine oxidase inhibition of phenoxyethylcyclopropylamines (46). Equation 24 shows that specific steric effects as well as electron-attracting and hydrophobic properties of substituents are responsible to the activity. Recently, it has been shown that the E8 value can be used as an index for intermolecular steric effects (46). In Equation 24, E8mm is the sum of E8 values of substituents at the two meta positions. The positive sign of the coefficient of this term means that the corresponding positions on the receptor site cannot accommodate larger substituents because of steric restraint. 

Two of the hydrophihcity scales in Table 2 were derived from experimental measures of the behavior of amino acids in various solvents, namely partitioning coefficients [K-D index of Kyte and Doolittle (30)] or mobility in paper chromatography [Rf index of Zimmerman et al. (31)]. By contrast, the Hp index was obtained from quantum mechanics (QM) calculations of electron densities of side chain atoms in comparison with water (32). The Hp index is correlated highly with these two established hydrophobicity scales (Table 4). Therefore, like the polarizability index, it is possible to represent fundamental chemical properties of amino acids (hydrophUicity, Hp) with parameters derived from ab initio calculations of electronic properties. However, in contrast to polarizabihty (steric effects), hydrophihcity shows significant correlation with preference for secondary structure. Thus, hydrophobic amino acids prefer fi-strands (and fi-sheet conformations) and typically are buried in protein structures, whereas hydrophilic residues are found commonly in turns (coil structure) at the protein surface. 

Several scales presented in Table 3 show promise as measures of fundamental electronic properties of amino acids as does the Hp index of hydrophilicity. Nevertheless, additional improvements are desirable. The polarizability index of steric effects should include hyperconjugation as a component. Clearly, the movement of electrons into antibonding orbitals contributes to molecular deformation (24). The Hp index is based on PM3 calculations of electron densities for the component atoms of amino acid side chains. A more integrative approach with higher-order theory is likely to refine this measure additionally. 

Specific combinations of k indices were also proposed as indices of molecular flexibility (- Kier molecular flexibility index) and steric effects (- Kiersteric descriptor). 

A substituent size descriptor defined as the volume V of the portion of the substituent within a sphere centred at the link atom [Meyer, 1986b], Tlie radius of the sphere was chosen equal to 0.3 nm to comprise the substituent portion responsible for the steric effect of the substituent. It was used, together with the - ovality index calculated on the substituent, to estimate substituent steric effects for substituents with equal volume V , much larger steric effects are observed for globular substituents. 

Lall, R.S. (1982) Steric effect I — Charton s parameter vs topological index. Curr. Sci. -India, 51, 775—777. 

In chemical phenomena, two classes can be described as being influenced by steric effects. First, groups of atoms affect the reactivity of a nearby part of a molecule. This steric effect frequently manifests itself as a repulsive influence toward a reagent attacking a nearby reaction center. These effects are analyzed and encoded into substituent indexes from a standard reaction, based on their relative inhibitory effect. 

In a later paper, Kier expressed the equation above as a new graph-based index capable of predicting steric effects of any group irrespective of its chemical role in the Taft model, This index, H, is calculated as... 

A new structure-activity relationship, Xjj =yS+l, where y is a negative constant, S is the total steric effect, and 4 is the total inductive effect, correlated strongly with available measurements of ozonolysis. New rate coefficients were measured for ozonolysis of a number of unsaturated heteroatomic compounds and it has been emphasized that the inductive effect rather than the steric effect is important in predicting their reactivity %, the inductive effect index, was compared with the Taft a constant and rates of reaction of hydroxyl radical with a given species it correlated strongly in both cases (which should be unaffected by steric factors) suggesting a universal response by olefinic species towards electrophilic addition. ... 

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