Steric index

Beyond the parameter sets, described above, a steric index, Hj, is introduced, which represents the steric hindrance of the ith atom by other atoms in the molecule. By definihon, the H value of the ith atom ranges from a minimum value of 0 to a maximum value of 1 proportional to its shielding by all other atoms of the molecule. Thus, a funchonal group next to large subshtuents will weakly contribute to the eshmated log P. Including the steric index yields a small, but significant improvement in model accuracy. 

An empirical steric index, denoted as 2D, for substituted benzene rings and defined as the 6-term sum of the distances, given by the L Sterimol length parameter, between the six atoms bonded to the benzene carbon atoms, i.e. the value of the external perimeter of the benzene ring [Taillander et ai, 1983 Ravanel et al., 1985]. It represents the perimeter of the efficacious section and describes the steric properties of aromatic compounds. 

The shape of an alkane molecule is a structure feature often discussed but difficult to quantitate. It is generally believed that molecular shape influences chemical events, however the encoding of this feature has not been well developed up to this time. One approach to quantifying shape has been to measure the effect of its presence upon some standard chemical reaction. From this effort has come the steric index of Taft and later contributions by Charton, Hancock ", Dubois and coworkers and DeTar. Other approaches have regarded molecules as hard-sphere geometric objects from which standardized dimensions can be calculated (Verloop and coworkers or size estimated from radii (Pearlman , Hermann and Bondi ). Austel and coworkers" have proposed a scheme using branching and atom differences. 

Pure, low temperature organic Hquid viscosities can be estimated by a group contribution method (7) and a method combining aspects of group contribution and coimectivity indexes theories (222). Caution is recommended in the use of these methods because the calculated absolute errors are as high as 100% for individual species in a 150-compound, 10-family test set (223). A new method based on a second-order fit of Benson-type groups with numerous steric correctors is suggested as an alternative. Lower errors are claimed for the same test set. 

The A//het(R-R ) value, whieh is derived from Arnett s empirical linear (28) and (29), might be used as an index of feasibility of salt formation. The values calculated by use of (28) or (29) for the combination of [2 ] with [1+], [24+], [26+], [28+] and [40+] are not less than zero but in the range of 10-18 kcalmoP. Tliese A//het(R-R ) values seem to be too large for salt formation. Therefore, the salt formation might be controlled not only by electronic factors, but also by steric hindrance to bond formation. 

The behavior of the different amines depends on at least four factors basicity, nucleophilicity, steric hindrance and solvation. In the literature (16), 126 aliphatic and aromatic amines have been classified by a statistical analysis of the data for the following parameters molar mass (mm), refractive index (nD), density (d), boiling point (bp), molar volume, and pKa. On such a premise, a Cartesian co-ordinate graph places the amines in four quadrants (16). In our preliminary tests, amines representative of each quadrant have been investigated, and chosen by consideration of their toxicity, commercial availability and price (Table 1). 

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. 

On the basis of this equation, an index of nucleophilicity pt can be assigned to each nucleophile Y (see Table 4.13). It is found, moreover, that a plot against pt of logfcy, for reaction of Y with another Pt(II) neutral substrate, is also often linear. Thus, Eq. (2.168) applies, and 5 is termed the nucleophilic discrimination factor (Sec. 4.7.1). Some of the departures from linearity of plots of Ary vs p, which have been observed, disappear if the Pt reference substrate chosen is of the same charge as the Pt reactants. The value of p, for a bulky nucleophile has also to be modified to allow for steric hindrance features. 

A methodology that can classify reactions by using similarity measures has recently been introduced and has now been extended to include a steric similarity index. Both substitution and elimination reactions are included. 

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)... 

Through the I Haya electropy was meant as an electronic information index, it is essentially topological in nature, but takes also into account electronic and steric factors through the intermediacy of topology. 

Recently, Tichy investigated 41) the dependencies of the steric constants, Es, v, L, Bj, B4, MV (molar volume), [P] (parachor), MR (molar refraction), MW (molecular weight), and % (molecular connectivity index) on lipophilicity, as it is measured by n 42) and f43) constants. The data were treated by factor analysis methods. 

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