Nature of the steric ΩS, ER and [formula omitted] substituent constants. Comparison with the aid of NBO and STERIC analysis

Abstract

The nature of the major steric substituent constant scales for alkyl substituents, i.e. ΩS, ER and ES′ scales, was studied with the aid of the NBO and the natural steric (STERIC) analyses. Cyclohexyl esters R3CCOOC6H11 (R=alkyl or H) were used as the model compounds. Special emphasis was laid on the potential contribution of the polar component in these steric substituent parameters. In the light of our model the ΩS scale seems to be dominantly a steric substituent constant scale as is seen on the strengths of the good correlation between the ΩS constants of the CR3 group and the total steric exchange energy values ETSEE for the model compounds. However, the ΩS values also seem to include a minor electronic component due to the varying electrostatic effect via the Cα atom. On the other hand, ER and ES′ parameters largely hinge on the size dependent polar effect of the CR3 alkyl group. By way of our model this repulsive interaction can be quantified by descriptor ΔqOCO, the natural charge difference qC(CO)−ΣqO for the OC(O) functional group. ΔqOCO depends on the ETSEE values, on qCα and on the polarization coefficients of the oxygen hybrid in the NBO of the πCO bond. The size sensitivity of the kinetic ES′ constants can be connected to variation of the Bürgi–Dunitz angle in the transition state for the standard reaction used. A comparison is made for the qC(CO) or ΔqOCO values computed on the one hand with the NBO formalism and on the other hand with the Hirshfeld formalism. A practical novel substituent constant qC(CO) for the size of the alkyl groups is introduced.