Steric effects parameterization

Segmental equation A steric effect model that separately parameterizes each segment of a substituent. It requires fewer parameters than the XB equation and is generally more effective than the SB equation. 

MR, the molar refractivity, which parameterizes polarizability and steric effects and Verloop s parameters, which are steric substituent values calculated from bond angles and distances. 

Stewart" proposed a parallel between the rate of esterification of 2-substituted benzoic acids and the molecular weights of the substituents. The nitro group deviated strongly from this relationship. It is the first work to attempt to relate the steric effect of a group to some property that might at least in part be a measure of size. Kindler made the first attempt at defining a set of steric parameters. These parameters were later shown to be a function of electrical effects. The first successful parameterization of the steric effect is due to Taft, who defined the steric parameter Es for aliphatic systems by equation 27 ... 

Despite the success in parameterizing acid/base and many other properties for a range of different compounds, it is obvious that the simple electrostatic model used by Taft and extended by others [27, 28] have fundamental weaknesses - both with regards to the domain of validity and at a more fundamental level. The model is more intuitive than physical, in the sense that the inductive effect, polarisation effect, resonance effect, mesomeric effect and steric effect have no proper quantum mechanical definition, and can therefore not be derived directly from the system s wave function [29,30]. 

The principal aim of the reported studies was to model structures, conformational equilibria, and fluxionality. Parameters for the model involving interactionless dummy atoms were fitted to infrared spectra and allowed for the structures of metallocenes (M=V(II), Cr(II), Fe(II), Fe(III), Ru(II), Os(II), Co(II), Co(III), Ni(II)) and analogues with substituted cyclopentadienyl rings (Figure 14.3) to be accurately reproduced [160]. The preferred conformation and the calculated barrier to cyclopentadienyl ring rotation in ferrocene were also found to agree well with the experimentally determined data (Table 14.1). This is not surprising as the relevant experimental data were used in the parameterization procedure. However, the parameters were shown to be self-consistent and transferable (except for the torsional parameters, which are dependent on the metal center). An important conclusion was that the preference for the eclipsed conformation of metallocenes is the result of electronic, and not steric effects, van der Waals and electrostatic terms were similar... 

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