Steric effects segmental

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. 

Vi A steric parameter based on van der Waals radii that is a measure of the steric effect of the i-th segment of a substituent. The i-th segment consists of the i-th atom of the longest chain in the substituent and the groups attached to it. The MSI principle is assumed to apply and the segment is assigned the conformation that gives it the smallest possible steric effect. 

Set 0X14, l-oximino-3-X-5-methyl-l,2-benzoquinones, is of interest because the substituents in this data set do not differ significantly from each other in electrical effects. As chemical reactivities in water normally do not depend on polarizability, it would seem that the variations in pXa in this data set should be a function of steric effects. Correlation with the two parameter segmental steric effect equation gave best fit with the v/ parameters. The correlation is significant at the 99.0% confidence level and is good. It seems probable that the pXa values of these compounds are dependent on steric effects. 

End-to end dimers of 210 incorporating a fluorinated spacer segment (compound 211) form only a nematic and two different types of SmA phases, one with bilayer structure, the other an interdigitated one. This phase transition was attributed to a change of the molecular conformation from folded (bent), stabilized by segregation of the RF-segments (SmA(HT)), to linear, favored by reduced conformational mobility and steric effects (SmA(LT)) [368]. 

There is a significant dependence on the steric effect of the first segment which is adjacent to the active site. The positive sign indicates steric acceleration of the rate. There may possibly be a small steric effect due to the second segment which has a negative sign indicating rate deceleration. 

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