Steric effects branching models

Wy The number of j- th branches on the i th atoms of a substituent. These are the steric parameters used in the XB model of steric effects. 

The acid-catalyzed ester hydrolysis provides a good target for MM treatments. DeTar first used hydrocarbon models in which an ester was approximated by an isoalkane (74) and the intermediate (75) by a neoalkane (76). He assumed that if the rate of reaction truly is not influenced by polar effects but is governed only by steric effects of R, as has been generally postulated, the rate must be proportional to the energy difference (AAH ) between 74 and 76. The AAH f is mainly determined by the van der Waals strain in these branched alkanes. Nonsteric group increment terms were carefully adjusted, and statistical mechanical corrections for conformer populations... 

The use of sterlo parameters such as and of methods such as the branching equations to represent sterlo effects on bio-activity Is Justified. Transport parameters are composite they are a function of differences In Intermolecular forces. The function of bulk and area parameters Is to provide the proper mix of Intennol-eoular forces required by a particular mode of bloaotlvlty. In the absence of parabolic or bilinear behavior bloactlv-Ity can be modeled by an equation based on Intermolecular forces and steric effects. 

The simple branching model for the steric effect is given by equation 9 ... 

The F -I- H2 reaction continues to be of interest (75-80). Calculations on F + D2 have been carried out recently on an extremely accurate potential surface (78) and are in remarkably good agreement with crossed beam experiments. Branching between the products in F + HD has been explored (79,80). The F + H2 reaction was recently revisited using an improved surface. Komweitz et al. (82) have probed steric effects in O -I- HC1. Peripheral chemical reactions have also been modeled (83). Collision-induced dissociation in Rg + X2 was recently studied (84). 

A study of the lithium enolate of pinacolone with several a-phenyl aldehydes gave results generally consistent with the Felkin model. Steric, rather than electronic, effects determine the conformational equilibria.77 If the alkyl group is branched, it occupies the large position. Thus, the f-butyl group occupies the large position, not the phenyl. 

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