Natural steric analysis

Badenhoop JK, Weinhold F (1997) Natural steric analysys ab initio van der Waals radii of atoms and ions. J Chem Phys 107 5422-5432... 

The most effective approach to interpreting the barriers for a wide range of compounds lies in the consideration of the relative interactions within the Dewar, Chatt, Ducanson model of metal alkene bonding. An extended Hiickel MO approach has explored the interactions of the valence orbitals and examined the important interactions. A comprehensive extended Hiickel MO treatment of ethylene bonding and rotational barriers by Albright, Hoffmann et a/. presents an excellent analysis and the reader is referred to their paper for further discussiou. We have found that the following approach, which considers oifly three orbitals on the metal and the n and y orbitals of the alkene, provides the essential elements for understanding the barriers to rotation. Naturally, steric effects and secondary interactions with other orbitals modulate these primary iuteractious. 

Monod, J., Wyman, J., and Changenx, J.-R, 1965. On die nature of allo.steric tran.sitions A plan.sible mo(. Journal of Molecular Biology 12 88-118. The cla.ssic paper diat provided the first theoretical analysis of allo.steric regulation. 

Scheme 9.7 gives examples of each of these types of stereoselectivities. The analysis of any particular system involves determination of the nature of the reactant, e.g., has transmetallation occurred, the coordination capacity of the Lewis acid, and the specific steric and stereoelectronic features of the two reactants. 

In this section, we present a unified picture of the different electronic effects that combine to determine methyl rotor potentials in the S0, Sp and D0 electronic states of different substituted toluenes. Our approach is based on analysis of ab initio wavefunctions using the natural bond orbitals (NBOs)33 of Weinhold and cowork-ers. We will attempt to decompose the methyl torsional potential into two dominant contributions. The first is repulsive steric interactions, which are important only when an ortho substituent is present. The second is attractive donor-acceptor interactions between CH bond pairs and empty antibonding orbitals vicinal to the CH bonds. In the NBO basis, these attractive interactions dominate the barrier in ethane (1025 cm-1) and in 2-methylpropene (1010 cm-1) see Figure 3. By analogy, donor-acceptor attractions are important in toluenes whenever there is a substantial difference in bond order between the two ring CC bonds adjacent to the C-CH3 bond. Viewed the other way around, we can use the measured methyl rotor potential as a sensitive probe of local ring geometry. 

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