Rotational barriers steric effects

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. 

Valence Isomerism. Restricted Rotation and Permutational Isomer i sm. - The variable temperature spectra of bicyclic tetraphosphines (29) has been analysed in terms of valence isomerism involving the corresponding two coordinate open-chain isomers.62 A report on ab initio MO calculations of the inversion barriers for a series of methyl phosphines includes a discussion of the electronic consequences of steric effects.63 Inversion barriers of 1,2-diphosphinobenzenes are in the range 100 - 110 kJ mol". 66 The ring inversion barrier for a dibenzophosphorin has also been measured.63... 

Mannschreck et al. (44) examined the effect of substituents on the barriers to rotation in 2,4,6-trisubstituted benzamides. In /V-benzyl-A(-methyl-2,4,6-tri-bromobenzamide, the rotational barrier (AG ) is 23.8 kcal/mol at 35.8 to 40.6°C for the Z -> E process in quinoline (44). This should be compared with AG of 23.4 kcal/mol for the same process with the trimethyl compound (5). It is seen that steric effects are of primary importance, inasmuch as the van der Waals radii of the methyl and bromo groups are almost the same. 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

Furthermore, the estimation of the rotational barriers of ortho-substituted 6-aryl-1,1,5-trimethylindanes (Fig. 1a) [24] by NMR as well as conformational NMR-studies of (9-anthryl)carbinol derivatives (Fig. 1b) [25] have shown the CF3 substituent to have steric effects that are comparable to those of an isopropyl group (Fig. 1). 

Whereas the van der Waals radius of the fluorine atom is the smallest one after that of hydrogen, its volume is actually closer to that of oxygen (Table 1.16). Note that if the volume is an intrinsic property, steric effects are dependent on the observed phenomena. They frequently appear in dynamic processes. This allows comparison of steric parameters of various groups, fluorinated or not. These parameters show that the CF3 group is at least as bulky as an isopropyl or isobutyl group (Table 1.17). These data are confirmed by the values of the rotation, or of inversion barriers, of fluorinated diphenyl-type compounds (Figure 1.6). 

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