Steric energy barriers

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

What other factors might be responsible for difference in activation energies Compare atomic charges anc electrostatic potential maps for the Sn2 transition states Does the increase in steric crowding lead to enhanced o diminished charge delocalization Explain. How, if at all would this be expected to affect the energy barrier Why ... 

Tetramethylbisdialine 86 has been used to synthesize substituted pentaheli-cenes characterized by a relatively high energy barrier to racemization due to the marked steric interactions between the methyl groups at the terminal aromatic rings [68b] (Scheme 2.39). 

Identifying the transition state and the associated energy barrier is essential for understanding the course of a reaction. Of course, details of the shape of the potential material, e.g. steric hindrance and entropic effects, may impede the system from crossing the barrier. The barrier energy (which is not very different from the activa-... 

In such systems the requirement of the electrostatic contribution to colloidal stability is quite different than when no steric barrier is present. In the latter case an energy barrier of about 30 kT is desirable, with a Debye length 1/k of not more than 1000 X. This is attainable in non-aqueous systems (5), but not by most dispersants. However when the steric barrier is present, the only requirement for the electrostatic repulsion is to eliminate the secondary minimum and this is easily achieved with zeta-potentials far below those required to operate entirely by the electrostatic mechanism. 

Among catalysts I-IV, which predominantly catalyze the generation of cyclodimer products, the overall lowest intrinsic free-energy barrier of 20.5 kcal mol-1 (AG nt) for 2a -> 8a appears for catalyst IV with L = P(OPh)3, where both electronic and steric factors are seen to assist the formation of VCH to a similar amount. Reductive elimination involves a higher intrinsic barrier (AG nt) for catalysts bearing moderately bulky, donor phosphines,... 

Figure 3.56 The rotation-barrier profile for CH3CH3 in (a) fully optimized and (b) idealized rigid-rotor geometry, showing the total energy Etotai (circles, solid line), vicinal cch-cch stabilization iw (circles, dotted line), and steric energy -Enteric (squares, dashed line).

In addition to the electronic difference between PR3 and PH3, bulkier ligands on the phosphine can change the reaction through their steric effect. Using the R = Bu on the anthraphos system, Haenel et al. calculated the available molecular surface (AMS) around the metal center as a measure of the space available to the alkane (13b). They correlated the AMS to the relative reactivities of the catalysts and the results show that two bulky tert-butyl groups on each P certainly limit the access to the metal center, and thus, may reduce the reactivity. Other theoretical studies on the pincer complexes showed that this steric contribution/ limitation plays a less important role than the activation barriers introduced by the catalyst itself (22), where the increase in energy barrier induced by the bulky 4Bu is smaller than the original barriers calculated... 

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