Steric repulsion energies

ADF calculates various chemically meaningful terms that add up to the bond energy, with an adaptation of Morokuma s bond-energy decomposition to the Kohn-Sham MO method. The individual terms are chemically intuitive quantities such as electrostatic energy, steric repulsion, Pauli repulsion, and orbital interactions. The latter are symmetry decomposed according to the Ziegler transition-state method. ... 

Alkanes prefer structures that stagger bonds on adjacent tetrahedral carbons (see Chapter 5, Problem 1). However, steric repulsion can affect the relative energies of staggered conformations. 

Examine the maximum energy conformations. What evidence is there for steric repulsion between methyl groups in the conformation that eclipses these groups ... 

Internal rotation in isooctane (2,2,4-trimethylpentane) creates a large number of staggered conformations. However, only rotation about the C3-C4 bond produces conformations with different structures. Plot the energy of isooctane (vertical axis) vs. HCCCtBu torsion angle, i.e., about the C3-C4 bond (horizontal axis). How many minimum energy structures are there Are they all fully staggered Draw Newman projections that show the conformation of these structures. How does steric repulsion affect isooctane conformation ... 

Try to explain the conformational preference in terms of steric repulsion. Which ring atom(s) in the higher-energy conformer approach the CH3 group most closely (Make sure that you find all significant nonbonded interactions.) Which of these interactions are absent in the lower-energy conformer Can interactions that appear in both conformers account for the conformational preference ... 

Compare energies for both diaxial and diequatorial chair conformers of trans-2-fluoromethyIcyclohexane (X = Me). Which conformer is preferred Examine a space-filling model of each conformer. Which group is largest methyl, fluorine, or hydrogen Which is smallest Does the preferred conformer minimize steric repulsion Explain. 

The parameter redundancy is also the reason that care should be exercised when trying to decompose energy differences into individual terms. Although it may be possible to rationalize the preference of one conformation over another by for example increased steric repulsion between certain atom pairs, this is intimately related to the chosen functional form for the non-bonded energy, and the balance between this and the angle bend/torsional terms. The rotational banier in ethane, for example, may be reproduced solely by an HCCH torsional energy term, solely by an H-H van der Waals repulsion or solely by H-H electrostatic repulsion. Different force fields will have (slightly) different balances of these terms, and while one force field may contribute a conformational difference primarily to steric interactions, another may have the... 

For the 2-1-2 pathway the FMO sum becomes (ab — ac) = a b — c) while for the 4 -I- 2 reaction it is (ab-I-ab) — a (2b). As (2b) > (b — c), it is clear that the 4 + 2 reaction has the largest stabilization, and therefore increases least in energy in the initial stages of the reaction (eq. (15.1), remembering that the steric repulsion will cause a net increase in energy). Consequently the 4 - - 2 reaction should have the lowest activation energy, and therefore occur easier than the 2-1-2. This is indeed what is observed, the Diels-Alder reaction occurs readily, but cyclobutane formation is not observed between non-polar dienes and dieneophiles. 

In Table 5 are summarized theoretical lower-excitation energies and corresponding intensities for nonalternant hydrocarbons for which experimental data are available. Theoretical values are in general agreement with experimental ones. The predicted excitation energies for heptalene (VII) are rather small as compared with experimental data. This may arise as a consequence of nonplanarity due to the steric repulsions between ortho-hydrogen atoms. 

An estimate of the energy of steric repulsion may be obtained by comparing the observed heat of polymerization with the value calculated on the assumption that the interaction energy between substituents is zero. For the latter purpose, we consider the following steps... 

The observed heats evolved ( AHp) in polymerization are equal to or smaller than those calculated in all cases investigated. Their differences given in the last column of Table XXVI may be considered to represent the energy of steric repulsion between neighboring groups within the polymer chain. For mono-substituted units the difference... 

Tertiary alkyl substituents in the ori/zo-position to the phenolic group cause an additional repulsion in TS and, hence, diminish the reactivity of phenols. Using values of Ee0 for ArjOH and sterically hindered Ar2OH, we can estimate the contribution from such a steric repulsion AES to the activation energy E [33,34],... 

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