Conformation steric repulsion

The number of polyphosphazenes that have been studied by X-ray dif-fraetion are not very large. However, these studies have been quite informative. Two different measurements on [NPCyn revealed difference in metric parameters, although the main structural features are similar. Most polyphosphazenes seem to adopt the cis-trans-planar conformation over the trans-trans-planar conformation. Steric repulsion between the substituents present on phosphorus is minimized in the cis-trans planar conformation. Thus, if one considers a P2-N3 bond (bond labeled A, Fig. 3.52) as a reference, the disposition of the immediate back-bone substituents atoms N1 and P4 are cis with respect to each other. On the other hand, with respect to the adjacent N3-P4 bond (bond labeled B, Fig. 3.52), the disposition of the backbone substituents P2 and N5 are trans with respect to each other. Thus, the stereochemical orientation of the pol5mier chain alternately varies from cis to trans. 

The //NMR spectrum (Fig. 2.19) displays anAB system for the protons adjacent to this bond the coupling constant = 72 Hz. From this can be deduced first that the dihedral angle 9 between the C7/bonds is about 180°, second that conformer 14b with minimised steric repulsion between the substituents predominates and third that there is restricted rotation around this CC bond. 

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. 

Which conformation is more stable, anti or gauchel What evidence is there for steric repulsion between methyl groups in the gauche conformation (Hint Look for distortions in the CCC bond angles as a function of conformation.)... 

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 ... 

Internal rotation in cycloalkanes is restricted by the need to maintain bonding between adjacent ring atoms. Aside from this restriction, though, cycloalkanes obey the same structural rules as alkanes staggered conformations that tninimize steric repulsion are preferred. 

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 ... 

The conformation of alkylcyclohexanes is determined largely by steric repulsion (see Chapter 5, Problems 6 and 7). More polar substituents may show different conformational preferences due to a combination of steric and electronic factors. 

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... 

The sense of the asymmetric induction at the /J-carbon of 10 is opposite to that of 7, indicating that 10 reacts with the Gilman reagents in a conformation where the carbonyl is s-trans to the a,(i double bond to avoid steric repulsion of the a-substituent and the camphor residue. 

A less obvious explanation is that the observed residual structure is not due to attractive interactions, but rather to repulsive ones. The steric repulsion between atoms forced to partially overlap is a dominant, if not the dominant, force in all of chemistry. These highly local interactions are known to be important in polymer conformations (Flory, 1969 Ramachandran and Sasisekharan, 1968). For homopolymers or simple alternating polymers, they can often be safely neglected by assuming they confer no net directionality to the chain. Polypeptide chains, however, are chiral and support specific sequences of 20 differently shaped... 

Textbook discussions of the steric effects of side chains on backbone conformation convey the impression that, except for glycine and proline, all side chains restrict the allowed range of phi and psi to approximately the same extent. If this were true, encoding of long-range structure by local steric repulsion would seem unlikely. 

Open-chain vicinal dihalides are apparently reduced conceitedly via a conformation in which the two halogen atoms are anti to each other. This conclusion is based upon the fact that meso isomers of structures 47 ait generally more easily reduced than the dl isomers, presumably because steric repulsions are less for meso than for dl in the transition state for electron transfer. The stereochemistry of the products of reduction of simple open chain vicinal di-... 

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