Staggered states

Figure 2.2 Newman and saw horse projections for n-butane. (a) A staggered state (planar trans) with angle of bond rotation (j> = 0 and (b) an eclipsed state (planar cis) with = 180°.

Rotation through syn state Libration through perfectly staggered state Rotation through eclipsed state Libration through the anti conformation Rotation between gauche conformations Rotation between gauche and anti conformations... 

If perfectly staggered states t(0°), g ( 120°) are assigned in the calculation of intomethylene proton-proton distances, then the results in the bottom row of Table 2.11 are obtained, Le., all interactions r are approximately the same. It is apparent from Figure 2.14 that this is not the case. 

Example Hthanc is stable in the staggered gauche) con formation. The transition state for rotating a methyl group in ethane has the eclipsed con form at ion, A gcom etry optim i/ation start in g from an eclipsed eon formation yields th e tran sition state. 

One of the frmdamental structural facets of organic chemistry, which has been explained most satisfactorily in MO terms, is the existence of a small barrier to rotation about single bonds. In ethane, for example, it is known that the staggered conformation is about 3kcal/mol more stable than the ecl sed conformation so that the eclipsed conformation represents a transition state for transformation of one staggered conformation into another by rotation. 

In the solid state sulfamic acid forms a strongly H-bonded network which is best described in terms of zwitterion units +H3NSO3" rather than the more obvious formulation as aminosulfuric acid, H2NS02(0H). The zwitterion has the staggered configuration shown in Fig. 15.50a and the S-N distance is notably longer than in the sulfamate ion or sulfamide. 

In the transition state, the torsional strain involving the partially formed bond between the nucleophile and the carbonyl group represents a substantial fraction of the total strain, even when the degree of bonding is low. Thus, in the case of acyclic carbonyl compounds, a staggered conformation is preferred in the transition state (Figure 6). 

In solution the compounds exhibit solvent-dependent colours in dilute solution in non-polar solvents, planar monomers are present but in more concentrated solutions oligomerization occurs. In the solid state a dimeric structure has been identified (X-ray, Figure 2.30) with R = Ph there is a staggered configuration (Rh-Rh 3.193 A) but with other isocyanides (R = 4-FC6H4) the configuration is eclipsed. 

In the case of cooperative processes, the formation of a nucleus, already discussed from the kinetical point of view, plays a crucial role. The steady state described by Eq. (1) depicts the formation of a triple helix as the simplest model by the formation of a nucleus Hx through fast pre-equilibria and subsequent propagation steps, Hx in this case is a triple-helical intermediate with x tripeptide units (that means x hydrogen bonds) in the helical state. The final product H3n 2 possesses two hydrogen bonds less than tripeptide units because the three single chains are staggered at one amino add residue each. 

Fig. 33. Unzippering of a staggered helical state according to model II,48)...

Fig. 14. Staggered isomeric rotational states of the CH—CH bond in erythrodiisotacic poly(l,2-dimcthyltetramethylcnc)...

Fig. 18. Staggered rotational isomeric states of the CH—CH bond of threodiisotactic poly(l,2-dimethyltetramethylene)...

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