Molecular distortion strain

As with 12, it might appear that there is nothing unusual about 14. But a ball-and-stick model of 14 reveals that the carbon-carbon double bond is in a strained configuration like 15. Some of the properties of 14 are given in Table 12-6. That compounds with a double bond to a bridgehead carbon, such as 14, should be highly strained is known as Bredt s Rule. The most spectacular example of this form of molecular distortion reported so far is bicyclo[2.2.1]-l-heptene, 16, for which evidence has been adduced that it is an unstable reaction intermediate ... 

Because bipyridines substituted at the 3 and 3 positions exhibit a large steric repulsion between substituents while in the cis configuration,27 they bind metals more weakly and form strained, nonplanar structures.28 However, a series of 3,3 -disubstituted bipyridines were coordinated to ruthenium, and it was demonstrated that molecular distortions could be used to advantage in modulating physical properties of the resulting complexes.29... 

Rondan, N. G., Paddon-Row, M. N., CarameUa, R, Houk, K. A. (1981). Nonplanar Alkenes and Carbonyls A Molecular Distortion which Parallels Addition Steroselectivity. J. Am. Chem. Soc., 103,2436. Ess, D. H. Houk, K. N. (2007). Distortion/Interaction Energy Control of 1,3-Dipolar Cycloaddition Reactivity. J. Am Chem. Soc., 129, 10646-10647. Lopez, S. A., Houk, K. N. (2013). Alkene Distortion Energies and Torsional Effects Control Reactivities, and Stereoselectivities of Azide Cycloadditions to Norbomene and Substituted Norbomenes. J. Org. Chem., 78(5), 1778-1783. Hong, X., Liang, Y, Griffith, A. K., et al. (2013). Distortion-Accelerated Cycloadditions and Strain-Release-Promoted Cycloreversions in the Organocatalytic Carbonyl-Olefin Metathesis. Chem. Sci., 5(2), 471-475. 

The distorted structure can be replaced by a more reasonable structure using an empir ical molecular mechanics calculation This calculation which is invoked m Spartan Build by clicking on Minimize, automatically finds the structure with the smallest strain energy (m this case a structure with realistic bond distances and a boat conformation for the SIX membered ring)... 

The various elastic and viscoelastic phenomena we discuss in this chapter will be developed in stages. We begin with the simplest the case of a sample that displays a purely elastic response when deformed by simple elongation. On the basis of Hooke s law, we expect that the force of deformation—the stress—and the distortion that results-the strain-will be directly proportional, at least for small deformations. In addition, the energy spent to produce the deformation is recoverable The material snaps back when the force is released. We are interested in the molecular origin of this property for polymeric materials but, before we can get to that, we need to define the variables more quantitatively. 

Various other interactions have been considered as the driving force for spin-state transitions such as the Jahn-Teller coupling between the d electrons and a local distortion [73], the coupling between the metal ion and an intramolecular distortion [74, 75, 76] or the coupling between the d electrons and the lattice strain [77, 78]. At present, based on the available experimental evidence, the contribution of these interactions cannot be definitely assessed. Moreover, all these models are mathematically rather ambitious and do not show the intuitively simple structure inherent in the effect of a variation of molecular volume considered here. Their discussion has to be deferred to a more specialized study. 

The active molecule must be regarded as in a state of strain or as a distorted molecule and the ease of distortion or the energy of activation will be dependent on the molecular structure. The following values for the energies of activation as calculated from the temperature coefficients indicate clearly the effect of substitution in a simple molecule on the energy of activation. 

The molecular mechanics energy of a molecule is described in terms of a sum of contributions arising from distortions from ideal bond distances ( stretch contributions ), bond angles ( bend contributions ) and torsion angles ( torsion contributions ), together with contributions due to non-bonded (van der Waals and Coulombic) interactions. It is commonly referred to as a strain energy , meaning that it reflects the strain inherent to a real molecule relative to some idealized form. 

Molecular Mechanics Models. Methods for structure, conformation and strain energy calculation based on bond stretching, angle bending and torsional distortions, together with Non-Bonded Interactions, and parameterized to fit experimental data. 

---