Strain, Pitzer

Common rings (5, 6, and 7 membered). Largely unstrained. The strain that is present is mostly Pitzer strain. 

INITIAL RATE ENZYME ASSAYS Pitzer strain,... 

Comparison of the heats of combustion of cycloalkanes (Table 9.1) shows that cyclopropane, cyclobutane, and cyclononane yield more energy per methylene group than the other cycloalkanes. This can be attributed to strain resulting from bond-angle distortion (Baeyer strain), eclipsed conformations (Pitzer strain), and trans-annular, repulsive van der Waals interactions. Common (five- and six-membered) rings and large (more than twelve-membered) rings have little or no strain. This... 

Internal strain in cyclic systems 93,95) incorporates angle strain (Baeyer strain), torsional strain (Pitzer strain) and transannular strain. In the case of nitrogen inversion, large effects are expected to arise from angle strainb>, the nature of the cyclic system imposing a value of the angle 0... 

We now assess the state of all of these contributions. In Table 3.23, we list the contributions attributed to Bayer strain, Pitzer strain, Dunitz-Shomaker strain, and rehybridization that lead to strengthened C-H bonds. We list two values for this... 

In so far as a half chair of type 1 has been postulated as the transition state for the chair-chair interconversion its relative enthalpy represents the barrier to this interconversion, and the values of Bayer strain, Pitzer-strain and van-der-Waals-strain for this form represent the contribution from these factors to the barrier. 

It has been shown in the introduction how a single substituent is not expected to raise the barrier to ring inversion. Nor certainly should it lower this barrier at least in terms of Pitzer strain, since it can readily be envisaged that such strain increases in passing from a more or less perfectly staggered chair conformation to the transition state for ring inversion. Thus there is no obvious rationalisation of results quoted in Table 2. 

Another means of studying the contribution of Pitzer strain is to compare the barrier to rotation about bond RA— BR and the barrier to ring... 

Note Added in Proofs. There has been additional work on the calculation of conformational energies for cyclohexane, see Section II. Wiberg and Boyd 129) conclude that non-bonded interactions make little contribution to the barrier, the most important component of which is torsional strain, in agreement with earlier work. Quantum mechanical calculations of various conformations which do not by their nature allow a spUtting of energies into Bayer strain, Pitzer strain, and van der Waals strain, have also been made I30,i3i), it has been concluded 12D that transition state conformations 1 and 2 are of similar energies i. e. that there is pseudorotation in the transition state as proposed by Pickett and Strauss 29,30),... 

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