Cyclohexane conformational energies

Quantitatively, the anomeric effect is defined as the difference in conformational free energy for the process shown in Scheme 28 and the corresponding process in cyclohexane. Conformational energies for a range of substituents are available (82JA3635). 

Disubstituted Cyclohexanes Conformational Energies of Substitutents Conformation and Chemical Reactivity Six Membered Heterocyclic Rings Cyclooctane and Cyclononane Cyclodecane... 

Figure 3 Shapes and relative energies of cyclohexane conformations...

The conformational energy of an alkyl group at C—3 of cyclohexanone is substantially less than that of an alkyl group in cyclohexane because of reduced... 

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

Conformational Energies (94MI2) in kJ tnoL of the Axial Methyl Groups in Diheterocyclohexanes (cf. Cyclohexane 7.5 kJ mol" )... 

This chapter assesses the ability of molecular mechanics and quantum chemical models to properly assign preferred conformation, and to account quantitatively for differences in conformer energy as well as for barriers to rotation and inversion. The chapter ends with a discussion of ring inversion in cyclohexane. 

The conformational energies derived from gas phase NMR of 1,2-dimethoxyethane can be most reliably compared with the results of theoretical calculations where a single molecule is treated. Adoption of the Jt and Jg values obtained leads to the energy difference Eio) = - 1.26 kJ mol-1 between the gauche and the trans state. This result is consistent with E(c) < 2.1 kJ mol-1 obtained from measurements in cyclohexane. 

Theoretical anaiyses, performed using the RIS model, prove that the only parameters that have an appreciable effect on the calculated values of ha are those concerning the cyclohexane ring. Conformational energies, geometrical parameters, and contributions to the optical anisotropies from the oxymethylene oxide have no noticable effect on the value of Aa calculated for the polymer. The theoretical values of a are roughly one order of magnitude lower than the experimental results,... 

Draw the conformational isomers of cis-1,2-dimethylcyclohexane and cis-3,4-dimetliylcyclohexanone. While the cyclohexane conformers are of equal energy, the cyclohexanone conformers are not. Indicate which con-former is favored and explain why. 

Conformational energy of cyclohexane. The chair conformation is most stable, followed by the twist boat. To convert between these two conformations, the molecule must pass through the unstable half-chair conformation. 

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),... 

Conformational energies for axial-equatorial interconversion for a number of monosubstituted cyclohexanes have been reported. These are often referred to as A values and allow us to estimate steric effects in reactions. 

The conformational energies of many acyclic compounds are not precisely known. Cyclohexane derivatives would be expected to represent a much simpler case. They... 

In contrast with the cyclohexanes and the decalins, evaluation of the conformational energies of the octalins seems not to have been attempted by either experimental or theoretical approaches. The experimental equilibria in Table IX permit calculation of the free energy changes for isomerization of the various isomers, but are of little help for estimating the enthalpies unless the corresponding entropy changes can be assessed. 

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