Trans-gauche energy difference

The decalin (bicyclo[4.4.0]decane) ring system provides another important system for study of conformational effects in cyclohexane rings. Equilibration of the cis and trans isomers reveals that the trans isomer is favored by about 2.8 kcal/mol. Note that this represents a change in configuration, not conformation. The energy difference can be analyzed by noting that the cis isomer has three more gauche butane interactions that are... 

It is possible to include in the above the effect of trans/ gauche energy difference, but this will entail conditional probability matrices of much greater complexity, leading to severe computational difficulties. 

In the first row the relative dielectric constant for the compound is given. In the second row the valency of the unit is given. The other rows give the values for the various FH parameters. Remaining parameters the characteristic size of a lattice site 0.3 nm the equilibrium constant for water association K — 100 the energy difference for a local gauche conformation with respect to a local trans energy it/ 8 — 0.8 A T the volume fraction in the bulk (pressure control) of free volume was fixed to (pbv = 0.042575... 

Figure 4. Symmetry breaking of the ethanol torsion potential (top, two gauche and one trans conformation) by interaction with a chiral acceptor molecule (dimethyl oxirane, bottom), in this case RR trans 2,3 dimethyloxirane [128]. Note that trans ethanol is less stable in the complex and that the two gauche (g) forms differ in energy.

By equilibrating (BF3) the cis/trans isomers of 2-Ph-5-OR-l,3-dioxanes, the conformational free energy differences were determined [-AG° = 0.24 kcal mol- (OMe), 0.01 (OTs), -0.34 (OSO2C6H4-PNO2), -0.48 (OSO2 CH3)] and discussed with respect to the steric and the attractive gauche O-C-C-O effect (92T5941). 

The three-state RIS model of conformer statistics is used to analyze the 16 independent dipole coupling constants measured in a proton NMR study of n-hexane in a nematic liquid crystal solvent. The orientational ordering of the n-hexane molecule is treated in the context of the modular formulation of the potential of mean torque. This formulation gives an accurate description of alkane solute orientational order and conformer probabilities in the nematic solvent. Consequently, substantially more accurate calculated diplar couplings are obtained, and this is achieved without the need to resort to unconventionally high values of the trans-gauche energy difference E(g) in the RIS model. 

Stress-temperature coefficients are determined for cross-linked networks of PE and polyisobutylene elongated in the amorphous state. Interpretation of the indicated temperature coefficient of 0 for PE according to the three-fold potential model for rotation around the C—C bonds is consistent with an energy difference of 2.1 kJ mol-1 between gauche and trans states. The small temperature coefficient for isobutylene is due to steric interactions affecting bond rotations. 

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. 

The energy difference AF between gauche and trans conformers, resulting from rotation about single bonds of the chain backbone in trans-1,4-polybutadiene, c/s-1,4-polybutadiene and 1,5-hexadiene, is evaluated from IR measurements on the bands characteristic of bending vibration of CH2 groups. Consideration of the experimental results, on the basis of the RIS model, leads to the conclusion that a value of 0.4 to 0.8 kJ mol-1 is the best estimate of AF. 

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