Gauche to trans conformations

The authors prefered the latter explanation. They argue that the difference in the dielectric constant of the medium, when a hydrocarbon rich mixture is compared with that rich in sulfur dioxide, affects the ratio of gauche to trans conformations of the polymer chain. Such effects were observed in other systems, e.g. in 1,2 dichloroethane (38). Hence,... 

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. 

Such a theoretical model has been quite successfully checked by Fourier transform infra-red spectroscopy (FTIR) on polystyrene [11,12]. Indeed, the FTIR spectrum of polystyrene contains vibrational lines associated with out-of-plane modes of the phenyl ring, whose position depends on the gauche or trans conformation of the main-chain C - C bonds. By recording FTIR spectra during sample stretching, it is possible to determine the amount of gauche or trans conformation as a function of strain. 

The structure of tetramethyldistibine (22) has been investigated in the gas phase by electron diffraction (27). Although gauche and trans conformations are probably present, it was not possible to determine the conformational preference. Of major interest was the finding that the Sb—Sb bond length of 2.818(4) A was shorter than found in the crystal (2.862 and 2.831 A) (25,26). Presumably, the extended bonding in the crystal along the Sb—Sb—Sb—Sb chain reduces the intramolecular Sb—Sb bond order so that it is less than in the monomeric gas. A similar effect has been observed for I2 (vide infra) (54,55). 

High energy stable rotamers of hexatriene have also been theoretically studied . Two possible Cis/Trans isomers are possible with respect to the Cl=C2 bond (see Figure 2). For each of them, the rotation around the Cl—C3 and C2—C4 bonds can lead to s-trans and gauche conformations. The gauche-Trans-trans, trans-Cis-trans gauche-Cis-trans conformers have been found to be 3.0, 2.0 and 5.1 kcalmol above the most stable all-trans conformation, respectively. ... 

In the next paper 10 Czuchajowski and Kucharski extended their calcii lations of valence force field (Vf F) to compounds of the general formula T C II2-Cl Ij-ONOj, where V was NO2, CN. ONO2. Their calculated results in eluded the difference between gauche and trans conformers. the values o which were 1655-1652 and 1644-1641 cm respectively and arc also in agree mem with the experimental data [7]. 

The most extensive studies of phenylethylamines have been done by Pullman and coworkers (59,65,67/68). Their PCILO calculations indicated that gauche and trans conformers have nearly identical energies in the majority of phenylethylamines and phenylethanolamines studied. AI studied were carried out using three different basis sets (67) these all indicated a slight but definite preference for gauche conformers. These authors noted the preponderance of extended conformers in crystal structures and in solution they attributed this to environmental forces. To test this hypothesis, computations were repeated with the inclusion of water molecules, and a tendency toward the extended conformation was indeed observed. 

Figure 31. Gauche and trans conformations of p-propyl aniline. These illustrations are the result of structural optimization using a molecular mechanics computer program. Atoms were drawn (not to scale) in order to improve spatial perspective.

Energy elastic components may be either positive or negative (Table 11-5). With polymers having a/ram conformation as lowest energy conformation, a transition from gauche to trans will therefore lead to an energy... 

The analysis of the resonance data in terms of this model makes it possible to estimate the probabilities of gauche and trans conformations about carbon-carbon single bonds at various positions in fatty acid chains in phospholipid bilayers. The probability of the gauche conformation increases for instance with increasing n by a rotation about the single bond when one end of the chain, the carboxy group, is... 

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