Gauche isomer

As was shown by Floryl), starting from a certain concentration, it is impossible in principle (for purely geometric reasons) to fill a larger fraction of the volume with these chains when they are randomly arranged. This critical concentration depends on chain flexibility which Flory characterized by the fraction f of folded (gauche) isomers in a polymer molecule... 

The planar civ-butadiene is not a minimum but rather represents the transition structure for the degenerate isomerization of the two identical gauche isomers. 

Fig. 14 Experimental (a) and calculated (b) conductance values of Au-n-alkanedithiol-Au junctions vs number n of methylene units in a semilogarithmic representation. The three sets of conductance values - high (H), medium (M), and low (L) - are shown as squares, circles, and triangles. The straight lines were obtained from a linear regression analysis with decay constants (3n defined per methylene (CH2) unit. The conductances of many different, nonequivalent gauche isomers cover the window below the medium values in (b) [64]...

Experimental support for the above analysis of 2,3-disubstituted 1,3-dienes is available. For example, 2,3-dichlorobutadiene has been shown to exist preferentially in a planar tram geometry 20S A more interesting case is the conformational isomerism exhibited by oxalyl bromide and chloride. It is found that in the gas phase, these molecules exist in an equilibrium mixture of tram and gauche isomers but no detectable concentration of the cis conformation was observed206, 207 ... 

The orientation induced by drawing has also been followed for partially oriented PET fibers. In this case where unaxial orientation can be presumed, it is possible to calculate the relative amounts of the trans and gauche isomers. The measurements indicate that increased orientation of the amorphous phase leads to an increase in the trans isomer 288). 

Fig. 28. Response of gauche isomer to thermal environment. Difference spectra of gauche isomers at elevated measurement temperatures, from 1650 cm-1 to 1320 cm-1...

Due to shape restrictions, n-butane forms sll (with a small help gas) in the gauche isomer (Davidson et al., 1977b Subramanian and Sloan, 2002)—rather than the trans isomer that is preferred in the gas phase. At most temperatures of interest to the natural gas processor above 100 K, sll guest molecules have only small restrictions to reorientation. 

Fig. 15. Calculated values of d In W(r)/dr as a function of relative elongation r/nl for DGER molecules. 1 Langevin approach 2-4 Monte-Carlo analysis with energy difference between trans- and gauche-isomers E, = Eg (2) Et - E, = 0.5 kcal/mol (3) Et — Es = —0.5 kcal/mol...

Ab initio molecular orbital theory is utilized to study the hydrogen abstraction reaction of n-bromopropane with hydroxyl radical and chlorine atom. The stability of the trans and gauche isomers of n-bromopropane is explored. The potential energy surface of both reactions is characterized by pre- and post-reactive complexes, as well as transition state structures in both trans and gauche isomeric forms. The importance of these two reactions relies on the ultimate product distribution from both reactions. Differences in the reactivity of 1-bromopropane toward OH and Cl are observed. The reaction of n-bromopropane with OH radical favors the abstraction of hydrogen atoms while the reaction with Cl atoms favors the abstraction of hydrogen atoms at the a and p carbon sites. 

In this study, we explored the stability of the n-bromopropane trans and gauche isomers by calculating the isomerization enthalpy at the levels of theory of interest. The results are summarized in Table 11.1. The values vary from —0.12 to... 

Tm and at low concentration, gramicidin A slightly decreases the lipid gauche isomers and increases the chain motion. This effect is more pronounced below Tm. Moreover, gramicidin A broadens the transition (Lee et al., 1984). Raman spectroscopy also reveals an increase in the fluidity of the gel phase of DPPC (Susi et al., 1979). 

N2F4 (liquid) (mixtures of staggered and gauche isomers) R JCP 1968 48 3216... 

C-CI stretching, (trans isomer) 750-700 cm-1 C-CI stretching, (gauche isomer) near 650 cm-1 C-CI stretching, (cyclohexane rings) ... 

---