Trans conformations, ratio

With all-atom simulations the locations of the hydrogen atoms are known and so the order parameters can be calculated directly. Another structural property of interest is the ratio of trans conformations to gauche conformations for the CH2—CH2 bonds in the hydrocarbon tail. The trans gauche ratio can be estimated using a variety of experimental techniques such as Raman, infrared and NMR spectroscopy. 

Looked at in a different way, the orientation of gauche conformations is always very low, and the oriented chains are predominantly in the trans conformation so that the increase in the trans/gauche ratio is accompanied by a rapid rise in trans orientation. In fact, it has been pointed out 27) that to a very good approximation the overall orientation... 

The analysis of relations between intensities in the region of double bond stretching vibrations >c=n in the Raman spectra, allows one to arrive at a conclusion about s-cis- or s-trans -conformation of multiple bonds. The ratio between intensities of the high-frequency band to the low-frequency one for s-trans -conformers appears usually to be more than 0.5, whereas for s-cis -conformers it is less than 0.25 (393, 394). 

Z/E ratios have been reported for [Ak—CH—CH=CH2] M+ with M = Li, Na, K and Cs, in various solvents at various temperatures - (set CP2, Table 10). These ratios are equivalent to the ratios Cz/Ce, where Cz and Ce are the concentrations of the cis and trans conformers, respectively. They are equal to the conformational equihbrium constants for the process depicted in equation 47 ... 

Owing to the inequality of the bond angles, a decrease in the trans population increases 0lnl2. Hence, the positive temperature coefficient for this ratio denotes a lower energy for the trans state. This energy is attributed to favorable interaction between CH3 pairs separated by 380 pm in the planar (trans) conformation. The larger value observed for the ratio in a less polar medium is in the direction predicted for enhanced electrostatic interaction within a chain of partially ionic Si—0 bonds. 

Mechanism A is a generalised mechanism which was proposed for those metals where the frans-but-2-ene cis-but-2-ene ratio was around unity. This mechanism contains a variety of reversible steps which permit the conformational interconversion of the diadsorbed buta-1 3-diene. Consequently, the trans cis ratio will depend upon the relative rates of these reversible steps and the ratio may be much lower than would be expected if the relative surface concentrations of anti- and syn-diadsorbed buta-1 3-diene, species I and III, respectively, in Fig. 37, were similar to the relative amounts of anti- and syn-buta-1 3-diene in the gas phase. It was also suggested that the relative importance of the various steps in mechanism A may be different for different metals. Thus, for example, the type A behaviour of nickel and cobalt catalysts, as deduced from the butene distributions and a detailed examination of the butene AAprofiles [166], was... 

Mechanism B was originally proposed for the palladium-catalysed reaction [208], which is characterised by a high trans cis ratio. In this mechanism, conformational interconversion of adsorbed species does not occur and the trans cis ratio is a direct reflection of the relative surface concentrations of the anti- and syn-diadsorbed buta-1 3-diene, which are dependent upon the nature of the sites available at the surface and upon the relative stabilities of the two conformers. In addition to palladium, mechanism B has been proposed, along with a contribution from mechanism A,... 

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,... 

Morton et al.135,141) were the first to study the poly(butadienyl)lithium anionic chain end using (b). They found no evidence of 1,2-chain ends and concluded that only 1,4-structures having the lithium cr-bonded to the terminal carbon were present. A later study by Bywater et al.196), employing 1,1,3,4-tetradeuterobutadiene to minimize the complexity of the spectrum that arises from proton-proton coupling, found that the 1 1 adduct with d-9 fert-butyllithium in benzene exists as a mixture of the cis and trans conformers in the ratio 2.6 1. Glaze et al. 36) obtained a highly resolved spectrum of neopentylallyllithium in toluene and found a cis trans ratio of about 3 1. 

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