Hydrocarbons conformations

Besides the above examples of hydrocarbons, conformational energy calculations have also been carried out for synthetic polymers (De Santis et al., 1963), perfluoroalkanes (Bates, 1967), nucleotides (Sasisekharan et al., 1967 Scott, 1967), and polysaccharides (Rao et al., 1967). 

A. Scarso, L. Trembleau, J. Rebek Jr., Hydrocarbon conformation in self-assembled nanocapsules, Angew. Chem. Int Ed., 2003, 42, 5499-5502. 

A Raman spectroscopic study of the amylose-monoglyceride complex, using monostearin as the model compound, indicated that the hydrocarbon conformation inside the amylose helix was ordered as if in a crystalline state. Three helical turns existed for each hydrocarbon chain. 

A point which should be addressed here, that is not peculiar to hydrocarbon conformational analysis, is the meaning of a conformational minimum. The organic chemist tends to write structures in terms of the conformers situated at the bottom of potential minima, so that a few very well defined conformational states describe the system. It is desirable that he think or be aware of the less certain nature of things in reality, that aside from zero-point energy, thermal energy implies that a series of vibrational and torsional states, close to the minimum, are significantly populated. Even an adjacent minimum separated by a small barrier may be so readily accessible that the distinct identity of the two minima is subverted. 

A detailed review of the experimental methods used to determine hydrocarbon conformations and dynamics will not be given, since two fine accounts are available. In their 1965 textbook Conformational Analysis Eliel, Allinger, Angyal and Morrison give a sixty page survey of physical methods which had been used until then. Berg and Sandstrom have given an up-to-date account of experimental techniques in a recent review. 

Conformational analysis should not be discussed without some attention being paid to any differences in results in the solid, in the gas phase and in various types of solvents. It may be reasonable to assume that with saturated hydrocarbons, conformational results peculiar to the solid state reflect some property of that state rather than of the conformation. 

It is not clear whether for hydrocarbons larger than butane the more compact conformation is always relatively more favoured in the liquid state. The phase-dependence of hydrocarbon conformational equilibria merits further experimental investigaton. 

Ab initio calculations relevant to saturated hydrocarbon conformational analysis have been carried out, but do not extend much beyond simple compounds "". Satisfactory results require the inclusion of electron correlation effects and big basis sets" so that even simple problems require extensive calculation. 

This form is obeyed fairly well above x values of 5-10 dyn/cm in Fig. Ill-15c. Limiting areas or a values of about 22 per molecule result, nearly independent of chain length, as would be expected if the molecules assume a final orientation that is perpendicular to the surface. Larger A values are found for longer-chain surfactants, such as sodium dodecyl sulfate, and this has been attributed to the hydrocarbon tails having a variety of conformations [127]. 

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. 

Extensive discussions have focused on the conformation of the alkyl chains in the interior ". It has been has demonstrated that the alkyl chains of micellised surfactant are not fully extended. Starting from the headgroup, the first two or three carbon-carbon bonds are usually trans, whereas gauche conformations are likely to be encountered near the centre of tlie chain ". As a result, the methyl termini of the surfactant molecules can be located near the surface of the micelle, and have even been suggested to be able to protrude into the aqueous phase "". They are definitely not all gathered in the centre of tire micelle as is often suggested in pictorial representations. NMR studies have indicated that the hydrocarbon chains in a micelle are highly mobile, comparable to the mobility of a liquid alkane ... 

Ethane is the simplest hydrocarbon that can have distinct conformations Two the stag gered conformation and the eclipsed conformation, deserve special mention and are illustrated with molecular models m Figure 3 1... 

We can relate the conformational preference for an equatorial methyl group m methylcyclohexane to the conformation of a noncyclic hydrocarbon we discussed ear her butane The red bonds m the following structural formulas trace paths through four carbons beginning at an equatorial methyl group The zigzag arrangement described by each path mimics the anti conformation of butane... 

Freundlich Isotherm. The isotherms for some systems, notably hydrocarbons on activated carbon, conform more closely to the Freundhch... 

Table 2 Conformational Defects m the Hydrocarbon Chains in Fluid Phase Lipid Bilayers...

This effect is also observed with some polymers. The trans form of a hydrocarbon chain requires an energy about 0.8 kcal/mole less than the gauche. The trans form leads to an extended molecule and in hydrocarbons this becomes more favoured as the temperature is lowered. Linear polyethylenes take up this conformation in the crystalline state. 

The conformations of simple hydrocarbons can be interpreted by extensions of the principles illustrated in the analysis of rotational equilibria in ethane and butane. The staggered... 

In a class of reahstic lattice models, hydrocarbon chains are placed on a diamond lattice in order to imitate the zigzag structure of the carbon backbones and the trans and gauche bonds. Such models have been used early on to study micelle structures [104], monolayers [105], and bilayers [106]. Levine and coworkers have introduced an even more sophisticated model, which allows one to consider unsaturated C=C bonds and stiffer molecules such as cholesterol a monomer occupies several lattice sites on a cubic lattice, the saturated bonds between monomers are taken from a given set of allowed bonds with length /5, and torsional potentials are introduced to distinguish between trans and "gauche conformations [107,108]. 

The size and shape of micelles also are affected by fluormation Whereas hydrocarbon surfactants usually form spbencal micelles, linear fluorocarbon surfactants tend to produce larger rodhke speacs [31, 32 This is attnbuted to two inherent charac-tenshcs of the (CF2) chain (1) it adopts a hehcal rather than a linear zigzag conformation [dd 34, 35, 36], and (2) it is much suffer than the (CH2) cham [d5 37, 38] The relatively sbff, helical (CFj) chains thus prefer cylindrical to sphencal packing... 

It is essential that the process hazardous atmosphere and the process system and handling of combustible hydrocarbons/chemicals be recognized in the physical designs by conforming to the appropriate class of atmos-phere/environment codes specified by the National Electrical Code [71, 83, and 84]. 

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