Alkanes conformational properties

Warshel, Levitt, and Lifson derived a partially optimised consistent force field for amides and lactams (25). It is composed of an alkane part and an amide-part. The former was taken over from analogous earlier calculations for saturated hydrocarbons (17). The potential constants of the amide-part were optimised with the help of a large number of experimental frequencies (taken from TV-methylform amide, acetamide, iV-methylacetamide, and several deuterated species) as well as experimental geometry data for 7V-methylacet-amide. The resulting force field was used for the calculation of vibrational and conformational properties of 2-pyrrolidone, 2-piperidone and e-caprolactam. 

The conformational properties of alkanes have been reviewed recently in this series23. A brief reprise and expansion upon several important points made there will be a propos to the present discussion. 

Tasi, G., Mizukami, F., Csontos, J., Gyorffy, W., Palinko, I. Quantum algebraic-combinatoric study of the conformational properties of n-alkanes. II. J. Math. Chem. 27, 191-199 (2000)... 

It was shown in the previous section that different relatively stable conformations of a given molecule can result from internal rotation of a particular functional group. The possibility of the existence of various conformers is of extreme importance in many applications. It should be noted, for example, that the biological activity of an organic molecule often depends on its confonfia-tion - in particular the relative orientation of a specific functional grtmp. As another example, the thermodynamic properties of, say, an alkane are directly related to the conformation of its carbon skeleton. In this context the industrial importance of /sooctane is well-known. 

A short presentation of the Consistent Force Field is given, with emphasis on parametrization and optimization of energy function parameters. For best possible calculation of structure, potential energy functions with parameter values optimized on both structural and other properties must be used. Results from optimization with the Consistent Force Field on alkanes and ethers are applied to glucose, gentiobiose, maltose and cellobiose. Comparison is made with earlier and with parallel work. The meaning and use of conformational maps is discussed shortly. 

It may be desirable to define certain basic physical processes afresh, when we are dealing with systems essentially subject to two-dimensional conformations and hence two-dimensional constraints. This is the case for membranes, and also for a number of alkali salts of alkali -alkane carboxylates. These melt to give mesophases, in which the anions and cations are arranged in layerlike structures. At considerably higher temperatures the mesophases pass into isotropic ionic melts, but in the intervening temperature range they exhibit marked anisotropy of optical and physical properties. In these mesophases, which are ordered fluid... 

The configuration-bias Monte Carlo (CB-MC) technique (112) has also been extensively applied to characterize the sorption of alkanes, principally in silicalite (111, 156, 168-171) but also in other zeolites (172-174). Smit and Siepmann (111, 168) presented a thorough study of the energetics, location, and conformations of alkanes from n-butane to n-dodecane in silicalite at room temperature. A loading of infinite dilution was simulated, based on a united-atom model of the alkanes and a zeolite simulation box of 16 unit cells. Potential parameters were very similar to those used in the MD study of June et al. (85). As expected, the static properties (heat of adsorption, Henry s law coefficient) determined from the CB-MC simulations are therefore in close agreement with the values of June et al. The... 

Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

In a combined experimental and theoretical study, Borovkov et alP examined the properties of w-alkane radical cations in solution. Here, we shall just focus on one single aspect of their work, i.e., the conformers of one of the alkanes, w-nonane, C9H2o-In its simplest form of the highest symmetry, its structure can be described as a zigzag chain formed by the 9 carbon atoms. To each of the carbon atoms, 2 hydrogen atoms... 

Table 4.14 Structure (top) and physical properties [41] table below) of semi-fluorinated n-alkanes (60-63) and the homologous dialkyl bicyclohexyl liquid c stals 64 and 65 from which they are structurally derived. The spacefill model of 63 shows the helical conformation of the central perfluoroalkylene segment in contrast with the pentyl side-chains with their typical hydrocarbon zigzag conformation. The differences in charge distribution (red and blue denote negative and positive partial charges, respectively) are visualized by mapping of the electrostatic potential on to the electron density of 63 (B3LYP/6-31C //PM3 level of theory) [44, 50].

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