[Polyunsaturated hydrocarbon chains: computer study of the characteristics of intramolecular ordering of chains]

The conformational properties of isolated unbranched hydrocarbon polyunsaturated molecules of cis-C18:4 and cis-C18:5 under theta-conditions (T = 298 K) were studied using Monte Carlo simulations. The conformations were generated by a computer (the continuum model was used; the energy of nonbonded interactions and torsion and electrostatic terms were taken into account). A molecule-fixed coordinate system with the axes along inertia tensor eigenvectors of each molecule conformation (principal axes of inertia) were used for the calculations. C-H and C-C bond orientation distribution functions rho and ordering parameters S with respect to the maximum molecule span axis were calculated. It was shown that the presence of five methylene-interrupted cis double bonds in C18 chain has a maximum effect on the intramolecular ordering properties of the molecule. The widths of function rho CH for pentaenes differed significantly from those of other C18-chains: the widths of function rho for all CH2-groups were nearly twice as large as that for C-H-bonds flanking the double bonds C=C, and roughly constant along the chain sequence. The mean magnitudes of magnitudes of SCH in the molecule decreased when unsaturation increased.