Dipole Moments in Relation to Configuration of n-Alkane Chains Bearing alpha, omega Dipolar Substituents

The dipole moments of alpha,omega-dibromo-n-alkanes Br-(CH(2))(n-1)-Br have been treated by exact methods applicable to a linear sequence of bonds each constrained to choice among several discrete rotational states, due account being taken of neighbor dependence in assignment of statistical weights to the various configurations. Computations have been carried out on the basis of threefold potentials with one trans and two gauche states (+/- 120 degrees) for each bond. Successive gauche states of opposite sign are excluded by steric overlaps. An energy of 500 cal mole(-1) has been assigned to gauche relative to trans, in keeping with spectroscopic evidence for n-alkanes and the scheme successfully applied to the mean-square dimensions of polymethylene chains. Nonbonded interactions in which a Br atom is involved have been assigned statistical weights consistent with van der Waals radii and with evidence from Raman and infrared studies on n-alkyl bromides. The mean-square dipole moments averaged over all configurations are appreciably affected by the dipole-dipole interaction energy for n-1<6, but not for longer chains. Calculated values are in good accord with experimental results for n-1 = 3 to 10, inclusive. The applicability of the rotational-isomeric state-approximation to chains of comparatively short length is thus demonstrated.