Orientations of linear nonpolar molecules in a regular chain

Energetically preferable orientational states for linear nonpolar molecules are determined which result from the competition of quadrupole-quadrupole and Van der Waals dipole-dipole interactions. It is shown that with the dec reasing intermolecular distance a and hence with the increasing Van der Waa ls contribution to the total energy, the system concerned successively pass es through the following orientational phases: alternating longitudinal and transverse molecular orientations relative to the chain axis (phase I), pl anar arrangement of molecules inclined to the chain axis at some angle whic h tends to diminish with decrease in a (phase II), molecules aligned parall el to the chain axis (phase III). In contrast to the continuous transition between phases II and III, the nature of the transition I --> II depends on the radius of the interactions taken into account. In the approximation of short-range interactions, the phase transition represents a jump, whereas long-range interactions, if included, lead the system to change continuousl y from inhomogeneous to homogeneous structure in a narrow range of intermol ecular distances. The applicability of the results obtained in the descript ion of orientational structures of adsorbed nonpolar molecules is discussed . (C) 1999 American Institute of Physics. [S0021-9606(99)51111-2].