to be 112°. Molecular mechanics calculations yield a COC bond angle of 111.78°, more in agreement with experimental data.49 In-phase ring stretching vibrational frequencies resulting from Raman spectroscopy for cycloalkanes and cyclic aliphatic ethers have been found to be roughly proportional to the bond angle.47,50 A study on the correlation of planarity of rings with the substitution pattern in chlorinated dibenzodioxin was conducted by Chen, using IR data. 44 The infrared vapor phase spectrum for 2,3,7,8-tetrachlorodibenzodioxin was recorded, and its COC bond angle (a) was calculated from IR data by using mass approximations for the terminal atom in a nonlinear XY2 model and by neglect of the valence force field equations of the symmetric stretch-bending term. Details of these calculations, describing the molecular geometries using IR data for 2,3,7,8-tetrachloro-and other chlorinated dibenzo-

No abstract

The conformational behaviour of the three macrocyclic octaols (4)-( 6), obtained b y acid-catalysed condensation of resorcinol with heptanal, is elucidated for the first time. T w o of them, namely the diamond (5a) and the chair (6a) stereoisomers, undergo a ring-inversion conformational process in acetone to give the corresponding crown conformers (5b) and (6b). In DMSO or on addition of acetic acid to an acetone solution of diamond octaol ( 5 ) , conformer (5a) is favoured. The presence of such equilibria and solvent effects are interpreted as an interplay between the tendency of the phenolic OH groups to form intramolecular hydrogen bonds and the alkyl chains to assume the endo position, avoiding steric repulsions and allowing self-aggregation.

The effect of molecular structural changes on the mesomorphic nature of a new class of macrocyclic columnar liquid crystal is reported. Twenty-six new compounds with general molecular structure I were prepared, characterized and compared.Only dodecasubstituted ester derivatives 2d-h exhibit thermotropic mesophases. The design of mesogens based on these new, unusual macrocyclic cores requires the presence of the following structural elements: twelve aliphatic side chains, esters as bridging units and small R groups on the core.

The mesomorphic properties of a new class of columnar liquid crystals, consisting of a bowl-shaped metacyclophane core surrounded by twelve flexible alkyl chains are reported. These mesogens form D,, columnar mesophases having negative optical and dielectric anisotropy. Both planar and homeotropic alignment is obtained. An antiferroelectric pairing of the molecules within the columns is observed, ruling out the possibility of a ferroelectric self-organization of the cores. For all the mesogens a complex thermal behaviour is observed. Two different solid phases C, and C2 have been characterized and their thermal behaviour elucidated: C, is the mesophase precursor, while C,, more ordered, melts directly to an isotropic liquid. Mesophase stability over time depends on the length of the side chains. The overall mesomorphic complexity is attributed to the combination of the different possible core packing geometries with the numerous conformations available to the alkyl chains.