THE STRUCTURE AND CONFORMATION OF 4-HYDROXYPHENYL TEREPHTHALATE - A MODEL-COMPOUND FOR A LIQUID-CRYSTALLINE POLYESTER

A theoretical analysis using ab initio calculations of the possible conformations of 4-hydroxyphenyl terephthalate which represents the repeat unit for poly(p-phenylene terephthalate) is reported. The study includes complete geometry optimizations and calculation of vibrational frequencies at the SCF/STO-3G level of ten distinct stationary points on the molecular potential surface. The calculations show that both the terephthalic and the hydroquinone fragments of the system prefer planar conformations. For the entire molecule, the low energy form is a twisted conformation where the hydroquinone portion is twisted by about 65 degrees relative to the terephthalic portion of the molecule. The barrier to internal rotation around the ester bond is about 2 kcal mol(-1).