MOLECULAR-STRUCTURE OF P-AZOXYANISOLE, A MESOGEN, DETERMINED BY GAS-PHASE ELECTRON-DIFFRACTION AUGMENTED BY AB-INITIO CALCULATIONS

As the first attempt to determine the gas-phase structure of molecules forming liquid crystals, the molecular structure of p-azaxyanisole (P AA, CH3O-C6H4-NO=N-C6H4-OCH3), a mesogen, has been studied by gas elec tron diffraction. A high-temperature nozzle was used to vaporize the s ample. The temperature of the nozzle was about 170 degrees C. Structur al constraints were taken from HF/4-21G() ab initio molecular orbital calculations on PAA, Vibrational amplitudes and shrinkage corrections were calculated from the harmonic force constants given by normal coo rdinate analysis, The structural model assuming four conformers well r eproduced the experimental data. Five bond distances, six bond angles, and two dihedral angles were determined, Mean amplitudes were adjuste d in five groups. The dihedral angles between the phenylene rings and the azoxy plane have been determined to be 11(26)degrees and 11(11)deg rees, and these values are in agreement with those in the solid phase determined by X-ray diffraction within experimental errors. The confor mation of the core of this mesogen is mainly ascribed to the interacti on between the st-electrons of the azoxy group and the aromatic rings.