Molecular Structure of<i>p</i>-Azoxyanisole, a Mesogen, Determined by Gas-Phase Electron Diffraction Augmented by ab Initio Calculations

Abstract

As the first attempt to determine the gas-phase structure of molecules forming liquid crystals, the molecular structure of p-azoxyanisole (PAA, CH3O−C6H4−NON−C6H4−OCH3), a mesogen, has been studied by gas electron diffraction. A high-temperature nozzle was used to vaporize the sample. The temperature of the nozzle was about 170 °C. Structural 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 coordinate analysis. The structural model assuming four conformers well reproduced the experimental data. Five bond distances, six bond angles, and two dihedral angles were determined. Mean amplitudes were adjusted in five groups. The dihedral angles between the phenylene rings and the azoxy plane have been determined to be 11(26)° and 11(11)°, and these values are in agreement with those in the solid phase determined by X-ray diffraction within experimental errors. The conformation of the core of this mesogen is mainly ascribed to the interaction between the π-electrons of the azoxy group and the aromatic rings.