Gas-phase molecular structure of MBBA (4-methoxybenzylidene-4′-n-butylaniline), a mesogen, by electron diffraction combined with ab initio calculations

The molecular structure of a typical mesogen, 4-methoxybenzylidene-4'-n-butylaniline (MBBA, CH(3)O-C(6)H(4)-CH=N-C(6)H(4)-(CH(2))(3)-CH(3)). has been studied by gas-phase electron diffraction (GED). The nozzle temperature was about 150 degrees C. Structural constraints in the GED data analysis were obtained by the ab initio MO calculation at the HF/4-21G(*) level of theory. Vibrational amplitudes and shrinkage corrections were calculated from the harmonic force constants given by a normal coordinate analysis. The phenylene ring attached to the C(=N) atom and the azomethine group (-CH=N-) are essentially on the same plane, i.e., the dihedral angle is 0(12)degrees. The phenylene ring bonded to the nitrogen atom is found to be out of the plane of the azomethine group and the determined value of the dihedral angle, 48(9)degrees, in the gas phase is larger than that in the crystalline state. This is mainly due to the steric interaction between the hydrogen atoms of the azomethine group and the phenylene ring. In the gas phase the four rotational conformers with respect to the configurations of the n-butyl group were assumed to exist. Their conformational abundance was fixed, as calculated from the ab initio relative energies. The principal bond distances and angles (r(g)/Angstrom and angle(alpha)/deg) determined by GED are r(N=C) = 1.290(12), r(C-N) = 1.413(12), r(C(az)-C(ring)) = 1.467(3), [r(C(ring)-C(ring))] = 1.400(6), angle C-N=C = 119.0(18), angle N=C-C = 121.6(13), angle NC(ring)C(14) = 128.5(25), angle C(az)C(ring)C(4) = 121.2 (dependent), [CCC(ring)] = 120.0(3), [angle CCC(butyl)] = 116.2(11), angle C(5)C(ring)O = 129.3 (16), where C(az), C(ring), and C(butyl) denote the carbon atoms of the azomethine, phenylene and butyl groups, respectively. C(14) and C(4) are the C atoms of the rings synclinal to the C(=N) atom and cis to the H(az) atom, and C(5) is the C atom of the ring cis to the C atom of the methoxy group. The values' in parentheses are three times the standard deviations. The notation [ ] represents the average value. The transition temperature from the nematic to liquid phases was discussed on the basis of the determined molecular structure.