The molecular and crystal structure of N-(4-n-pentyloxybenzylidene)-4 '-heptylaniline (50.7) and N-(4-n-nonyloxybenzylidene)-4 '-butylaniline (90.4)

The crystal and molecular structures of the homologous mesogenic compounds N-(4-n-pentyloxybenzylidene)-4'-heptylaniline (50.7) and N-(4-n-nonyloxyben zylidene)-4'-butylaniline (90.4) have been determined at room temperature, the starting model obtained by direct methods. The crystals of both compoun ds belong to the triclinic system with space group P -1: a = 5,645(3), b = 18.92(1), c = 21.85(1) Angstrom, alpha = 80.07(3), beta = 85.95(4), gamma = 84.63(4)degrees with four molecules per unit cell for 50.7 and a = 5.570(2 ), b = 8.227(4), c = 26.614(12) Angstrom, alpha = 88.40(3), beta = 87.84(3) , gamma = 74.66(4)degrees with two molecules per unit cell for 90.4. The st ructures were refined by full-matrix least-squares calculations to R = 0.06 1 for 1792 unique observed reflections for 50.7 and to R = 0.074 for 983 un ique reflections for 90.4. The conformations of 50.7 and 90.4 differ consid erably from each other. In the two crystallographically independent molecul es of 50.7 the phenyl rings of one mesogenic group are twisted by 58.0 degr ees and 56.1 degrees, respectively. In contrast to these conformations. in 90.4 they are nearly coplanar. The molecular packing in 90.4 and 50.7 diffe rs as well. 90.4 shows a tilted smectic-like layer structure. The structure of 50.7 consists of blocks, in which the mesogenic units overlap almost co mpletely. These blocks are arranged in a stepwise shifted fashion. But both compounds transform to a smectic C phase at higher temperature.