X-ray Structure at 150 K of the Polar Alkyl Mesogenic Compound 7CBB: 4-Cyanobiphenyl-4′-heptylbiphenyl Carboxylate

Abstract

The compound under study 7CBB consists of two biphenyl moieties A–B and C–D linked by a carboxylate group. Ring A terminates in a 4-cyano group and C–D is linked to the terminal alkyl chain. A previously reported room temperature determination of the crystal structure employing MoKα X-radiation was thought to contain serious errors in describing the alkyl chain as being disordered. The X-ray structure factors were not deposited with this published structure and as a consequence two new X-ray data sets have been collected: (1) using MoKα radiation at room temperature 295 K (a repeat of the previous study) and (2) using CuKα radiation at 150 K in an attempt, which proved successful, to improve the overall quality of the structure determination. The corrected MoKα structure and the CuKα structure both reported here have enabled the previous errors to be identified, concluding that there is no disorder in the alkyl chain. The alkyl chain in the MoKα structure at 295 K was found to have anisotropic thermal factors slightly exaggerated with respect to the remainder of the structure. However, this effect is not observed in the CuKα structure at 150 K, all anisotropic thermal factors including those in the alkyl chain being reduced to cover a much smaller overall range of values. Consequently, it can be concluded that the effect observed in the alkyl chain at 295 K is merely one of a thermal nature, not one of disorder. There are no unusual bond lengths or angles present. The biphenyl moieties are planar within 0.035 A and 0.020 A respectively. The dihedral angles of all ring pairs have been calculated. Calculation of intermolecular distances between molecules related by a centre of symmetry reveals the existence of a number of van-der Waals interactions. The H-bonding motif typical of crystal structures of cyanobiphenyl compounds is observed. The molecular packing mode corresponds to that of a precursor of the smectic phase. The alkyl chain thermal ellipsoids for 7CBB (a) the Mo RT structure are noticeably exaggerated compared to those for the remainder of the structure, an effect which is not present in (b) the Cu LT structure (Drawn with Ortep/Raster (Barnes in J Appl Cryst 30:568, 1997; Merritt and Bacon in Methods Enzymol 277:505–524, 1997)). It may be concluded therefore that in the Mo RT structure this is a thermal effect and cannot be explained in terms of static disorder.