Biphenyl, crystal structure

In this section, we will describe the crystal structures of the mesogenic A -n-alkoxy-4-cyano-biphenyls (Fig. 1) [53-57]. The compounds have the phase sequences crystal-nematic-isotropic for CB05—CB07 and monotropic nematic for CB01-CB04 crystal-smectic A-nematic-isotropic for CB08. The crystal and molecular data of the investigated compounds CBOn are summarised in Table 2. 

Hori and her collaborators [157, 158] also investigated the crystal structures of 4-[(S)-l-methyl-pentyloxycarbonyl]phenyl 4 -octyloxybiphenyl-4-carboxyl-ate, 4- [ (S)-1 -methyl-hexyloxy-carbonyl] phenyl 4 -octyloxybiphenyl-4-carbox-ylate, and (R)-l-methylheptyl 4-(4 -octyloxy-biphenyl-4-yloxymethylene) benzoate. All three crystal structures are isomorphous with the structure of MHPOBC. 

Hori and Ohashi [162] found three crystal forms for the compound 4 -hexyloxy-4-biphenyl p-[(S)-2-methylbutyl]benzoate. The structures of two crystal forms I-T (triclinic) and II-M (monoclinic) of this compound were determined. The crystal structure of I-T contains four crystallographically independent molecules. The dihedral angles between the phenyl rings of the... 

Fig. 29. Crystal structure of 4-[(2S)-chloro-3-methylbutanoyloxy]biphenyl-4 yl 4-undecy-loxy-2,3-difluorobenzoate along tbe x-axis (Reprinted from [164])... 

In order to understand the rates of racemization of biphenyls and bihetero-cyclics, an accurate knowledge of their geometric structure is essential. Such knowledge makes it possible to estimate the amount of interference caused by substituents in the vicinity of the pivot bond in an assumed coplanar transition state for rotation. A study of the crystal structure of 1 (X = Se) found that the selenophene rings have a small but significant deviation from planarity and are nearly perpendicular to each other.17 The deviation from 9(T is such that the carboxyl groups are in transoid positions. 

Biphenyl, terphenyl, and quaterphenyl all have room-temperature structures that behave similarly, and also all have low-temperature phases in which only one conformer is present. The torsion angle about the central C-C bond in biphenyl is estimated to be about 10° in the low-temperature form, which is appreciably less than that in the gas phase. In the case of terphenyl, it has been established (41) that the room-temperature structure is disordered each molecule librates in a double-well potential, with the barrier height being about 0.6 kcal/ mol. In the low-temperature form the molecule is stabilized in one of the two minima of the well, and has its terminal rings rotated in the same sense, so that the molecule conserves its center of symmetry. This alternation of rotations between adjacent rings is found (42) also in the low-temperature form of quaterphenyl and results in the molecule being noncentric (even though the crystal structure has a center of symmetry). 

Those crystalline salicylideneanilines that are not thermochromic are photo-chromic that is, they undergo a reversible color change on exposure to light. In these materials the molecules are markedly nonplanar (45b see discussion on biphenyls in Section II-D) and the crystal structures are very open. It has been suggested that the color change here is due to a two-step process (102) ... 

Buemi (1990) has also carried out semi-empirical AMI calculations on 3-(4 -biphenyl)pentane-2,4-dione whose molecular parameters have been fully determined by X-ray diffraction (Emsley et al., 1988b). The most stable configuration predicted by theory was not that found in the crystal structure. The calculated hydrogen-bond length (7 o o 275.9 pm) differed from the observed value (241.1pm). The conclusion was reached that the short hydrogen bond observed in the crystal is a peculiarity of the solid state. 

X-Ray structural analysis of bridged biphenyls containing sulfoxide, and sulfone functions in the bridge, 1,11 -dimcthyl-5,7-dihydrodibenzo[rv ]thiepine. V-oxide 24 and. Y,.Y-dioxide 25 in racemic form, was undertaken <2000HGA479>. The crystals of both 24 and 25 are centrosymmetric (RAY). The crystal structures of 24 and 25 (Table 6) showed similar twisted biphenyl moieties with dihedral angles (9) (C(2)-C(l)-C(8)-C(9) and C(6)-C(l)-C(8)-C(13)) in the range of 62.5-65.0°. 

The single-crystal structure of 36 revealed that it too adopts a folded solid-state structure (Fig. 3.5). Analysis of the 1H-1H COSY, 1H-1H COSYLR and ROESY spectra of 36 allow an unambiguous assignment of all the protons in the molecule. Figure 3.6 shows the aromatic region of the proton H- H ROESY NMR spectrum of the terphenyl dimer 2,5,2, 5 -tetra(4-tert-butylphenyl)-l,l -biphenyl (36). The most notable feature is again the high-field position of two directly... 

Fig. 3.5 Single-crystal structure of 2,5,2, 5 -tetra(4-tert-butylphenyl)-l, 1 -biphenyl (36) [50].

In a molecular crystal, the idealized symmetry of the molecule is often not fully expressed in other words, the molecule occupies a site of lower point symmetry. For example, in the crystal structure of naphthalene, the CsHio molecule (idealized symmetry Z>2h) is located at a site of symmetry I. On the other hand, the hexamethylenetetramine molecule, (CH2)6N4, retains its T symmetry in the crystalline state. Biphenyl, C6H5 —C6H5, which exists in a non-planar conformation with a dihedral angle of 45° (symmetry >2) in the vapor phase, occupies a site of symmetry I in the crystalline state and is therefore completely planar. 

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