• Chowdhury, Rituparno
• Dey, Somnath
• Sarkar, Debashrita
• Mondal, Saikat
• Reddy, C. Malla
• Kumar, Santosh
• Roth, Georg
• Sasmal, Supriya
• Haldar, Debasish
• Peters, Lars

Abstract

<jats:p>Amongst the derivatives of 4-biphenylcarboxylic acid and amino acid esters, the crystal structure of 4-biphenylcarboxy-(L)-phenylalaninate is unusual owing to its monoclinic symmetry within a pseudo-orthorhombic crystal system. The distortion is described by a disparate rotational property around the chiral centers (φ<jats:sub>chiral</jats:sub> ≃ −129° and 58°) of the two molecules in the asymmetric unit. Each of these molecules comprises planar biphenyl moieties (φ<jats:sub>biphenyl</jats:sub> = 0°). Using temperature-dependent single-crystal X-ray diffraction experiments we show that the compound undergoes a phase transition below <jats:italic>T</jats:italic> ∼ 124 K that is characterized by a commensurate modulation wavevector, <jats:bold>q</jats:bold> = δ(101), δ = ½. The (3+1)-dimensional modulated structure at <jats:italic>T</jats:italic> = 100 K suggests that the phase transition drives the biphenyl moieties towards noncoplanar conformations with significant variation of internal torsion angle (φ<jats:sup>max</jats:sup><jats:sub>biphenyl</jats:sub> ≤ 20°). These intramolecular rotations lead to dimerization of the molecular stacks that are described predominantly by distortions in intermolecular tilts (θ<jats:sub>max</jats:sub> ≤ 20°) and small variations in intermolecular distances (Δ<jats:italic>d</jats:italic><jats:sub>max</jats:sub> ≃ 0.05 Å) between biphenyl molecules. Atypical of modulated structures and superstructures of biphenyl and other polyphenyls, the rotations of individual molecules are asymmetric (Δφ<jats:sub>biphenyl</jats:sub> ≈ 5°) while φ<jats:sub>biphenyl</jats:sub> of one independent molecule is two to four times larger than the other. Crystal-chemical analysis and phase relations in superspace suggest multiple competing factors involving intramolecular steric factors, intermolecular H—C...C—H contacts and weak C—H...O hydrogen bonds that govern the distinctively unequal torsional properties of the molecules.</jats:p>

Topics

• impedance spectroscopy
• compound
• phase
• x-ray diffraction
• experiment
• Hydrogen
• phase transition
• ester