Dipole Parallel-Alignment in the Crystal Structure of a Polar Biphenyl: 4'-Acetyl-4-Methoxybiphenyl (AMB)

Rainer Glaser,* Nathan Knotts, Zhengyu Wu, and Charles Barnes

Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri, 65211 (USA)

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Figure 1. The crystal structure of (MeO, Acetyl)-biphenyl syn-AMB features perfect dipole-parallel alignment along the long axis within 2-dimensional layers and also near-perfect dipole-parallel alignment in the stacking direction. In contrast, the crystal structure of the analogous "(MeO, Acetyl)-phenyl", 4-methoxyacetophenone (syn-MAP), is not polar.

Figure 2. The MP2(full)/6-31G* optimized structures of (MeO, Acetyl)-biphenyl AMB with syn conformation, planar and twisted (t = 42.9o, a = -0.3o, m = 0.3)o and anti conformation, planar and twisted (t = 42.9o, a = -0.5o, m = 0.0o).

Figure 3. Space-filling models of a 2d-layer viewed down the c direction with methoxy and acetyl groups in the foreground, respectively.

Figure 4. Each AMB molecule engages in two (ff|ee) contacts A and two (ff|ee) contacts B as well as two (ff|ff) contacts C and two (ff|ff) contacts D. For comparison, the crystal structure of biphenyl itself at room temperature (BIPHEN) also contains planar biphenyls with (ff|ee) and (ff|ff) contacts.

Figure 5. (in paper only) The dipole moments of the AMB molecules are additive in the c direction.

Figure 6. Across the layer interface, each AMB molecule engages in two EN-CH2-H...O contacts with another AMB molecule. These interactions are responsible for the syn conformation and the directionality of the layer stacking in the crystal structure.

Figure 7. Perspectives of the EN-CH2-H...O contacts E that contribute to interlayer bonding; d(OCH2-H...O=C) = 2.672 Ang. and d(COCH2-H...O) = 2.715 Ang.