Non‐Covalent Interactions in the Biphenyl Crystal: Is the Planar Conformer a Transition State?

A combined experimental and theoretical study of the biphenyl (BP) crystal is presented. The X‐ray diffraction data collected at 100 K were subjected to Hirshfeld atom and multipole refinements of the electron density, ρ(r). A theoretical exploration of the potential energy surface (PES) of the crystal was also carried out. This investigation challenges the common assumption that the planar structure of BP in the phase I crystal is an average of two twisted configurations in a double‐well potential. The theoretical computations provide compelling evidence that this structure corresponds to a minimum on the PES hence to a stable molecular arrangement. Consistently, the experiment showed no evidence of positional or dynamic disorder. The intramolecular hydrogen‐hydrogen bonds detected are not repulsive. The topological analysis of the experimental and theoretical ρ(r) reveals that both the intra‐ and intermolecular H⋅⋅⋅H and the C−H⋅⋅⋅π contacts stabilize the BP crystal. Planar biphenyl is a stable minimum on the quantum mechanical potential energy surface of the crystal. The lack of thermal disorder in the phase I at 100 K and the properties of the experimental and theoretical electron densities support this conclusion. The crystal packing is mostly determined by intra and intermolecular hydrogen‐hydrogen bonding, non‐covalent C-H⋯π interactions and internal rotation of the biphenyl molecule.