High pressure studies on the planarity of para-hexaphenyl

We present experimental and theoretical findings on the geometry of para-he xaphenyl (PHP) molecules in polycrystalline powder. A new method to assess the planarity of PHP via Raman spectroscopy is presented. Based on this met hod we describe a W-shaped potential energy curve which governs the torsion al motion between neighboring phenyl rings. We determine the activation ene rgy to promote PHP from a non-planar to a planar state to be 0.04 eV, in go od agreement with our quantum chemical calculations. Finally we are able to experimentally planarize the molecules by the application of hydrostatic p ressure, which modifies the W-shaped potential energy curve to a U-shaped o ne.