Planarity of para hexaphenyl

We present experimental and theoretical findings on the geometry of polycrystalline para hexaphenyl via Raman scattering. The planarity of the molecule is affected by hydrostatic pressure and temperature. Our studies indicate that the potential energy curve which governs the torsional motion between neighboring phenyl rings is "W" shaped. We determine the activation energy to promote the molecule from a nonplanar to a planar state to be 0.04 eV, in good agreement with our quantum chemical calculations. From the relative intensities of the 1280 cm(-1) to the 1220 cm(-1) Raman modes we show that high pressure planarizes the molecules, modifying the "W"-shaped potential energy curve to a "U"-shaped one. [S0031-9007(99)09073-0].