Pressure dependence of Raman lines below 600 cm-1. The green shaded triangles correspond to the 95.5 cm-1 Raman line at ambient pressure. The analysis of the Grueneisen parameter of the low energy modes reveals the intramolecular nature of this 95.5 cm-1 mode. From polarization measurements under pressure this 95.5 cm-1 line has B1 symetry. This is in excellent agreement with the normal coordinate analysis performed by Kertesz et al.  who calculate the fluorene butterfly mode at 95 cm-1. From the amplitude of this mode along the C···N direction this is also a very likely reaction promoting mode together with the 125 cm-1 libration.