Figure (a) shows the static spectra of each of the four polymorphs. Each polymorph has a unique crystal packing which should be reflected in the low frequency infrared response and this seems to be the case, with very different features for each of the four polymorphs. Form II has the lowest frequency absorption feature, below 40 cm -1 , and it shows two prominent absorptions just above 60 cm -1 and 120 cm -1 . Both Forms I and IV show a low frequency absorption peak around 56 cm -1 , with weaker transitions at lower frequencies broadening the absorption peaks. Form III shows less absorption in the terahertz region than any of the other polymorphs while Form II has the most intense spectral feature.