In all the static calculations the signature of the S-H stretch appears to be a doublet of absorptions separated by about 50 cm -1 . Figure shows the spectra from the static calculations of each of the four polymorphs. Form I has the lowest frequency absorption for the S-H stretch, followed by Forms IV, Form II and Form III in that order. Form III is the only polymorph that does not exhibit the doublet splitting of this vibration and it is also the polymorph with the highest frequency S-H stretching vibration. Form II shows strong absorption at 2560 cm -1 which is owing to an N-H stretching vibration which is part of the N-H...O hydrogen bonded network in this polymorph. In the high frequency region associated with N-H and S-H stretching, only Form I shows any significant absorption associated with the S-H stretching motion at around 2400 cm -1 . Experimentally 40 the S-H stretching frequency in Form I is 2551 cm -1 and the N-H stretching frequency is 3064 and 3166 cm -1 .