To For Form I, which has a SH...S hydrogen bonding pattern as its most stable structure, the other minimum, consistent with SH...O bonding, occurs 1.0 kJ mol -1 higher in energy at an angle of about -70 • . The barrier to inter-conversion is about 6.3 kJ mol -1 at -20 higher than its global minimum, with a barrier at 180 • of 3.9 kJ mol -1 . The other group, which is part of a SH...S hydrogen bond in the optimised structure has another minimum 2.1 kJ mol -1 higher in energy with a barrier height of 6.2 kJ mol -1