This symmetric system, C s and 1 A , exists in two forms with the 180 • some 22.25 kJ mol -1 more stable than the 0 • at the G4 level where the dihedral angle is defined by H-O-N to the C-C midpoint. A relaxed potential energy scan faces a barrier of 23.6 kJ mol -1 leading to the 0 • conformer at 22.0 kJ mol -1 .