The lowest energy conformer has a cis or syn arrangement with C s symmetry with two gauche forms, ∠CCNO = ±123.4 • , lying very close at ≈ 2 kJ mol -1 . The difference between syn and anti forms (described as anti but in reality gauche) is slight, ranging from 1-3 kJ mol -1 according to Fu and co-workers. Detailed explorations of the microwave spectrum and the potential functions have been carried out by Cox et al. In careful relative intensity measurements they determined a cis/gauche zero-point energy difference of 175 ± 35 cm -1 or 2.1 ± 0.4 kJ mol -1 . This is in excellent agreement with WMS calculations which yield ∆{∆ f H -• (0 K)|syn -gauche|} = |58.72 -60.79| = 2.07 kJ mol -1 .