The conformer with cis Hs, technically this is the (Z ) conformer, is some 5.5 kJ mol -1 more stable than that with trans Hs or (E ) conformer in agreement with earlier work and with a study of nitrogen species by a series of composite methods where the difference was reported as 5.38 ± 0.8 kJ mol -1 at 298. in accord with all recent theoretical determinations, viz. this work 9.79 eV, Frigge et al. 9.80 eV.