(12/11)CASSCF and (12/11)CASPT2 ab initio electronic Structure calculations with both the cc-pVDZ and cc-pVTZ basis sets find that there is a barrier to the very exothermic hydrogen shift that converts singlet methylnitrene, CH3N, to methyleneimine, H2C=NH. These two energy minima are connected by a transition structure of C-s, symmetry, which is computed to lie 3.8 kcal/mol above the reactant at the (12/11)CASPT2/cc-pVTZ//(12/11) CASSCF/cc-pVTZ level of theory. The (12/11)CASSCF/cc-pVTZ value for the lowest frequency vibration in the transition structure is 854 cm(-1), and CASPT2 calculations concur that this a" vibration does indeed have a positive force constant. Thus, there is no evidence that this geometry is actually a mountain top, rather than a transition structure, an the global potential energy surface or that a CI pathway of lower energy connects the reactant to the product. Therefore, our computational results indicate that the bands seen for singlet methylnitrene in the negative ion photoelectron spectrum of CH3N- an due to singlet methylnitrene being an energy minimum, rather than a transition State. Our results also Lead us to predict that, at least in principle, singlet methylnitrene should be an observable intermediate in the formation of methyleneimine.