Total geometry optimizations, at the RHF/3-21G level of theory, for all relevant and computable minima on the unprotonated and protonated potential energy hypersurfaces (PEHSs) of the ground and low-lying valence excited states (i.e. 1 , 3 (π -> π * ) and 1 , 3 (n -> π * )) of HCH=NOH were determined, using ab initio molecular orbital methods, for the purpose of modelling their aromatic analogues. Reference to both energetics and nuclear and electronic structure aspects of adiabatic proton transfer was made in terms of proton affinities and diagrammatic representations in which the role of geometry and electronic configuration is expressed qualitatively with the aid of Lewis/resonance structures.