Valence bond curve-crossing model of the 1,2-hydrogen shift in HCN and isovalent systems

A valence-bond curve-crossing model is proposed to account for the shape of the energy profile of the hydrogen exchange in HCN and isovalent molecules HXY. The adiabatic energy profile associated to the 1,2-hydrogen shift as a function of the bending angle might display one, two or three minima. We propose that these energy profiles can be modeled as the avoided-crossing of three diabatic curves. Ab initio valence-bond calculations performed on HCN/CNH support this model suggesting that the preference for the bent HXY structures versus the linear ones essentially correlates with the 2Σ–2Π splitting of the XY moiety.