Abnormal abundance of HNC in interstellar spaces has been the motivation of many experimental as well as theoretical studies of the branching ratio [HNC]/[HCN] in the dissociation of HCNH, right after its formation from electron capture by HCNH+, available in the upper atmosphere. In the present work we were interested in nonadiabatic studies involving the dissociation channel of HCNH leading to the formation of HNC. This study reports for the first time that the conical intersection (CI) between the states 1 2Σ+ and 22Σ+ exists only in some bent geometry (and not in the collinear geometry) where both of these states have A′ symmetry. This finding is important as this CI is crucial in the dissociation of HCNH. We further report that these two states strongly couple with 12Π, the lowest electronic state of collinear HCNH. Hence we construct a three-state Hilbert subspace (HSS), comprising of the states 12Π, 12Σ+, and 2 2Σ+, in a configuration space where these states interact very strongly and the adiabatic-to-diabatic transformation angles (mixing angle) yield meaningful values of topological (Berry) phase. This leads to the construction of the corresponding three-state diabatic potentials. We advocate that these diabatic potentials, considering both the linear as well as bent configurations, nicely elucidate the formation of the HNC molecule by the CH bond dissociation of HCNH molecule. © 2013 American Chemical Society.