We present a joint theoretical and experimental investigation of the isomerization dynamics of doubly ionized acetonitrile $[{\mathrm{H}}_3\mathrm{C}─\mathrm{C}\equiv \mathrm{N}{]}^{2+}$ in the gas phase. Molecular dynamics simulations show that the dication retains its linear geometry for a few hundred femtoseconds (fs) before undergoing a fast cyclization process, where the carbon and nitrogen atoms arrange into a triangular geometry. It occurs in less than 30 fs, a time comparable to or even shorter than that required for the migration of the much lighter hydrogen atoms. As a result, the dication later adopts an almost irreversible linear isonitrile ($\mathrm{C}─\mathrm{N}─\mathrm{C}$) configuration, where the positions of the neighboring nitrogen and carbon atoms are swapped with respect to their initial arrangement. These findings are compatible with the outcomes of fs pump-probe experiments in which the produced charged fragments are detected in coincidence as a function of the pump-probe delay via Coulomb explosion imaging.

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• Atomic & molecular structure
• Chemical reactions
• Electronic excitation & ionization
• Molecular dissociation