Ultrafast Transient Cyclization of Doubly Charged Acetonitrile

We present a joint theoretical and experimental investigation of the isomerization dynamics of doubly ionized acetonitrile [H_{3}C─C≡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 (C─N─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.