Experimental versus simulated Coulomb-explosion images of flexible molecules: Structure of protonated acetylene C2H3+

The most probable structure of protonated acetylene C2H3+ (the "vinyl cation") is inferred from both Coulomb-explosion experiments and finite-temperature ab initio quantum simulations followed by simulations of foil effects. It is found that C2H3+ features significant deviations from the planar bridged equilibrium structure as well as from the planar Y-shaped local minimum structure, which are known from static electronic structure calculations. In particular, the "axial protons" feature a significant out-of-plane trans-bending due to fluctuation effects. This implies that the nonplanarity has to be taken into account in theoretical treatments that aim at describing this fluxional molecule