Structural Dynamics of Protonated Methane and Acetylene

The structural stability of protonated methane (CH₅⁺) and acetylene (C₂{H}₃⁺$) has been investigated with ab initio molecular dynamics. In C${H}₅⁺$, the hydrogen atoms execute large amplitude pseudorotational motions. The hydrogen migrations are not disturbed by complexation with a hydrogen molecule. In contrast, for ${C}₂H₃⁺, the three hydrogen atoms permutate cyclically among the end carbon atoms and the bridging position. The theoretical results are consistent with and explain recent spectroscopic observation of these molecules.