Structural Dynamics of Protonated Methane and Acetylene

The structural stability of protonated methane (C${\mathrm{H}}_5^{+}$) and acetylene (${\mathrm{C}}_2$ ${\mathrm{H}}_3^{+}$) has been investigated with ab initio molecular dynamics. In C${\mathrm{H}}_5^{+}$, the hydrogen atoms execute large amplitude pseudorotational motions. The hydrogen migrations are not disturbed by complexation with a hydrogen molecule. In contrast, for ${\mathrm{C}}_2$ ${\mathrm{H}}_3^{+}$, 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.