The dynamics of the carbonium ion (CH5+), a highly reactive intermediate with no equilibrium structure, was studied by measuring the infrared spectra for internally cold CH5+(H-2)(n) (n = 1,2,3) stored in an ion trap. First-principle molecular dynamics methods were used to directly simulate the internal motion for these ionic complexes. The combined experimental and theoretical efforts substantiated the anticipated scrambling motion in the CH5+ core and revealed the effect of the solvent molecular hydrogen in slowing down the scrambling. The results indicate the feasibility of using solvent molecules to stabilize the floppy CH5+ ion in order to make it amenable to spectroscopic study.