The protonated betaine/ammonia complex: how stable is the gas-phase zwitterion structure?

The ammonium ion stabilizes a betaine zwitterion in the gas phase forming a salt-bridge structure, [(CH₃)₃N⁺CH₂COO^–NH₄⁺] that is 3.7 kcal/mol less stable than the ion/molecule complex between protonated betaine and neutral ammonia, (CH₃)₃N⁺CH₂COOH/NH₃. DFT calculations have reversed the previously determined relative stability based on PM3 calculations and are in agreement with black-body infrared radiative dissociation experiments. A double-well potential energy surface is formed with a rather low central barrier separating the two complexes. This is conducive to efficient hydrogen/deuterium exchange in agreement with experiment. It prevents the existence of the salt-bridge complex as a distinct species under thermal conditions.