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)(3)N+CH2COO-NH4+] that is 3.7 kcal/mol less s table than the ion/molecule complex between protonated betaine and neutral ammonia, (CH3)(3)N+CH2COOH/NH3. DFT calculations have reversed the previous ly determined relative stability based on PM3 calculations and are in agree ment with black-body infrared radiative dissociation experiments. A double- well potential energy surface is formed with a rather low central barrier s eparating the two complexes. This is conducive to efficient hydrogen/deuter ium exchange in agreement with experiment. It prevents the existence of the salt-bridge complex as a distinct species under thermal conditions. (C) 20 00 Elsevier Science B.V. All rights reserved.