Binding Energies of Protonated Betaine Complexes: A Probe of Zwitterion Structure in the Gas Phase

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Abstract
The dissociation kinetics of proton-bound dimers of betaine with molecules of comparable gas-phase basicity were investigated using blackbody infrared radiative dissociation (BIRD). Threshold dissociation energies were obtained from these data using master equation modeling. For bases that have comparable or higher gas-phase basicity, the binding energy of the protonated base·betaine complex is ∼1.4 eV. For molecules that are ∼2 kcal/mol or more less basic, the dissociation energy of the complexes is ∼1.2 eV. The higher binding energy of the former is attributed to an ion−zwitterion structure which has a much larger ion−dipole interaction. The lower binding energy for molecules that are ∼2 kcal/mol or more less basic indicates that an ion−molecule structure is more favored. Semiempirical calculations at both the AM1 and PM3 levels indicate the most stable ion−molecule structure is one in which the base interacts with the charged quaternary ammonium end of betaine. These results indicate that the measurement of binding energies of neutral molecules to biological ions could provide a useful probe for the presence of zwitterions and salt bridges in the gas phase. From the BIRD data, the gas-phase basicity of betaine obtained from the kinetic method is found to be 239.2 ± 1.0 kcal/mol. This value is in excellent agreement with the value of 239.3 kcal/mol (298 K) from ab initio calculations at the MP2/6-31+g** level. The measured value is slightly higher than those reported previously. This difference is attributed to entropy effects. The lower ion internal energy and longer time frame of BIRD experiments should provide values closer to those at standard temperature.

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