Binding energies of protonated betaine complexes: a probe of zwitterion structure in the gas phase

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 approximately 1.4 eV. For molecules that are approximately 2 kcal/mol or more less basic, the dissociation energy of the complexes is approximately 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 approximately 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.

Figure 1

Blackbody infrared radiative dissociation data of the protonated dimers of (a) DBU·betaine, (b) betaine·betaine, and (c) TMDB·betaine fit to unimolecular kinetics at the temperatures indicated.

Figure 2

Zero-pressure limit Arrhenius plot for the dissociation of the protonated dimers of (▴) betaine, (○) DBN·betaine, (▪) DBU·-betaine, (▵) TMG·betaine, (♦) DMPA·betaine, (•) TMDB·betaine, (▾;) TMDP·betaine, and (*) TBA·betaine.

Figure 3

Minimized structures for TMG·betaine dimer and energies calculated at both the AM1 and PM3 semiempirical levels. Structure I is an ion–zwitterion complex; II–IV are ion–molecule complexes. All energies are referenced to the betaineH⁺·TMG exit channel.

Figure 4

Kinetic method plots of the logarithm of the ratio of protonated monomers versus gas-phase basicity of reference compounds. Protonated betaine dimers were dissociated using blackbody infrared radiation in a Fourier transform mass spectrometer. Gas-phase basicity of betaine is determined using GB values for reference compounds from both the old (dotted line) and the revised (solid line) scales of Lias.

Figure 5

Calculated Boltzmann distributions for the TMG·betaine dimer at 450 K and 780 K (top) representing the average dissociation temperatures of blackbody infrared radiative dissociation in a Fourier transform mass spectrometer (FTMS) and collisionally activated dissociation in a quadrupole mass spectrometer, respectively. Wahrhaftig diagram (bottom) illustrating hypothetical dissociation processes, as indicated, for the protonated TMG·betaine dimer. Kinetic windows for FTMS (dotted–dashed line) and quadrupole mass spectrometer (dotted line) are displayed for reference.

Figure 6

Zero-pressure limit Arrhenius plots of the experimental data (•) and limiting case master equation modeled fits for the dissociation of protonated TMG·betaine dimer with (a) transition dipole moments calculated at the AM1 level with A^∞ = 10^15.0 s⁻¹ and E_o = 1.28 eV (dashed line) and A^∞ 10^17.4 s⁻¹ and E_o = 1.38 eV (dotted line) and (b) transition dipole moments 2 times larger than AM1 values with A^∞ = 10^15.0 s⁻¹ and E_o = 1.25 eV (dashed line) and A^∞ = 10^17.4 s⁻¹ and E_o = 1.35 eV (dotted line). These master equation fits represent the maximum acceptable limit within the error of the experiment.

Figure 7

Energy diagrams for the dissociation of proton-bound dimers of betaine and reference bases of decreasing gas-phase basicity: (a) DBU, (b) DBN, (c) TMG, (d) TMDB, TMDP, DMPA, and TBA. The ΔGB values reported correspond to the differences in gas-phase basicity between betaine and the reference bases. The measured value of E_o for betaine with each base is also indicated on the diagram.

Figure 8

Energy diagrams for the dissociation of proton-bound dimers of betaine with a base of identical gas-phase basicity: (a) homodimer of betaine and (b) a heterodimer of betaine with an equally basic nonzwitterionic compound.

Scheme 1