Ultrafast Proton Transfer Pathways Mediated by Amphoteric Imidazole

Abstract

Imidazole, being an amphoteric molecule, can act both as an acid and as a base. This property enables imidazole, as an essential building block, to effectively facilitate proton transport in high-temperature proton exchange membrane fuel cells and in proton channel transmembrane proteins, enabling those systems to exhibit high energy conversion yields and optimal biological function. We explore the amphoteric properties of imidazole by following the proton transfer exchange reaction dynamics with the bifunctional photoacid 7-hydroxyquinoline (7HQ). We show with ultrafast ultraviolet-mid-infrared pump-probe spectroscopy how for imidazole, in contrast to expectations based on textbook knowledge of acid-base reactivity, the preferential reaction pathway is that of an initial proton transfer from 7HQ to imidazole, and only at a later stage a transfer from imidazole to 7HQ, completing the 7HQ tautomerization reaction. An assessment of the molecular distribution functions and first-principles calculations of proton transfer reaction barriers reveal the underlying reasons for our observations.

Figure 1

Comparison of the acid–base reaction pathways between the N and Z tautomers and the ionic A and C species of 7HQ, reacting with (a) formate ion/formic acid or (b) the amphoteric H₂B⁺/HB/B^–, where HB can be imidazole, or the solvent H₂O or CH₃OH. Excess proton transfer pathway I can occur with the bases formate anion and imidazole but also with the solvent reacting with 7HQ. Note that for formate solutions proton vacancy pathway II can occur only with 7HQ exclusively reacting with the solvent, not with formate as the active reaction partner, whereas for imidazole solutions, both pathways are possible for imidazole and the solvent reacting with 7HQ. The transient UV-pump-IR-probe spectra are shown as a function of the base added to the deuterated methanol solution at particular pulse delay times for 7HQ reacting with (c) the formate anion or (d) imidazole. The dashed lines in the plots indicate the transient response recorded at −100 ps, showing the baseline in these measurements.

Figure 2

(a) Full widths at half-maximum (fwhm) of 7HQ marker bands for the A* and Z* species as a function of pulse delay time for the 7HQ stock and 4 M imidazole solutions. (b) Integrated areas of the A* and Z* marker bands (at 1422 and 1530 cm^–1, respectively), as a function of pulse delay time. (c) Transient kinetics of the 7HQ N* and Z* tautomers as a function of the DIm imidazole concentration. (d) Absolute population fractions of the N* and Z* tautomers and the A* anion at a 1 ns pulse delay time, derived from the transient UV/IR pump–probe spectra, as a function of imidazole concentration. Note that the curves depicted in panels a and b are shown with logarithmic scaling of the x-axis, whereas in panel c, a normal scaling has been used, to highlight the early time components of “tight” and “loose” complexes in panels a and b and the long time components in panel c.

Figure 3

Average particle density of imidazole (HIm) and methanol (CH₃OH) molecules around a cis- or trans-7HQ molecule (see the color scale) together with the average orientation of the displayed vectors of HIm and CH₃OH (see the arrows).

Figure 4

Combined distribution function depicting the probability of finding a certain distance (see the sketch) between 7HQ and CH₃OH and between CH₃OH and HIm on the horizontal and vertical axes, respectively.

Figure 5

Energy paths for (a) oxygen site deprotonation onto HB = CH₃OH, HIm (N* + HB → A* + H₂B⁺) and (b) proton abstraction from HB onto the nitrogen site (A* + HB → Z* + B^–) in both cases with different solvation influences. (c) Energy path for proton abstraction of HIm onto A* and N*.