Parallel proton transfer pathways in aqueous acid-base reactions

Abstract

We study the mechanism of proton transfer ͑PT͒ between the photoacid 8-hydroxy-1,3, 6-pyrenetrisulfonic acid ͑HPTS͒ and the base chloroacetate in aqueous solution. We investigate both proton and deuteron transfer reactions in solutions with base concentrations ranging from 0.25M to 4M. Using femtosecond midinfrared spectroscopy, we probe the vibrational responses of HPTS, its conjugate photobase, the hydrated proton/deuteron, and chloroacetate. The measurement of these four resonances allows us to follow the sequence of proton departure from the acid, its uptake by the water solvent, and its arrival at the base. In recent studies it was shown that proton transfer to carboxylate bases proceeds via Grotthuss conduction through a water wire connecting the acid and the base ͓Mohammed et al., Science 310, 83 ͑2005͒;Agnew. Chem. Int. Ed. 46, 1458 ͑2007͒;Siwick and Bakker, J. Am. Chem. Soc. 129, 13412 ͑2007͒; J. Phys. Chem. B 112, 378 ͑2008͔͒. Here we show that, for the weaker base chloroacetate, an alternative channel for proton transfer arises. In this channel the proton is first transferred to the water solvent and only later taken up from the water by the base. We study the base concentration dependence of the two competing channels.

Influence of Ions on Aqueous Acid–Base Reactions

“…[9][10][11][12][14][15][16] In the presence of a base, the proton/deuteron transfer reactions speed up significantly because proton/deuteron transfer between the acid and base becomes the dominant pathway. [17][18][19][20][21][22][23][24][25][26][27][28] Herein we studied how the mechanism of the proton-transfer reaction between aqueous HPTS and acetate changes when bromide salts are added to the aqueous solution.…”

“…[17][18][19][20][21][22][23][24][25][26][27][28] We also probed the carbonyl stretch of acetic acid, which marks the arrival of the proton/deuteron at the base. (Figure 4 a), the decay of the deuteron signal shows practically the same dynamics as the decay of HPTS*, which means that there is little delay between the release of the deuteron by HPTS and the take-up of the deuteron by acetate.…”

“…[26][27][28] The rise of the carbonyl absorption and the depletion of the deuteron signal mark the arrival of the deuteron at the acetate base to form acetic acid.…”

Water-wire catalysis in photoinduced acid–base reactions

“…[10][11][12][13][14][15][16][17][18][19][20] In this regard, the proton translocation in bifunctional heteroaromatic molecules, featuring both proton donor and acceptor groups distal to each other, can serve as a simplest model reaction to experimentally examine the catalytic function of water wires. [17][18][19][20] 7-Hydroxyquinoline (7HQ) and related probe molecules are excellent candidates among those because proton donating and accepting groups are at well-defined positions forming proton-transfer coordinates.…”

“…The lifetime of a transient hydronium ion has been reported to be several ps in a proton-relay process involving a weaker acetate base of pK b = 9.25. 14, 15 The transient hydronium cation complexed with intermediate A* may be effectively in the Zundel-like configuration, in which the proton is symmetrically shared by two water molecules, due to strong H-bonds along the wire at the expense of the interaction with surrounding bulk water (Scheme 1). 11,12 Essentially the above picture is in line with ultrafast infrared (IR) spectroscopic studies, in which sequential proton hopping is reported to take place for photoacid-(H 2 O) n -base systems.…”