TY - JOUR
T1 - Aqueous bimolecular proton transfer in acid-base neutralization
AU - Mohammed, Omar F.
AU - Pines, Dina
AU - Pines, Ehud
AU - Nibbering, Erik T.J.
N1 - Funding Information:
This work has been supported by the German–Israeli Foundation for Scientific Research and Development (GIF 722/01 for E.T.J.N. and E.P.), the Israeli Science Foundation (ISF 562/04 for E.P.), the James Franck centre for Laser–Matter Interactions (E.P.), and by a long term mission fellowship by the Egyptian government (O.F.M.). We thank Prof. Dr. Andrius Baltuška in translating essential text sources on Theodor von Grotthuss from the Baltic region. This work is dedicated to Douwe A. Wiersma, who since 30 years has been a pioneer in ultrafast spectroscopy of condensed phase chemical dynamics. This work is just one demonstration of his unshakeable conviction that progress in the field ultimately will lead to essential insight into elementary condensed phase chemical reactions.
PY - 2007/11/15
Y1 - 2007/11/15
N2 - We report on a comparative femtosecond infrared study of the bimolecular proton transfer reaction dynamics of the photoacid pyranine (8-hydroxy-1,3,6-trisulfonate-pyrene, abbreviated as HPTS) with the family of carboxylate bases -OOCCH(3-x)Clx (x = 0-3) in aqueous solution. We use a generalized reaction model consisting of n proton transfer reaction pathways between photoacid and base, each pathway characterized by the number n of water molecules functioning as solvent switch facilitating an efficient transfer. The dynamics of the different proton transfer pathways have been followed by inspection of appropriate vibrational modes marking the progress of the reaction, and allow the direct observation of the transient relay mechanism of proton through one bridging water molecule in a loose photoacid-base configuration of the type HPTS-⋯H3O+⋯-OOCCH(3-x) Clx (x = 0-3). By applying a global fit procedure on the n-coupled reaction pathways, with full reversibility and detailed balance, we find for the reaction series that the encounter complex formed after mutual diffusion between photoacid and base, and subsequent desolvation, follows in general the loose complex pathway with one water molecule bridging acid and base. Only for the weakest base studied, trichloroacetate, larger solvent switches with n ≥ 2 water molecules are important in the transfer dynamics. Whereas transfer in the tight complex with no water spacer (n = 0) occurs within time resolution of 150 fs, the transfer in the loose (n = 1) complex proceeds in a sequential fashion where the first transfer occurs within time resolution, irrespective of base strength, whereas the second transfer from the water to the base is activated and conforms to a correlation between free energy and reaction rate.
AB - We report on a comparative femtosecond infrared study of the bimolecular proton transfer reaction dynamics of the photoacid pyranine (8-hydroxy-1,3,6-trisulfonate-pyrene, abbreviated as HPTS) with the family of carboxylate bases -OOCCH(3-x)Clx (x = 0-3) in aqueous solution. We use a generalized reaction model consisting of n proton transfer reaction pathways between photoacid and base, each pathway characterized by the number n of water molecules functioning as solvent switch facilitating an efficient transfer. The dynamics of the different proton transfer pathways have been followed by inspection of appropriate vibrational modes marking the progress of the reaction, and allow the direct observation of the transient relay mechanism of proton through one bridging water molecule in a loose photoacid-base configuration of the type HPTS-⋯H3O+⋯-OOCCH(3-x) Clx (x = 0-3). By applying a global fit procedure on the n-coupled reaction pathways, with full reversibility and detailed balance, we find for the reaction series that the encounter complex formed after mutual diffusion between photoacid and base, and subsequent desolvation, follows in general the loose complex pathway with one water molecule bridging acid and base. Only for the weakest base studied, trichloroacetate, larger solvent switches with n ≥ 2 water molecules are important in the transfer dynamics. Whereas transfer in the tight complex with no water spacer (n = 0) occurs within time resolution of 150 fs, the transfer in the loose (n = 1) complex proceeds in a sequential fashion where the first transfer occurs within time resolution, irrespective of base strength, whereas the second transfer from the water to the base is activated and conforms to a correlation between free energy and reaction rate.
KW - Acid-base neutralization
KW - Bimolecular reaction dynamics
KW - Femtochemistry
KW - Grotthuss mechanism
KW - Hydrated proton
KW - Proton transfer
KW - Ultrafast infrared spectroscopy
UR - http://www.scopus.com/inward/record.url?scp=35748964154&partnerID=8YFLogxK
U2 - 10.1016/j.chemphys.2007.06.040
DO - 10.1016/j.chemphys.2007.06.040
M3 - Article
AN - SCOPUS:35748964154
SN - 0301-0104
VL - 341
SP - 240
EP - 257
JO - Chemical Physics
JF - Chemical Physics
IS - 1-3
ER -