title: Proton transfer in concentrated aqueous hydroxide visualized using ultrafast infrared spectroscopy.
abstract: While it is generally recognized that the hydroxide ion can rapidly diffuse through aqueous solution due to its ability to accept a proton from a neighboring water molecule, a description of the OH(-) solvation structure and mechanism of proton transfer to the ion remains controversial. In this report, we present the results of femtosecond infrared spectroscopy measurements of the O-H stretching transition of dilute HOD dissolved in NaOD/D(2)O. Pump-probe, photon echo peak shift, and two-dimensional infrared spectroscopy experiments performed as a function of deuteroxide concentration are used to assign spectral signatures that arise from the OH(-) ion and its solvation shell. A spectral feature that decays on a ∼110 fs time scale is assigned to the relaxation of transiently formed configurations wherein a proton is equally shared between a HOD molecule and an OD(-) ion. Over picosecond waiting times, features appear in 2D IR spectra that are indicative of the exchange of population between OH(-) ions and HOD molecules due to deuteron transfer. The construction of a spectral model that includes spectral relaxation, chemical exchange, and thermalization processes, and self-consistently treats all of our data, allows us to qualitatively explain the results of our experiments and gives a lower bound of 3 ps for the deuteron transfer kinetics.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Sean T
authors.initials: ST
authors.lastname: Roberts
authors.forename: Krupa
authors.initials: K
authors.lastname: Ramasesha
authors.forename: Poul B
authors.initials: PB
authors.lastname: Petersen
authors.forename: Aritra
authors.initials: A
authors.lastname: Mandal
authors.forename: Andrei
authors.initials: A
authors.lastname: Tokmakoff
mesh_terms: D006878:Hydroxides / Q000737:chemistry*; D011522:Protons; D013055:Spectrophotometry, Infrared; D013997:Time Factors; D014867:Water / Q000737:chemistry
chemical_list: D006878:Hydroxides; D011522:Protons; D014867:Water