Proton Transfer in Concentrated Aqueous Hydroxide Visualized Using Ultrafast Infrared Spectroscopy

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 115; no. 16; pp. 3957 - 3972
• Main Authors: Roberts, Sean T, Ramasesha, Krupa, Petersen, Poul B, Mandal, Aritra, Tokmakoff, Andrei
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 28.04.2011
• Subjects: Hydroxides - chemistry; Protons; Spectrophotometry, Infrared; Time Factors; Water - chemistry
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp108474p

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/D2O. 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.