title: Collective vibrations of water-solvated hydroxide ions investigated with broadband 2DIR spectroscopy.
abstract: The infrared spectra of aqueous solutions of NaOH and other strong bases exhibit a broad continuum absorption for frequencies between 800 and 3500 cm(-1), which is attributed to the strong interactions of the OH(-) ion with its solvating water molecules. To provide molecular insight into the origin of the broad continuum absorption feature, we have performed ultrafast transient absorption and 2DIR experiments on aqueous NaOH by exciting the O-H stretch vibrations and probing the response from 1350 to 3800 cm(-1) using a newly developed sub-70 fs broadband mid-infrared source. These experiments, in conjunction with harmonic vibrational analysis of OH(-)(H2O)n (n = 17) clusters, reveal that O-H stretch vibrations of aqueous hydroxides arise from coupled vibrations of multiple water molecules solvating the ion. We classify the vibrations of the hydroxide complex by symmetry defined by the relative phase of vibrations of the O-H bonds hydrogen bonded to the ion. Although broad and overlapping spectral features are observed for 3- and 4-coordinate ion complexes, we find a resolvable splitting between asymmetric and symmetric stretch vibrations, and assign the 2850 cm(-1) peak infrared spectra of aqueous hydroxides to asymmetric stretch vibrations.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Aritra
authors.initials: A
authors.lastname: Mandal
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Krupa
authors.initials: K
authors.lastname: Ramasesha
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Luigi
authors.initials: L
authors.lastname: De Marco
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Andrei
authors.initials: A
authors.lastname: Tokmakoff