Vibrational Spectroscopy of Microhydrated Conjugate Base Anions

Conjugate-base anions are ubiquitous in aqueous solution. Understanding the hydration of these anions at the molecular level represents a long-standing goal in chemistry. A molecular-level perspective on ion hydration is also important for understanding the surface speciation and reactivity of aeros...

Full description

Saved in:
Bibliographic Details
Published in:Accounts of chemical research 2012-01, Vol.45 (1), p.43-52
Main Authors: Asmis, Knut R, Neumark, Daniel M
Format: Article
Language:English
Subjects:
Aerosols
Anions
Anions - chemistry
Bicarbonates
Bicarbonates - chemistry
Computer Simulation
Conjugates
Gases - chemistry
Hydration
Hydrogen Bonding
Infrared
Molecular Structure
Phase Transition
Solutions - chemistry
Speciation
Spectroscopy
Spectrum Analysis - instrumentation
Spectrum Analysis - methods
Sulfates - chemistry
Tandem Mass Spectrometry - instrumentation
Tandem Mass Spectrometry - methods
Vibration
Water - chemistry
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:Conjugate-base anions are ubiquitous in aqueous solution. Understanding the hydration of these anions at the molecular level represents a long-standing goal in chemistry. A molecular-level perspective on ion hydration is also important for understanding the surface speciation and reactivity of aerosols, which are a central component of atmospheric and oceanic chemical cycles. In this Account, as a means of studying conjugate-base anions in water, we describe infrared multiple-photon dissociation spectroscopy on clusters in which the sulfate, nitrate, bicarbonate, and suberate anions are hydrated by a known number of water molecules. This spectral technique, used over the range of 550–1800 cm–1, serves as a structural probe of these clusters. The experiments follow how the solvent network around the conjugate-base anion evolves, one water molecule at a time. We make structural assignments by comparing the experimental infrared spectra to those obtained from electronic structure calculations. Our results show how changes in anion structure, symmetry, and charge state have a profound effect on the structure of the solvent network. Conversely, they indicate how hydration can markedly affect the structure of the anion core in a microhydrated cluster. Some key results include the following. The first few water molecules bind to the anion terminal oxo groups in a bridging fashion, forming two anion–water hydrogen bonds. Each oxo group can form up to three hydrogen bonds; one structural result, for example, is the highly symmetric, fully coordinated SO4 2–(H2O)6 cluster, which only contains bridging water molecules. Adding more water molecules results in the formation of a solvent network comprising water–water hydrogen bonding in addition to hydrogen bonding to the anion. For the nitrate, bicarbonate, and suberate anions, fewer bridging sites are available, namely, three, two, and one (per carboxylate group), respectively. As a result, an earlier onset of water–water hydrogen bonding is observed. When there are more than three hydrating water molecules (n > 3), the formation of a particularly stable four-membered water ring is observed for hydrated nitrate and bicarbonate clusters. This ring binds in either a side-on (bicarbonate) or top-on (nitrate) fashion. In the case of bicarbonate, additional water molecules then add to this water ring rather than directly to the anion, indicating a preference for surface hydration. In contrast, doubly charged sulfate dianio
ISSN:0001-4842
1520-4898
DOI:10.1021/ar2000748