DOI: 10.1002/cphc.200800650 Dynamic and Structural Properties of Aqueous Arsenic Solutions Orkid Coskuner* and Thomas C. Allison* [a] Exposure to arsenic (As) in drinking water has been linked to cancer, cardiovascular disease, diabetes, and hypertension. [1a] Hundreds of millions of people around the world are exposed to high levels of arsenic in their drinking water. [1a–c] Despite many techniques for removing As from water, [1d–f] much work remains to ensure a safe water supply. More efficient techni- ques for As remediation and more effective treatments for ar- senic poisoning may benefit from a detailed knowledge of As- ion solution properties. Furthermore, events such as water dis- sociation in As solutions are important in determining pH and chemical reactivity. Herein, Car–Parrinello molecular dynamics [2a,b] (CPMD) simu- lations with transition path sampling [2c–e] (TPS) used in previous studies of water dissociation [2f,g] are employed to study mecha- nisms of water dissociation in the presence of As 2 + , As 3 + , and As 5 + ions and structural properties of their solutions. Simula- tions were performed with 64 water molecules using periodic boundary conditions. [2h] The CPMD/TPS technique allows simu- lation of rare dynamical events and the resulting trajectory, which arises from exploring a large number of dynamical paths, gives detailed mechanistic information. [2c–e] Herein, the breaking of a H ÀOH bond influenced by an As ion and the subsequent formation of water complexes is observed. The water dissociation mechanisms for As ions begin with the breaking of the H ÀOH bond (typically when the bond length exceeds 1.2 ) in a water molecule coordinated to an As ion, leading to hydroxylation of the As ion (Figure 1, I-A, II- A) and a proton in the first solvation shell. The proton then ini- tiates formation of Zundel (H 5 O 2 + , Z) or Eigen (H 9 O 4 + , E) com- plexes. [3] For As 3 + and As 5 + solutions, a proton initiates formation of a Zundel complex by coordinating to oxygen atoms in two water molecules in the first shell (Figure 1, I-B) within 60 fs (As 3 + ) and 71 fs (As 5 + ). In the following 38 fs (As 3 + ) and 33 fs (As 5 + ), proton transfer leads to the disappearance of the first Zundel complex and the formation of another Zundel complex in the second shell (Figure 1, I-C). During the next 51 fs (As 3 + ) and 42 fs (As 5 + ), the migrating proton moves between the second and third shells and binds to a water molecule which, in turn, coordinates to three additional water molecules via hy- drogen bonding to form an Eigen complex (Figure 1, I-D). The As 2 + solution exhibits qualitatively different behavior, with proton fluctuation through hydrogen bonding (Figure 1, II-A) forming an Eigen-like complex before a Zundel complex. The Eigen-like complex (Figure 1, II-B) is formed within the first 57 fs when the migrating proton binds to a water molecule Figure 1. Illustration of proton transfer in water around the As 3 + ion (I) and As 2 + ion (II) from CPMD/TPS simulations. The As ion (dark grey), O atoms (light grey), H atoms (white) and hydrogen bonds (dashed) are depicted. [a] Dr. O. Coskuner, Dr. T. C. Allison National Institute of Standards and Technology 100 Bureau Drive, Stop 8320 Gaithersburg, Maryland 20899-8320 Fax: (+ 1) 301 869 4020 E-mail : orkid.coskuner@nist.gov thomas.allison@nist.gov Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200800650. ChemPhysChem 2009, 10, 1187 – 1189 # 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1187