Molecular Simulation -ol. ,. No. ' January 2('( .123, Local dynamics and structure of the solvated hydroxide ion in water Song Hi Leea* and Jayendran C. Rasaiahb1 aDepartment of Chemistry, Kyungsung University, Pusan 608-736, South Korea; bDepartment of Chemistry, University of Maine, Orono, " !##69, USA (Received 31 December 2008; final version received 13 June 2009) Details of the structural diffusion mechanism in proton transfer (PT) reactions involving a hydroxide ion in water and the structure of the solvated ions and transfer intermediates are in dispute. Here, we elucidate the mechanism of PT involving a 10 hydroxide ion in water by molecular dynamics simulations using a dissociating water model based on ab initio calculations. 2 We find that the hydroxide ion in bulk water is present as the four-coordinated OH (H2O)4 complex, which loses a water molecule before a !" occurs through the formation of proton sharing intermediates in general agreement with the previously disputed first principles studies of small systems. Keywords: proton transfer; hydroxide ion; OSS2 model; aqueous solution 02 22 February 20 1. Introduction transport in aqueous solutions is driven by solvent Proton transfer (PT) in $ater is of central importance to a fluctuations in the second solvation shell of the Eigen wide range of chemical and biological processes [1+,]. comple# as suggested earlier by Agmon [19,20- $ith PT occurring via a Zundel intermediate. 16427733] At: 00: he anomalously large mobility of hydrogen and hydroxide ions in aqueous solution compared to other Computational and theoretical studies *7+12,23] of the ions *.%/- is explained by structural diffusion in which hydroxide ion in bulk $ater are not in agreement with protons move successively between neighbouring mol- the mechanism of transport or even the identity of the dominant solvated hydro#ide ion species and intermedi0 ecules, one of which contains either an excess protonic ates in aqueous solution. In one scenario *7%(,-% the - Superceded by 9 charge or a negative charge defect, characterised as a oxygen of the hydroxide ion is four coordinated through 1proton hole2 *3%4-. Simple hole+particle symmetry hydrogen bonds in a stable and relatively inert comple# suggests that hydrogen ions move in the same direction 2 OH [H '- . Fluctuations lead to the loss of a water as that of PT while hydroxide ions move in the opposite ( . molecule to form the slightly less stable three-coordinated sense [7,8]. However, the mobility of the hydrogen ion is 2 comple# 'H [H('-, followed by PT to the oxygen of the ent Distribution about 1.8 times that of the hydroxide ion. This indicates a hydroxide ion from one of the three water molecules dynamical asymmetry in the transport mechanisms of in the comple# through a :undel0li>e intermediate hydrogen and hydroxide ions in liquid water *7]. The origin 2 2 [HO· A ·H· A ·OH] to create a new OH [H('-, complex of this is not fully understood, with conflicting opinions at the donor site. The whole cycle can be repeated again (in about the structure of the hydro#ide ion and the a large enough system! after a suitable solvent fluctuation 2 BSCOHost EJS Cont mechanism of PT *7+12]. in which an OH ion is virtually transported in a direction 8 hydrogen ion in a dilute aqueous solution exists as a þ opposite to that of PT without the actual movement of an range of structures from the Eigen cation [13+15] H7'. % oxygen atom in phase $ith PT. The process terminates consisting of a hydronium ion H 'þ hydrogen bonded to 2 , when the newly formed 'H [H('-, complex reverts back three water molecules, to the Zundel cation [16,17- H 'þ 2 / ( to the less active OH [H('-. by hydrogen bonding with Downloaded By: [E in which the excess proton is shared between two water an adjacent water molecule. molecules. Details of the mechanism of transport of the However, Quasi-chemical calculations and other ab þ H ion are broadly understood from first principles initio %uantum computer simulations &'( '') ha$e simulations of the excess proton in small systems of ,( suggested that the oxygen of the OH2 ion is three water molecules [18+(<- and from extended valence bond coordinated in its most stable form as expected from the theory calculations of larger systems [21,22- but the Lewis picture of electron pair bonding. These uncertainties corresponding details for hydroxide ion transport are in in the first principles simulations arise partly from small dispute *7+12,23]. Marx et al. [18] found that Hþ ion system sizes (usually of the order of about 32 water *Corresponding author. 9mail: [email protected] ISSN <57(04<(( print/ISSN 1029-0435 online q 2010 Taylor F Francis DOI: 10.1080/089(4<(<703115252 http://www.informaworld.com 4< S.H. Lee and J.C. Rasaiah molecules) and different approximations for the exchange system are nearly identical e$en though Ojama=e et al. used a functional in density functional theory which lead to different method for the 5wald sums in the calculation of the different conclusions about the proportions of the induced dipole moment. three-and four-coordinated hydroxide ion species in solution. "his translates into uncertainties in the mechanism of hydroxide ion transport. 3. Results and discussion Figure 1 shows four snapshots of a PT sequence from our simulations of 31 water molecules with a single OH2 after 2. Molecular model equilibration. In panel (a) the hydroxide ion is indeed 2 Stillinger, Lemberg, David and Weber *(.+(5- have made present as the 'H (H('!. comple# $hich in our several attempts at using dissociating water potentials in calculations is the most stable form before PT occurs as classical simulations. Recently% 'BamaJe et al. *(7- shown in panels (b) and (d). he four water molecules 10 designed a family of potentials for solvated proton (at the ends of the four yellow coloured bonds shown þ H (H('!n complexes in water called the OSSn potentials in panel (a)! are in a plane as predicted by Tuckerman wheren¼ '2, are fitted to ab initio 4!2 calculations. et al. [9]. Each of the four water molecules coordinated We use the OSS2 model of OjamaJe et al. for our with the oxygen atom of OH2 is on average tetrahedrally simulations of protons and hydroxide ions in water. coordinated; one to the 'H2 ion and the others to water 02 22 February 20 Although more accurate results could possibly be obtained molecules. One such $ater molecule is shown in panel (a). with the OSS3 potential, the OSS2 model also gi"es good o e#plain the mechanism, the four0coordinated 2 results judging by simulations of neat water. ?t is our oxygen atom of the OH ion is characterised as '@ and preferred choice because of faster and less elaborate the hydrogen atom as H0. The hydrogen atom of the 2 computer code implementation compared to the OSS3 (O@H0! ion can also be hydrogen bonded to a water 16427733] At: 00: model which contains three body dipole coupling terms. molecule. This ;"e coordinated structure (not shown) The OSS2 model [15] is a polarisable model in $hich e#ists only transiently in our simulations. hermal the induced dipole moment mi at each oxygen atom sitei is fluctuations cause a water molecule to be detached from 2 obtained self consistently from the four-coordinated ('@H0! ion comple# to form 02 *'@H (H('!,- which is accompanied by shrinking of - Superceded by 9 nOþnH nO the three hydrogen bonded '@Z' distances $hich rijqj cd Tijmj dd mi ¼ a , Sij ðrijÞ þ , Sij ðrijÞ ; " j–i rij j–i rij # X X by imposing the conditions d el !dr " ¼(("¼ ' 2,... n (), where el is the electrostatic ener#y n O andn H are the ent Distribution numbers of oxygen and hydrogen atoms, a is the polaris- cd dd ability of the atom andS ij ðrijÞ andS ij ðrijÞ are the cut-off functions for the charge2dipole and dipole2dipole inter- actions, respecti$ely. Ewald summations were used in our simulations with the parameter for k ¼ 5.(7# and the real- BSCOHost EJS Cont space cut distancer cut and$ max chosen as 0.6# and 3 respecti$ely where# is the length of the box (,9.83 9: for 32 water molecules correspondin# to a density of 0.118 # cm2,). "he double summations in reciprocal space, which cannot be reduced to a single summation due to the Downloaded By: [E cut-off functions, were i#nored. This is reasonable as the distances in reciprocal space are larger than the length# of the box. "o maintain electroneutrality a fictitious charge of opposite sign to the gi$en OH2 or Hþ ion was placed at a corner of the simulation box. "his charge ne$er mo$es and only interacts with other charges through a Coulomb Figure 1. Representative configurations of OH2 ion showing potential with 5wald summation. The time step was chosen the mechanism of PT leading to the transport of the hydroxide ion 2 as 0.2 fs. "he simulations were first $alidated by checking in a simulation of one OH ion in 31 water molecules using the 02 'SS2 potential. (a! '@H (H('!. comple# sho$ing four our results against Ojama=e’s work for pure water usin# the 2 0 coordination of '@. (b) OH (H('!, intermediate with H same OSS2 model. The calculated oxygen2hydrogen ^ between '@ and ' showing three coordination of '@. (c) Return (OZH) radial distribution function and the hydration to (a) from three-coordinated intermediate. (d) Completion of P 2 numbern(r) for hydrogen in the 21. molecule pure water via OH (H('!, intermediate (see the text).