27. K. K. Rao, M. Gravelle, J. S. Valente, F. Figueras, J. Catal. 173, 115 (1998). 28. V. Serr-Holm et al., Appl. Catal. A 198, 207 (2000). 29. B. M. Choudary et al., J. Mol. Catal. A 142, 361 (1999). 30. Supported by the U.S. Department of Energy Office of Basic Energy Sciences, NSF Chemical and Transport Systems Division of the Directorate for Engineering, and Conoco-Phillips. We thank M. Mavrikakis and R. Cortright for ongoing discussions, Y.-Y. Luk for help with aldol-condensation reactions, and E. L. Sughrue (Conoco-Phillips) and D. E. Resasco (University of Oklahoma) for helpful discussions about diesel fuel. Supporting Online Material www.sciencemag.org/cgi/content/full/308/5727/1446/ DC1 Materials and Methods Fig. S1 Tables S1 to S4 References 16 February 2005; accepted 15 April 2005 10.1126/science.1111166 Kinetic Evidence for Five-Coordination in AlOH(aq) 2þ Ion Thomas W. Swaddle, 1 Jo ¨ rgen Rosenqvist, 2 Ping Yu, 3 Eric Bylaska, 6 Brian L. Phillips, 7 William H. Casey 2,4,5 * Trivalent aluminum ions are important in natural bodies of water, but the structure of their coordination shell is a complex unsolved problem. In strong acid (pH G 3.0), Al III exists almost entirely as the octahedral Al(H 2 O) 6 3þ ion, whereas in basic conditions (pH 9 7), a tetrahedral Al(OH) 4 – structure prevails. In the biochemically and geochemically critical pH range of 4.3 to 7.0, the ion structures are less clear. Other hydrolytic species, such as AlOH(aq) 2þ , exist and are traditionally assumed to be hexacoordinate. We show, however, that the kinetics of proton and water exchange on aqueous Al III , coupled with Car- Parrinello simulations, support a five-coordinate Al(H 2 O) 4 OH 2þ ion as the predominant form of AlOH(aq) 2þ under ambient conditions. This result contrasts Al III with other trivalent metal aqua ions, for which there is no evidence for stable pentacoordinate hydrolysis products. Aluminum is the third most abundant ele- ment in Earth_s crust, after oxygen and sil- icon, and its chemistry in water is central to geochemistry, environmental science, and med- icine (1, 2). In particular, the speciation (3) and ligand substitution kinetics (4) of the Al III ions in the pH range 3 to 7 govern its toxicity toward plants, fish, and humans, yet the hydrolytic chemistry of Al III remains poorly understood. The structures of the oc- tahedral ion Al(H 2 O) 6 3þ , which dominates at pH G 3.0, and the tetrahedral aluminate ion Al(OH) 4 – , which dominates at pH 9 7, are well established (5). The first hydrolysis product, Al(H 2 O) n–1 OH 2þ , where n is the coor- dination number, becomes important at 3.0 G pH G 4.3. It coexists with Al(H 2 O) n–2 (OH) 2 þ above pH 4.3 and with Al(OH) 4 – at 5.2 G pH G 6.7 (3, 5–7). At high Al III concen- trations (9 0.05 mol L –1 ), oligomers such as (H 2 O) 4 Al(OH) 2 Al(OH 2 ) 4 4þ and the Keggin ion AlO 4 (Al(OH) 2 ) 12 (H 2 O) 12 7þ (which contains one four-coordinate and twelve six-coordinate Al III atoms) appear around pH 5 (5, 6, 8, 9). Thus, not only is the speciation of Al III (aq) complicated in the pH range 4.3 to 7.0, rendering quantitative studies difficult, but there is a shift from dominant six- to four- coordination over this range. It is generally assumed (5, 6) by analogy with several other trivalent metals but without experimental justification that octahedral coor- dination is retained in Al(H 2 O) n–1 OH 2þ and most other hydrolytic species. Martin (3, 7) pointed out that the remarkable closeness of the acid dissociation constants K a of Al(H 2 O) 6 3þ , Al(H 2 O) n–1 OH 2þ , Al(H 2 O) n–2 (OH) 2 þ , and Al(H 2 O) n–3 (OH) 3 (pK a 0 –log K a 0 5.5, 5.8, 6.0, and 6.2, respectively, in dilute solution) could be explained by a progressive reduction of coordination number n from 6 toward 4 across this sequence: Decreasing n shortens Al-O bond lengths, increasing the polarization and hence the acidity of the remaining aqua ligands. We report high-pressure 17 O-nuclear mag- netic resonance (NMR) data that support Martin_s basic hypothesis. We have studied the hydrogen-ion dependence of the rate of exchange of water ligands bound to the Al(H 2 O) 6 3þ with free solvent, and our results are consistent with coupling of proton and water dissociation from Al(H 2 O) 6 3þ via a five- coordinate Al(H 2 O) 4 OH 2þ ion. AlðH 2 OÞ 6 3þ ± AlðH 2 OÞ 4 OH 2þ þ H þ þ H 2 O ð1Þ The key data are volumes of activation DV M - E –RT(¯ ln k M /¯P) T , where k M is the corresponding rate constant for water exchange on aqueous metal ions M(aq ) zþ ^, because they are considered to be diagnostic of the reaction mechanism (10). These DV M - parameters are extracted from the pressure dependence of the measured rate constants. Formation of an intermediate in which the coordination number n is reduced is termed a dissociative (D) mech- anism, for which 0 ¡ DV M - e þ14 cm 3 mol –1 , whereas formation of an intermediate of expanded n is called an associative (A) mechanism and shows 0 d DV M - Q –14 cm 3 mol –1 (11). Cases in which the entry and departure of water molecules are coupled (no long-lived intermediates) are called inter- change mechanisms: dissociative interchange (I d ) for which, operationally, DV M - is positive, and associative interchange (I a ), for which it is negative. For the acid-independent water- exchange pathway on Al(H 2 O) 6 3þ (i.e., for the direct exchange of H 2 O with Al(H 2 O) 6 3þ ), DV Al - is þ5.7 cm 3 mol –1 (12), indicating a dissociatively activated mechanism, in agree- ment with the results of ab initio calculations (13). The DV - value and mechanism for the first hydrolyzed complex, AlOH(aq) 2þ , is re- ported here. The standard model (10) for water exchange via conjugate-base species EMOH(aq) (z–1)þ ^ holds that proton exchange is much more rapid than oxygen exchange, and that a preequi- librium state is established with retention of the first coordination sphere, characterized by an equilibrium constant, K a , and an equilibrium pressure dependence described by a volume of reaction, DV a 0 0 –RT(¯ ln K a /¯P) T : MðH 2 OÞ zþ n ± Ka ; DV 0 a MðH 2 OÞ nj1 OH ðzj1Þþ þ H þ ð2Þ The rate-determining step is then water exchange on the M(H 2 O) n–1 OH (z–1)þ ion (rate constant k MOH ). Experiments support this mechanism for a wide range of metal ions (5). From 17 O-NMR data in aqueous AlCl 3 , we obtained rate constants k obs for water exchange on aluminum species (Fig. 1A), and they vary inversely with EH þ ^, as expected Ee.g., (14)^. k obs 0 ðk 1 þ k 2 =EH þ ^Þ X 1 ð3Þ X 1 is the mole fraction of Al III present as Al(H 2 O) 6 3þ (the calculated pressure depen- 1 Department of Chemistry, University of Calgary, Calgary, AB T2N 1N4, Canada. 2 Department of Land, Air, and Water Resources, 3 Nuclear Magnetic Res- onance Facility, 4 Department of Geology, 5 Depart- ment of Chemistry, University of California, Davis, CA 95616, USA. 6 Fundamental Sciences, Pacific North- west Laboratory, P.O. Box 999, Richland, WA 99352, USA. 7 Department of Geosciences, State University of New York, Stony Brook, NY 11794, USA. *To whom correspondence should be addressed. E-mail: whcasey@ucdavis.edu 3 JUNE 2005 VOL 308 SCIENCE www.sciencemag.org 1450 R EPORTS