Probing the Local Structure of Liquid Water by X-ray Absorption Spectroscopy² Jared D. Smith, ‡,§ Christopher D. Cappa, ‡,§ Benjamin M. Messer, ‡,§ Walter S. Drisdell, ‡,§ Ronald C. Cohen, ‡ and Richard J. Saykally* ,‡,§ Department of Chemistry, UniVersity of California, Berkeley, California 94721, and Chemical Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94721 ReceiVed: June 12, 2006; In Final Form: July 14, 2006 It was recently suggested that liquid water primarily comprises hydrogen-bonded rings and chains, as opposed to the traditionally accepted locally tetrahedral structure (Wernet et al. Science 2004, 304, 995). This controversial conclusion was primarily based on comparison between experimental and calculated X-ray absorption spectra (XAS) using computer-generated ice-like 11-molecule clusters. Here we present calculations which conclusively show that when hydrogen-bonding configurations are chosen randomly, the calculated XAS does not reproduce the experimental XAS regardless of the bonding model employed (i.e., rings and chains vs tetrahedral). Furthermore, we also present an analysis of a recently introduced asymmetric water potential (Soper, A. K. J. Phys.: Condens. Matter 2005, 17, S3273), which is representative of the rings and chains structure, and make comparisons with the standard SPC/E potential, which represents the locally tetrahedral structure. We find that the calculated XAS from both potentials is inconsistent with the experimental XAS. However, we also show the calculated electric field distribution from the rings and chains structure is strongly bimodal and highly inconsistent with the experimental Raman spectrum, thus casting serious doubt on the validity of the rings and chains model for liquid water. 1. Introduction The unusual properties of liquid water are thought to result from its unique hydrogen-bonding (HB) network, traditionally viewed as being locally tetrahedral, wherein each water molecule forms ∼4 H bonds (two acceptors and two donors) with nearest neighbors. The HB structure of liquid water was first described as locally tetrahedral in 1933 by Bernal and Fowler, 1 who inferred this type of structure on the basis of water’s X-ray diffraction pattern. Since then, numerous other experimental and theoretical methods have supported this type of structure. 2,3 However, this time-honored picture has recently been challenged in favor of a structure in which most molecules (∼80%) form only two H bonds (one acceptor and one donor). In this purposed structure, water molecule would be linked in somewhat of an end-to-end pattern (similar to HF or methanol) and thus strongly favor the formation of hydrogen-bonded rings and chains. 4 It is currently unclear whether such a “ring and chain” model can account for any of water’s unusual thermodynamic properties. For instance, the well-known temperature of maximum density is thought to be a consequence of the open nature of its quasi- tetrahedral network structure. In fact, a density maximum appears to be a general characteristic of tetrahedrally coordi- nated liquids 5 and has not been found in liquids known to form only two H bonds per molecule (e.g., HF or methanol). Furthermore, the coordination number for water, derived from the area under the first peak in the O-O radial distribution function (RDF), is ∼ 4.4, 3 compared with values closer to 2 for HF and methanol. 6 A coordination number near 4 is thought to signify that the local tetrahedral structure found in solid ice, which also has a coordination number near 4, is to some extent preserved in the liquid. The position and temperature dependence of the second peak in the O-O RDF is also thought to evidence a local tetrahedral structure. 7,8 However, a recent analysis of X-ray and neutron difraction data by Soper 9 indicates that the diffraction data may indeed not distinguish between symmetric (tetrahedral) and asymmetric (rings and chains) bonding de- scription of water, whereas, a more recent analysis by Head- Gordon et al. 8 indicates that the X-ray scattering data can indeed distinguish between them and are inconsistent with an asym- metric bonding model. Strong theoretical support for a local tetrahedral structure also comes from simulations using both empirical potentials, fit to diffraction data and thermodynamic properties, as well as ab initio simulations. The underlying physics governing these computational methods is quite differ- ent, but they both predict hydrogen bonding in a similar locally tetrahedral structure. 10 Evidence in favor of a “rings and chains” structure has been claimed from theoretical and experimental X-ray absorption and X-ray Raman spectroscopic studies of the oxygen K-edge of liquid water. 4,11,12 In recent years, X-ray absorption spectroscopy (XAS) has emerged as a new tool for the investigation of the local structure of water and aqueous solutions 4,11-20 . XAS is an atom-specific probe in which a core electron is excited to an unoccupied electronic state. The electronic character of these unoccupied states is very sensitive to the local geometric structure, and therefore XAS can serve as an indirect structural probe. It is clear from both experimental and theoretical evidence that the pre-edge region (∼535 eV) of the XAS is a signature of distorted HB configurations, whereas the post-edge region (∼541 eV) is a signature of stronger “ice-like” configurations. 4,12 However, establishing quantitatively how sensitive the XA spectrum is to HB rearrangements, and the underlying structural ² Part of the special issue “Charles B. Harris Festschrift”. * Corresponding author: e-mail saykally@berkeley.edu. ‡ Department of Chemistry, University of California, Berkeley, California, 94721. § Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94721. 20038 J. Phys. Chem. B 2006, 110, 20038-20045 10.1021/jp063661c CCC: $33.50 © 2006 American Chemical Society Published on Web 08/19/2006