X-ray Absorption Spectroscopy of Liquid Methanol Microjets: Bulk Electronic Structure and Hydrogen Bonding Network Kevin R. Wilson, ²,‡ Matteo Cavalleri, § Bruce S. Rude, | R. D. Schaller, ‡ T. Catalano, | A. Nilsson, §,⊥ R. J. Saykally,* ,‡ and L. G. M. Pettersson* ,§ Department of Chemistry, UniVersity of California, Berkeley, California 94720, FYSIKUM, Stockholm UniVersity, AlbaNoVa UniVersity Center, S-10691 Stockholm, Sweden, AdVanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Stanford Synchrotron Radiation Laboratory, Menlo Park, California 94025 ReceiVed: February 17, 2004; In Final Form: September 22, 2004 We have measured the X-ray absorption (XA) spectrum of liquid (298 K) methanol at the oxygen and carbon K edges. The 4a 1 orbital at the O K edge exhibits a pronounced sensitivity to the formation of intermolecular hydrogen bonds, with significant differences observed between the vapor and bulk spectra, whereas the C K edge reveals only subtle corresponding spectral changes. Comparison with DFT computed spectra of model methanol clusters indicates that the bulk liquid comprises long chains (n > 6) and rings of hydrogen-bonded monomers. I. Introduction Our knowledge of the electronic and geometrical structure of liquids has advanced dramatically over the past decade due to rapid progress in both theory and experiment. However, the microscopic structure of hydrogen-bonded liquids remains a vigorously debated subject. Classical molecular dynamics simulations have offered some new microscopic insights, but developing realistic force fields that properly account for many- body effects in the bulk liquid continues to be a challenge. 1 Ab initio molecular dynamics simulations eliminate the need for predetermined force-fields and the method has become well- established as a simulation tool. Wilson et al., have recently reported a new X-ray spectroscopy technique for the study of bulk liquids and liquid-vapor interfaces. 2-4 In the initial applications, we described the first measurement of intermo- lecular surface relaxation in liquids (water and methanol), 4 as well as the presence of a previously unrecognized type of “acceptor only” species at the liquid water surface. 3 Recently, these experimental findings have been corroborated by new ab initio molecular dynamics (CPMD) simulations reported by Kuo and Mundy, 5 who find that 19% of surface molecules of 298 K water are “acceptor only”. Here we study the bulk properties of liquid methanol by X-ray absorption spectroscopy, using liquid microjets to introduce the volatile samples into the high vacuum environment of a synchrotron endstation and as an effective means of avoiding sample photodamage. Soft X-ray absorption spectroscopy (XAS) is a powerful structural tool commonly used to study the bonding of adsor- bates to metal surfaces through the ability to determine local geometry and bond lengths. 6 A main advantage of XAS is the ability to selectively excite individual atoms within a molecule and thereby examine specific chemical bonds (e.g., CdO, CsC, OsH, etc.). Traditionally, near-edge X-ray absorption fine structure (NEXAFS) experiments have characterized the rehybridization of covalent bonds, which can yield information about which orbitals are involved in the formation of a surface chemical bond. 7 Although NEXAFS is most sensitive to changes in intramolecular bond lengths and angles, there are a number of recent studies 3,4,8-15 examining the changes in electronic structure of liquid water, ice and H 2 O clusters upon the rearrangement of intermolecular hydrogen bonding (HB). Together these studies indicate that the O K edge XAS is particularly sensitive to broken donor hydrogen bonds located, e.g., within liquid water, 14,15 on the surface of ice, 15,16 or at the liquid-gas interface. 3 As expected, water in its various phases is a natural starting point to investigate the local changes in electronic structure induced by HB. In this paper, we extend these studies of intermolecular hydrogen bonds to methanol, which might be considered the next logical step in a detailed examination of the changes in electronic structure that occur upon condensation within the bulk hydrogen-bonded liquid. The general strategy outlined here is to first obtain high-resolution XA spectra of molecular (gas phase) methanol whereupon spectral changes induced by condensation can be evaluated. Subsequently, density functional theory calculations, utilizing small methanol clusters, are used to further interpret the experimental XA spectra, yielding new insights into the HB network of the bulk liquid. II. X-ray Absorption of Liquids XAS entails the removal of a core electron and its promotion into an unoccupied molecular orbital or continuum state. The excitation process obeys dipole selection rules yielding spectral intensity from transitions between the 1s (K shell) orbital to excited states of mainly p symmetry. The core hole is strongly localized, allowing individual atoms within molecules to be independently excited. For methanol, the carbon (∼289 eV) and oxygen K edge (∼530 eV) spectral intensity will be sensitive * Corresponding Authors. E-mail: R.J.S., saykally@uclink4.berkeley.edu; L.G.M.P., lgm@physto.se. ² Present address: Lawrence Berkeley National Laboratory, Chemical Sciences Division, Berkeley, CA 94720. ‡ University of California. § Stockholm University. | Lawrence Berkeley National Laboratory. ⊥ Stanford Synchrotron Radiation Laboratory. 10194 J. Phys. Chem. B 2005, 109, 10194-10203 10.1021/jp049278u CCC: $30.25 © 2005 American Chemical Society Published on Web 04/21/2005