Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Richard J. Saykally; Department of Chemistry, University of California and
Chemical Sciences Division, Lawrence Berkeley National Laboratory
Berkeley, CA 94720-1460
saykally@berkeley.edu http://www.cchem.berkeley.edu/rjsgrp/index.html
Project I. Soft X-ray Spectroscopy of Liquids and Solutions
Program Scope or Definition
The goal of this project is to explore, develop, and apply novel methodologies for
atom-specific characterization of volatile liquids, solutions, and and their surfaces,
employing combinations of liquid microjet technology with synchrotron X-ray and Raman
spectroscopies and close connection with state-of-the-art theory.
Recent Progress
Towards a Complete Understanding of CO 2 – Carbonate Equilibria [1]
The dissolution of carbon dioxide in water and the ensuing hydrolysis reactions are of
profound importance for understanding the behavior and control of carbon in the terrestrial
environment. Using liquid microjet technology, the first X-ray absorption spectra of aqueous
carbonate have been measured as a function of pH to characterize the evolution of electronic
structure of carbonate, bicarbonate, carbonic acid and dissolved CO2. The corresponding carbon
K-edge core-level spectra were calculated using a first-principles electronic structure approach
which samples molecular dynamics trajectories. Measured and calculated spectra are in excel-
lent agreement. Each species exhibits similar, but distinct, spectral features which are interpreted
in terms of the relative C–O bond strengths, molecular configuration, and hydration strength.
This work provides benchmarks for future studies of this system.
Electronic Structure of Aqueous Borohydride: A Potential Hydrogen Storage Medium [2,3]
With large weight percent hydrogen capacity, stable reactants, and benign reaction
products, borohydride salts have been considered as good prospects for transportable hydrogen
storage materials, with molecular hydrogen released via hydrolysis. We examine details of the
hydration of sodium borohydride by the combination of X-ray absorption spectroscopy of liquid
microjets and first principles’ theory. Compared to solid sodium borohydride, the aqueous
sample exhibits an uncharacteristically narrow absorption feature that is shifted to lower energy,
and ascribed to the formation of dihydrogen bonds between borohydride and water that weaken
the boron–hydrogen covalent bonds. Water also acts to localize the highly excited molecular
orbitals of borohydride, causing transitions to excited states with p character to become more
intense and a sharp feature, uncharacteristic of tetrahedral molecules, to emerge. The simulations
indicate that water preferentially associates with borohydride on the tetrahedral corners and
edges.
Towards a Predictive Theory of Core-Level Molecular Spectra [4,5, 11-12]
The development of near edge X-ray absorption fine structure spectroscopy (NEXAFS)
of liquid microjets has provided a useful new tool for characterizing the details of solvation for
increasingly complex systems. NEXAFS probes the unoccupied molecular orbitals, which are
highly sensitive to intermolecular interactions. This new approach to the study of liquids is
yielding important insights into the behavior of aqueous systems, but the chemical information
that can be extracted from the measurements is currently limited by the reliability of available
theoretical methods for computing core-level spectra. The accurate description of an absorption
event of several hundreds of eV of energy is an ongoing challenge in theoretical chemistry. In
collaboration with LBL scientist David Prendergast, we are using the excited core hole (XCH)
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