A debate on the structure of liquid water
Water is ubiquitous and arguably one of the most important liquid to investigate. Despite the
apparent simplicity of water molecule, liquid water has intrigued scientists for many years due to
its unusually strong hydrogen bonds, which leads to many mysterious properties of water.
Recently, two research papers on the structure of liquid water were published in journal
Science [ Ph. Wernet et al., 304 995 (2004), J. D. Smith et al., 306 851 (2004)]. These two
works used an identical experimental technique [X-ray absorption spectroscopy] to figure out the
solvation shell structure of liquid water, but their conclusions do not agree with each other.
Wernet and coworkers concluded that each water molecule has only two strong hydrogen bonds
(one donor and one acceptor), which contradicts to the traditional view of liquid water structure.
On the other hand, Smith et al. defended the traditional view by arguing that the data analysis
reported by Wernet et al. is misleading.
Questions to be addressed in your presentation:
1. Review the solvation shell structure of liquid water. In your review, discuss the definition and
the use of radial distribution functions. What kinds of experimental techniques have been used
traditionally to understand the structure of liquid water?
[You may start your literature search from Chem. Rev. 102 2651 (2002)]
2. Briefly discuss the basic principles of X-ray absorption spectroscopy.
3. What are the evidences presented by Wernet et al. to convince people that water molecule has
only two hydrogen bonds?
4. What are the main arguments presented by Smith et al. to show that the interpretation of
Wernet et al. is misleading?
5. Wernet et al. reported that the relative population of single hydrogen-bond donor species (SD)
and double hydrogen-bond donor species (DD) has a smaller temperature dependence
(SD/DD=80/15 at 25oC and SD/DD=85/10 at 90oC). Note that SD species has one less
hydrogen bond than DD species. Assuming that the populations of SD and DD species in
liquid water are determined by Boltzmann distribution, compute the hydrogen bond energy (i.e.
the energy needed to break one hydrogen bond) in liquid water. Does your result agree with
that of Smith et al.? What is the implication of your finding?
Consultant: Dr. Hee-Seung Lee