Copper clusters: Terahertz tunes in
- Author: David Bradley
Cluster headache cured
Details about the structure and properties of a
copper-rich metal cluster complex can be extracted using terahertz absorption spectroscopy according to scientists in Japan.
Writing in the journal Inorganic Chemistry, Hal Suzuki and Chiko Otani of the RIKEN research centre in Sendai, and Nobuto Yoshinari and Takumi Konno of Osaka University, explain how metal cluster complexes are important molecules in nature and industry alike. They are the cogs with which photosynthesis turns, nutrients are transported and are the catalytic workhorses of industry. However, to understand their structure and properties in detail there is a need for detailed and easily accessible ways to image these complexes at close to the atomic scale. "Most structural studies of these substances have used X-ray diffraction, but this technique has a weakness - the target material must be a crystal," says Suzuki, who works in the RIKEN Center for Advanced Photonics. "Since terahertz spectroscopy does not require crystals, it is expected to become a powerful analytical tool."
Terahertz spectroscopy uses radiation of shorter wavelength than infrared but extracts information from a molecule in a similar way, picking up the characteristic vibrations of the compound. However, whereas infrared spectroscopy detects the high-frequency bond vibrations, terahertz spectroscopy listens for the low-frequency "soft" vibrations, aggregates of atoms, such as those of a metal cluster.
Polymorphic proof
In order to show proof of principle, the team investigated a recently discovered copper cluster complex containing 14 metal ions. This cluster is large and porous and its voids soak up water molecules like a sub-microscopic, organometallic sponge. The team prepared the complex, D-penicillaminato (D-pen) CuI8CuII6, in three polymorphic forms: 1D helical, 2D hexagonal sheet and discrete cubic, forms that would be relatively easy to investigate with X-ray crystallography too for comparison purposes, although the strength of terahertz spectroscopy once proven would indeed be in studying non-crystalline materials too.
"By slightly tweaking its synthesis conditions, this cluster complex can be aggregated into different crystal structures," Suzuki explains. "It can thus be used as a model substance to reveal the general rules behind aggregation of cluster complexes."
Boiling complex
The team heated the clusters to 100 degrees Celsius to boil off the adsorbed water molecules and used terahertz spectroscopy to monitor the change sin structure as each aggregate lost its water molecules. The team explains that in this way they could show how disorder arises as evaporation proceeds. The helical cluster collapsed as water was lost, although the clusters in the other two polymorphic forms retained their structure.
"There is still more that terahertz spectroscopy, in combination with other techniques, can tell us about these complexes," Suzuki adds. "The next step is to reveal the role of water molecules in building up the cluster complexes as well as the crystalline forms." More specifically, he told SpectroscopyNOW, "I would like to reveal the role of loosely bound molecules in buliding up complicated structres."
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