Water can come in many different forms, whether it be tap water, bottled water, seawater or sparkling water, but at the molecular level, there are two types of water based on the spin directions of the two hydrogen atoms in H2O. When the nuclear spins are going in the same direction, this is referred to as ortho-water, and when they are in opposite directions, this is called para-water. A team led by researchers from the Institute of Industrial Science at the University of Tokyo recently conducted a study using spectroscopic techniques to examine the properties of a single water molecule and gain more information about this quantum phenomenon.
In the experiment, a single water molecule was placed within a C60 using a “molecular surgery” technique in order to open the carbon structure, inject the H2O and then close the cage again. The resulting H2O@C60 was then mounted between two gold electrodes as a single molecule transistor (SMT) to closely study the electron transport through the isolated molecule.
A Coulomb stability diagram was generated for the H2O@C60 SMT, which showed multiple tunneling-induced excited states for the water molecule, while an empty C60 cage only showed two excited states. Comparing the tunneling spectra of the H2O@C60 SMT to theoretical calculations, the researchers could attribute the conductance peaks measured to rotational and vibrational excitations of the water molecule.
Additionally, the quantum rotational excitations of both para- and ortho-water were observed simultaneously, which indicates that the single water molecule transitioned between these two states within the timeframe of the experiment, or approximately one minute. Terahertz spectroscopy experiments performed on the SMT showed results in agreement with the tunneling spectroscopy data. This research was published in Nano Letters.
“Our findings make an important contribution to the understanding of ortho-para fluctuation in water molecules,” said corresponding author Kazuhiko Hirakawa. “Because water plays such an important role in chemistry and biology, and even in understanding our universe, we expect our findings to have a wide-ranging impact.”
Photo: A collaborative team led by researchers from Institute of Industrial Science, The University of Tokyo used a single water molecule in a C60 cage to probe quantum mechanics.