MIT chemists uncovered one of the stubborn mysteries of water by seeing for the first time how a hydrogen bond vibrates right before it breaks apart. The work was reported in the Sept. 19 issue of Science.
To better understand how the bonds that weakly link adjacent hydrogen and oxygen molecules rearrange themselves, MIT chemists developed a model and conducted experiments using infrared spectroscopy, which measures the absorption and emission of different wavelengths of nonvisible light.
"Even though we encounter water everywhere and, as the fundamental solvent of biology and chemistry, it is the most studied liquid, there are many properties of water that are not understood," said Andrei Tokmakoff, associate professor of chemistry. "The rearrangement of hydrogen bonds in water is at the basis of fundamental processes such as the ability of molecules to dissolve in water, the motion of protons and other charges in water, and biological processes such as protein folding."
Hydrogen's positive charge pulls it near the negative charge of a nearby oxygen atom. Biochemists call this faint force between hydrogen and oxygen a hydrogen bond.
To see this process in action, the researchers used a sequence of extremely fast pulses of infrared light to follow the changes in hydrogen bonding structure in a unique type of water made from deuterium (a heavy form of hydrogen) and oxygen, instead of ordinary hydrogen and oxygen. Into this deuterated water, they placed a small number of hydrogen-oxygen-deuterium (HOD) water molecules.
The pulses excite the vibrational motion of the oxygen-hydrogen bond of the HOD molecule. The frequency of this motion is very sensitive to the hydrogen bonding structure that the hydrogen-oxygen bond makes to other deuterated water molecules. Initial pulses excite the oxygen-hydrogen vibration of water molecules at one frequency, and further pulses allow the scientists to observe how the frequency changes over time.
It turns out that the frequency shifts "were dictated by the geometrical configuration of positive and negative charges on the water molecule that accepted the hydrogen bond from the hydrogen-oxygen-deuterium molecule," Tokmakoff said.
"The experiments and model, put together, reveal a very uncharacteristic motion for liquids: for pairs of hydrogen-bonded molecules, the hydrogen bond actually vibrates periodically with a 170-femtosecond period [a femtosecond is 10-15 second] prior to breaking in roughly 1 picosecond. Our ongoing experiments, which build on these results, aim to separately watch the many-body mechanisms by which hydrogen bonds break and also form," he said.
In addition to Tokmakoff, authors include chemistry graduate students Christopher J. Fecko, Joel D. Eaves and Joseph J. Loparo, and Phillip L. Geissler, a former MIT Science Fellow.
This work is supported by the U.S. Department of Energy and the George R. Harrison Spectroscopy Laboratory at MIT.
A version of this article appeared in MIT Tech Talk on September 24, 2003.