Carbon dioxide molecules with different types of oxygen show up differently in Raman spectroscopy. Image: Pacific Northwest National Laboratory
Just as a wine glass vibrates and sometimes breaks when a
diva sings the right note, carbon dioxide vibrates when light or heat serenades
it. When it does, carbon dioxide exhibits a vibrational puzzle known as Fermi
resonance. Now, researchers studying geologic carbon storage have learned a bit
more about the nature of carbon dioxide.
The results provide clues to the nature of the Fermi
resonance in other molecules, and will help researchers better understand
details in chemical reactions. The team of researchers from the United States Department
of Energy’s Pacific Northwest National Laboratory (PNNL) report their findings
in Physical Chemistry Chemical Physics.
“We’re happy to be able to say something new about
something so old,” said PNNL chemist and author Charles Windisch, Jr.
“We figured out how the different carbon dioxide molecules are vibrating
at some of the Fermi resonance frequencies. And, of course, we can calibrate
our data with more accuracy now.”
“Even to this day, people mark Raman spectra
incorrectly,” said PNNL computational chemist Vassiliki-Alexandra
Glezakou. “It helps to know what we are looking at, if we are going to use
certain bands as guidelines to understand molecular interactions.”
Carbon dioxide conundrum
The PNNL researchers did not set out to study again a phenomenon that dates
back to the 1930s. Instead, they wanted to investigate what happens when carbon
dioxide is stored underground as part of a national research effort to reduce
carbon emissions from power generation. To do so, researchers plan to inject
carbon dioxide in an unusual form of the gas that behaves like a liquid due to
being under high pressure, called supercritical. To follow supercritical carbon
dioxide in chemical reactions, researchers often use a technique called Raman
Raman spectroscopy is a way of capturing a molecule’s
vibration. Simple molecules can vibrate in well-defined modes such as
stretching and bending, which correspond to peak frequencies on a graph. These
peaks are as unique and reproducible as a fingerprint.
The number and position of these peaks in a spectrum can
be predicted by quantum mechanics, but Fermi resonances result in unanticipated
peaks due to a combination of two different vibrations, such as stretching and
bending. First recognized in carbon dioxide and explained by Enrico Fermi in
1931, scientists agree that the Fermi peaks are the result of the mixing of the
two vibrational modes, but they often label one of them as ‘stretch’ and the other
as ‘bend’. This labeling became a problem when PNNL researchers observed a
‘flip’ in the Raman spectrum of supercritical carbon dioxide.
Shift or flip?
To follow reactions, researchers often use different versions of elements
called isotopes. Normally, carbon dioxide contains carbon plus the isotope
oxygen-16, the most common form of oxygen. By using a heavier isotope of oxygen
with its own fingerprint, oxygen-18, PNNL researchers can track the fate of
carbon dioxide when it reacts with minerals, particularly when there are other
sources of oxygen present such as water.
In the Raman spectra of the lighter supercritical carbon
dioxide, the pair of Fermi peaks included a weaker one at a lower frequency and
a stronger one at higher frequency. When they replaced all of the oxygens with
the heavier isotope, however, the peaks seemed to flip, with the stronger one
appearing at a lower frequency instead.
At first, it was not clear how the two sets of Fermi
peaks related to each other—whether the peaks were really a mirror image or if
the stronger oxygen-16 peak somehow morphed into a weaker peak when heavy
oxygen-18 was introduced. Typically, a heavier isotope will shift peaks to
lower frequencies, although different modes are not necessarily affected by the
The researchers needed to unambiguously identify the
peaks and to figure out how much bending and stretching modes contributed to
each one. To do so, the team decided to simulate the carbon dioxide molecules
with different oxygen isotopes on a computer and see if they could recreate the
Raman spectra they saw in their experiments.
To the computer
Using computing resources at EMSL, DOE’s Environmental Molecular Sciences
Laboratory at PNNL, Glezakou simulated carbon dioxide in supercritical conditions
similar to those in the experiment. The molecules were “made” with
either oxygen-16, oxygen-18.
They analyzed the motion of the molecules to produce
computational spectra that echoed the real spectra. In this way, the team was
able to determine the percent of bending and stretching modes expected in each
The results showed that with oxygen-16, the stronger peak
at the higher frequency is due mostly to the stretching mode, while the weaker
peak at the lower frequency is due mostly to the bending mode.
Oxygen-18, however, told a different story. The results
with heavy carbon dioxide showed unequivocally that the light- and heavy-oxygen
peaks were not exactly mirror images of each other. Carbon dioxide is mostly a
linear molecule, so the bending motion is much less affected than the stretch
when the oxygen-16 is replaced by its heavier isotope. As a result, the
composition of the peaks does not remain the same.
“The heavier oxygen doesn’t just shift the peaks. It
changes their identity,” said Glezakou. “And the bigger effects are
on the stretching, because the peak with the most stretching has the biggest
Windisch added that the experimental results matched the
computational ones nicely, in spite of the difficulty. “Our colleague Paul
Martin here at PNNL had to build equipment so we could do these experiments at
the pressures we needed. Not easy,” he said.
Having nailed down the vibrational pedigree of these
carbon dioxide molecules, they plan to use these results to understand better
other reactions between carbon dioxide and a variety of minerals.