Experiments Help Explain Mysterious 'Floppy' Space Molecule (Forwarded)

National Institute of Standards and Technology

CONTACT:
Laura Ost, (301) 975-4034

FOR IMMEDIATE RELEASE: Jan. 5, 2006

Experiments Help Explain Mysterious 'Floppy' Space Molecule

Boulder, Colo. -- A laboratory method developed for making and analyzing
cold, concentrated samples of a mysterious "floppy" molecule thought to
be abundant only in outer space has revealed new data that help explain
the molecule's properties.

The advance, described in the Jan. 6 issue of Science,[*] is a step
toward overcoming a decades-old challenge in chemistry -- explaining
reactions that occur within very cold clouds among the stars, and
perhaps for developing new chemical processes. The paper combines
experiments performed by David Nesbitt and colleagues at JILA, a joint
institute of the Commerce Department's National Institute of Standards
and Technology (NIST) and University of Colorado at Boulder, with
theoretical predictions made with Joel Bowman at Emory University in
Atlanta, Ga., and Anne McCoy at The Ohio State University in Columbus, Ohio.

Most molecules have a rigid three-dimensional (3D) structure. The
subject of the new study is "protonated" methane, which contains one
carbon atom and five hydrogen atoms, one of which is ionized, leaving
nothing but a proton (a particle with a positive charge). The five
protons from the hydrogen atoms scramble for four bonds around the
molecule as if playing a continuous game of musical chairs. In the
process, the molecule classically vibrates and rotates in a bizarre
manner, morphing between several 3D structures with nearly identical
energy levels. (Animation available at
http://www.nist.gov/public_affairs/images/floppy_animation.htm.)
Chemists have spent decades trying to explain why and how this occurs, a
challenge that has seemed insurmountable until recently.

Protonated methane is a so-called "super acid." This class of molecule
has been shown to be more than a million times more powerful than
conventional acids and is more effective in inducing reactions that
produce solvents and many other important industrial products.

Many theories have been published on the puzzling behavior of this
charged molecule (or ion), but experiments must be done to match the
ion's energy characteristics with its physical motions, and such data
are difficult to collect and understand. In particular, scientists are
interested in how the molecule absorbs different wavelengths of infrared
(IR) light, which provides clues about nuclear motion and chemical bonds
and structures.

The JILA method generates concentrated amounts of the ion at cold enough
temperatures to simplify the complex IR spectrum so it can be analyzed.
The data strike a balance between detail and simplicity, providing
useful information that is still challenging but easier to understand
than ever before. This enabled the authors of the Science paper to match
predicted changes in energy to specific vibrations and partially
characterize the ion's structure and dynamics. For example, they were
able to correlate one intense spectral feature to a transition between
two 3D structures with equivalent energy levels.

Previously published spectra of this molecule have either been too low
resolution to "see" this motion, or too hot (and therefore too complex)
to analyze.

"The experiments have provided the first jet-cooled, high-resolution
spectrum of this highly fluxional molecule," says Nesbitt, a NIST Fellow
who led the JILA experimental team. "This has been among the most
sought-after IR spectra since the first appearance of this molecule in
mass spectrometers over 50 years ago. This is a problem that has
occupied many careers; every piece helps."

The JILA method involves making methane gas at high temperature and
pressure, and expanding it into a vacuum to cool the molecules to 10 K
(-442 degrees F). The cold molecules then file through an opening just 1
millimeter wide, where they are hit with a "lightning bolt" of
electrical current that generates high concentrations of highly reactive
ions. The key to mass production is to surround the molecules with
enough electrons to make the entire gas mixture neutral in charge,
Nesbitt says.

For the analysis step, JILA scientists shine an infrared laser on the
cold ions, and detect the light that passes through. The light that is
lost, or the small amount absorbed by the molecules, is analyzed to
obtain a pattern of absorption at different wavelengths. The technique
is very sensitive, thanks to methods for detecting trace absorption of
the laser light and manipulating the electrical discharge to maximize
the ion concentration levels.

Future and ongoing studies will focus on matching the ion's IR
absorption characteristics with its rotational structure, including
end-over-end tumbling. "Protonated methane still has a few tricks up its
sleeve," Nesbitt cautions.

The research was supported in part by the National Science Foundation,
Office of Naval Research, and Air Force Office of Scientific Research.

As a non-regulatory agency of the Commerce Department's Technology
Administration, NIST promotes U.S. innovation and industrial
competitiveness by advancing measurement science, standards and
technology in ways that enhance economic security and improve our
quality of life.
[*] X. Huang, A.B. McCoy, J.M. Bowman, L.M. Johnson, C. Savage, F. Dong,
and D.J. Nesbitt. 2005. "Quantum deconstruction of the infrared spectrum
of CH5+". Science. Jan. 6.

IMAGE CAPTION:
[http://www.nist.gov/public_affairs/i...sbittGrpHR.jpg
(3.8MB)]
A research team at JILA has developed a new method for making and
analyzing an unusual floppy molecule. Shown above with their
experimental apparatus are (from left) Chandra Savage, Erin Whitney,
Feng Dong, and David Nesbitt.

Photo by Jeff Fal, University of Colorado, Boulder

*****

Research Communications
Ohio State University

Contact:
Anne B. McCoy, (614) 292-9694; mccoy.154 @ osu.edu

Written by:
Pam Frost Gorder, (614) 292-9475

Embargoed until 2:00 p.m. ET, Thursday, January 5, 2006, to coincide
with publication in the journal Science.

CHEMISTS CALCULATE STRUCTURE OF PUZZLING "SCRAMBLER" MOLECULE

COLUMBUS, Ohio -- Chemists have calculated the structure of a very
unusual molecule, one whose hyperactive atoms have earned it the
nickname "the scrambler."

This highly caustic "protonated methane," or CH5+, is also called a
"super acid," and it is a short-lived player in the chemical reactions
that make petroleum products.

CH5+ should also be present in interstellar clouds where stars and
planets form, said Anne B. McCoy, professor of chemistry at Ohio State
University. McCoy hopes that the work she and her team are publishing in
the current issue of the journal Science will one day give astronomers
the tools they need to determine once and for all whether the molecule
is really out there in space.

To identify chemicals on earth and in outer space, scientists record the
spectrum of light absorbed by a molecule. Each molecule ever identified
has its own unique spectrum, resembling lines in a bar code.

Since the 1960s, when petrochemical experiments suggested the existence
of CH5+, scientists have been trying to record a complete spectrum of
it, but the molecule won't sit still. Scientists who tried to image CH5+
have found that it's like a three-year-old child -- impossible to
photograph, except in a blur.

"CH5+ has five hydrogen atoms scrambling around a carbon atom that sits
at the center," McCoy explained. The hydrogen atoms are simultaneously
rotating and vibrating.

Because the atoms are always on the move, scientists have difficulty
interpreting the spectrum. Still, they have recorded several CH5+
spectra experimentally.

Study coauthors David Nesbitt, Chandra Savage, and Feng Dong of JILA, a
joint research institute of the University of Colorado at Boulder and
the National Institute of Standards and Technology, report the most
recent and best resolved of these spectra to date in the Science paper.
But in spite of this progress, researchers have not been able to match
the lines in the CH5+ bar code to any specific motions of the molecule.

That's what McCoy and Professor Joel M. Bowman of Emory University did
mathematically. For certain features on the spectrum, they calculated
what the motions must be.

The result is most complete vibrational spectra ever calculated -- a
theoretical picture of the molecule's structure.

The chemists' employed a unique strategy in their calculations.

"Although the hydrogen atoms are constantly scrambling, the overall
range of types of structures can be characterized by three basic
configurations," McCoy said. One configuration corresponds to a low
energy state for the molecule, and the other two to higher energy
states. McCoy, Bowman, Ohio State graduate student Lindsay M. Johnson
and Emory postdoctoral researcher Xinchuan Huang calculated spectra for
all three structures.

That in itself was standard procedure, she said -- but then they went on
to examine the probability that the molecule would assume each of those
three structures, and used that information to weight their calculations.

"It turns out that this was the crucial step," McCoy said.

She acknowledged that her team hasn't yet assembled a full picture of
CH5+, since their calculations accounted for the vibration of the
molecule but not its constant rotation. That will be their next step. If
successful, they'll have a complete theoretical view of what the
molecule's spectrum should look like.

"The ultimate goal of this work is to identify a kind of signature for
CH5+," McCoy said. "Once we have it, we can compare it to what is
observed from astronomical measurements to determine its abundance in
different regions of space."

"From a more fundamental perspective, one thing that intrigues me is how
we can characterize molecules like CH5+ that have no single well-defined
structure and how this lack of a well-defined structure impacts its
reactivity," she continued.

She and her coauthors have started calculating what would happen when
the hydrogen atoms in CH5+ are replaced with deuterium, also known as
"heavy hydrogen." They suspect that adding one or two heavy hydrogen
atoms will stabilize the remaining hydrogen atoms and settle "the
scrambler" down once and for all.