A molecule that won’t sit still
and World Science staff
Chemists say that after long efforts, they have managed to determine the structure of a molecule that won’t sit still.
The molecule’s hyperactive atoms have earned it the nickname “the scrambler,” and long kept chemists from finding out its shape, the researchers explained.
Called protonated methane, or CH5+, the substance is highly caustic and is also called a “super acid,” the researchers said. It’s a short-lived player in chemical reactions that make petroleum products.
CH5+ should also be present in interstellar clouds where stars and planets form, said Anne B. McCoy of Ohio State University in Columbus, Ohio. She said she hopes her new findings, published in the current issue of the research journal Science will one day help astronomers determine whether the molecule is out there in space.
To identify chemicals on earth and in outer space, scientists record the precise colors of light the molecule absorbs. This sequence is called its spectrum. Every known has its own unique spectrum, which can be represented like lines in a bar code.
Since the 1960s, when experiments suggested the existence of CH5+, scientists have been trying to record a complete spectrum of it, but the molecule won’t sit still, McCoy said. 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 simultaneously rotate and vibrate.
Because the atoms are always on the move, scientists have difficulty interpreting the spectrum. Still, they have recorded several CH5+ spectra experimentally.
Several colleagues of McCoy reported what they called the most precise of these spectra to date in the Science paper. Despite this, researchers have not been able to match the lines in the CH5+ bar code to any specific motions of the molecule.
So McCoy and Professor Joel M. Bowman of Emory University in Atlanta, Ga., did this mathematically. For certain features on the spectrum, they calculated what the motions must be. The result, they said, is most complete vibrational spectrum 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 and colleagues 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 said, they’ll have a complete theoretical view of what the molecule’s spectrum should look like.
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