Chemistry is often seen as the most magical science. You mix things together and suddenly a completely different substance appears. The aura of mystery has been helped along by the idea that it is impossible to directly observe the exact moment that reagents turn into products. That is, until now.
For the first time, scientists were able to observe these fleeting forms when the actual reaction takes place. The researchers from the Massachusetts Institute of Technology (MIT) measured the energy of the transition state, a feat which was supposed to be impossible due to the complexity of chemical reactions.
The transition state is the initial phase of a chemical reaction between the reactants and products. "Your reactants and products are stable valleys on either side of a mountain range, and the transition state is the pass," said Josh Baraban, the paper's lead author, in a statement. "It's the most convenient way to get from one to the other. Because it only exists as you go from as one thing to another, it's never really been thought of as something that you can easily study directly."
The team studied a chemical process called isomerization. In this reaction, one molecule is transformed into another molecule that has the same atoms but they are arranged in a different way. The researchers looked at acetylene, a molecule formed by two carbon atoms bound to each other, and each bound to a hydrogen atom.
In this work, published in Science, the team observed the molecules convert from a U-shaped configuration (with both hydrogens above the carbon atoms) to a zigzag shape (with one hydrogen above and one below the carbons). This is because atoms within molecules vibrate – the more energy given to a molecule, the faster it vibrates.
The researchers measured the vibration states in the molecule of acetylene and noticed a regular pattern as they provided higher energy. The predicted patterns were then suddenly seen to break as the molecule reached a certain energy level; beyond that energy level, the molecule showed vibrational energy at lower frequencies. The isomerization had taken place.
"We realized that where we saw the patterns breaking specifically involved the vibrations that were related to the kind of structural changes that should be happening at the transition state between these two conformations," Baraban said. "It looks exactly like what you'd expect."
The researchers didn’t simply observe this phenomenon for the first time; they also came up with a formula to calculate the transition state energy, which will help chemists work out the rate of a reaction. Although the formula was tested on isomerization, the researchers think that it could be applied to other chemical reactions as well.
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