DTU Chemistry - Breakthrough i Physical Chemistry

Breakthrough in physical chemistry

Monday 12 Jun 17
by Jan Wium


Esben Folger Thomas
PhD student
DTU Chemistry
+45 45 25 20 29


Niels Engholm Henriksen
Associate Professor
DTU Chemistry
+45 45 25 20 29

Scientists from DTU Chemistry have discovered a new method of transforming enantiomers in a racemic mixture so that all enantiomers will end up as the desired kind.

PhD student Esben F. Thomas uses a comparison to explain the findings: “Think of a table with a pile of coins on it, some with one side up, some with the other side up. Our results are analogous to finding a way to shake the table, where the outcome would be that all the coins ended with the same side up”.

The findings of Esben F. Thomas and supervisor Associate Professor Niels E. Henriksen have just been published in the high impact journal “The Journal of Physical Chemistry Letters”, and the Reviewers are impressed: “The manuscript has high potential to become a milestone publication in this demanding field of physical chemistry.”

Goodbye to inconveniences and tragedies
Having undesired enantiomers in a racemic mixture can cause all kinds of inconveniences - and even fatal tragedies. Just think of the Thalidomide Scandal.

It is well known that two molecules which consist of exactly the same atoms can exhibit vastly different chemical behavior – even if the molecules only differ in the very subtle way of being each other’s mirror image.

Each form of such molecules is called an enantiomer, and a mixture of equal amounts of enantiomers is denoted a racemic mixture.

Complete conversion

The video above shows the two enantiomers (i.e. the racemic mixture) of a bi-phenyl compound interacting with the time-dependent electric field of a tailored laser pulse. 

What Esben F. Thomas and Niels E. Henriksen have demonstrated theoretically is, that it is possible to construct a laser pulse that leads to increase in the amplitude of the torsional oscillations and, eventually, a transfer over the potential energy barrier separating the two enantiomeric forms. 

Moreover, this transfer can be limited so it occurs in only one type of enantiomer, leading to deracemization and complete conversion into a desired enantiomeric form within about 20 picoseconds.

Closing the gap
In order to close the gap between theory and experiment, the optimized laser pulses have been constructed in a way that uses the same control knobs as experimentalists have access to.

The optimized laser field was found using a so-called genetic algorithm where the time-dependent Schrödinger equation was solved in an iterative loop.

Thus, substantial cpu time of the order of many hundreds of hours was required. For the studied system the field couples to the molecule by way of the so-called (nonresonant) dynamic Stark effect.

In other words, Esben F. Thomas and Niels E. Henriksen are working on bridging the gap between the scientists who work with computer simulations and those who work with experiments in a laboratory

“What we have shown is a proof of principle by means of a computer. Our long term goal is to see the proof in an experiment in a laboratory. Actually we already have contact with scientists from Aarhus University”, says Niels E. Henriksen.

Read the article “Phase-Modulated Nonresonant Laser Pulses Can Selectively Convert Enantiomers in a Racemic Mixture” in “The Journal of Physical Chemistry Letters”

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