Our research explained

In a nutshell, our group wants to understand how molecules interact with each other, or—if they are large and flexible—how parts of them interact with other parts. We usually focus on the soft side of this interaction, where the molecules do not lose their identity. We are interested in more directed interactions called hydrogen bonds as well as more diffuse ones called London dispersion interaction. These have some analogy to social interactions between living beings, with important differences in terms of predictability and reproducibility. There is an old, tongue-in-cheek description of this field of molecular sociology (in German). In this place we will occasionally provide more recent examples from our research and visualize their analogies in social interactions.

Most of our experiments involve pushing small molecules through narrow slits into vacuum chambers and watching what they do when flying at supersonic speeds and cooling out.

supersonic expansion cartoon

Intermolecular balance experiments

We all have seen cows surrounded by lots of flies, in permanent motion, wandering from nose to tail and back. This is analogous to what solvent molecules do when they surround a large molecule in solution. They have some subtle preference for one or the other end of the molecule. But of course they will go for both, if there are many solvent molecules and few dissolved molecules. What can we do to find out whether the flies intrinsically prefer the tail or the nose of the large molecule? We could count and average them over a period of time, but that is not so easy to do experimentally and furthermore one fly might influence the other, hence we would not learn about individual preferences. We do not want to measure crowd preference, in the start, for reasons explained below.

Here is what we do in the lab, with molecules instead of cows and flies, because molecules don't suffer from our treatment: We pick a number of cows of a certain breed. We remove all the flies from the cows. We cool the cows, to stop all the motion of head and tail. We don't want our result to be influenced by a wagging tail. Strangely enough, this cooling can be achieved by forcing the cows through a small slit in the fence into a big empty chamber - this is called supersonic expansion into vacuum. The cows forget to wag their tails when running roughly in the same direction, but once they enter the chamber they spread like a fan. Now we dose a limited number of fast flies into the chamber and those pick their favourite docking site on the cows. We make sure there is at most one fly per cow, such that each fly is likely to pick its favourite site. Although the skin of the cows is cold, the flies still manage to wander around and decide for the best fit. As spectroscopists we can tell from the pitch of the buzzing sound of the flies whether they are close to the tail or to the nose. So we can actually measure how many flies choose the tail and how many the nose, without even looking, which is anyway difficult for molecule-sized cows.

We compare these experimental findings with theoretical predictions - that is our favourite pastime, as it allows us to discriminate poor from good predictions. Note that if the prediction is good, it does not necessarily mean that the underlying theoretical model is good as well. It could also be a lucky coincidence. However, if the prediction is poor, we can be sure the theory needs improvement. To rule out lucky coincidence, we repeat the experiment with different breeds of cow, aka different kinds of molecules. In chemistry, we can actually choose about millions of them. Only theories which succeed in many different types of cow are likely to be good for a good reason. After a lot of work, we end up with a grading of dfferent theoretical models for cow-fly interaction.

Now you may ask: why create this artificial situation: cold cows in despair running at supersonic speed, being visited by single flies? Why not apply the theory we want to test to an easygoing flock of cows at rest in the sun, surrounded by millions of flies? The answer is: Chances are that even poor theories succeed in a reasonable description of the overall situation, but for the wrong reason. When treating clouds of flies in the sun, further approximations have to made in the theory, which may hide other deficiencies it has. The worst case is if the theory has previously been adjusted uncritically to the complicated case of cows in the sun, surrounded by flies. It will appear to describe this situation well, but perhaps it fails completely for cold cows with single flies. Perhaps it already fails when the sun goes away. You may only get back what you have put in.

The really good theories come with very few or very clever approximations, but currently they can only be applied to cold cows with single flies due to limited computer power. Our experiments offer a means to test them. If the tests look good, more simplified theories can learn from them. If computers and algorithms get better, the advanced theories can even be applied to more complex situations. Our experiments thus provide a bridge between good, but expensive theories and cheaper theories which may have some accidental or deliberate error compensation.

We do not promise that our research will contribute to cows giving more milk. Our research is curiosity-driven. We have fun chasing molecular cows through slits into vacuum and probing where molecular flies tickle them. We are fascinated about the power of quantum mechanics in describing this and other processes and want to help in finding the best cost-saving approximations from an experimental point of view. This activity is called experimental benchmarking and we think there may be too little of it, these days.

Finally – why do we call our approach an intermolecular balance experiment? Because we measure the interaction between molecules, and we are able to measure closely balanced situations between nose and tail preference. We found breeds of cows and flies which slightly prefer the tail interaction, and we found other breeds which slightly prefer the nose contact. The more subtle the preference, the more rigorous is our test of theories. Remember that a theory which is slightly wrong for a single fly may be terribly wrong for hundreds of interacting flies, because errors could accumulate. That is why we are after the very delicately balanced situations. If you want to read about this topic in a more scientific way, try reference [160].