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Vibrational Spectroscopy and Intermolecular Dynamics

flag_germany.png (1 KB) Schwingungsspektroskopie und zwischenmolekulare Dynamik (in German)


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Whether we are enchanted by the scent of a rose, whether a drug blocks a specific enzyme in our body, whether genetic information is read from our DNA, or whether a thundercloud forms - it always has to do with specific interactions between molecules, which are mediated via hydrogen bonds and other attractive forces as well as repulsive contacts. In short, it has to do with molecular sociology. By studying simple model systems, we try to get to the bottom of such interaction mechanisms.

Bunsen.jpg (30 KB) In a case study of the simplest molecule which forms a hydrogen bond with itself - hydrogen fluoride - we have obtained a detailed knowledge of the underlying intermolecular interactions (see refs. [23, 29, 33] in the list of publication of M. Suhm). Today, we apply these insights to clusters of larger molecules.

 

It is well known that (ethyl) alcohol enhances sociability. We are interested in the social life of alcohol molecules, how they form hydrogen bonds among each other and to water. For this purpose, we cool the alcohol molecules in a supersonic jet expansion to temperatures near absolute zero and watch the molecular vibrations via infrared and Raman spectroscopy. These vibrations indicate with high sensitivity, whether and how one molecule interacts with others. liquor_small.jpg (25 KB)

 

handshake8.jpg (9 KB) Many molecules are chiral, i.e. there are left-handed and right-handed varieties. Contacts between two identical molecules are different from those between a molecule and its mirror image, much like in the case of handshakes. Using spectroscopy, we were recently able to detect this difference for several particularly elementary systems. Such molecular recognition phenomena are omnipresent in biochemistry and we want to understand them in detail.

 

When we audibly expand the molecules through a slit nozzle into a 23 m3 vacuum chamber, the molecules reach outer space-like temperatures around -260°C. We use these conditions to bind molecules to each other and to simplify their motion. By hitting them with thermal or laser radiation, we learn about their interactions. ragout_2_small.jpg (24 KB)

 

ragout.jpg (16 KB) A particularly elementary example is the interaction between hydrogen chloride (HCl) and water. Given sufficient water molecules, they can dissociate HCl into protons and chloride ions. This leads to hydrochloric acid, e.g. in the stomach. With only one or two water molecules per HCl, the HCl stays intact. How do we know and when does dissociation occur? Recently, we have succeeded in observing the vibration of an intact HCl molecule with one and two water molecules (doi:10.1039/b204840j). In the presence of many water molecules, we also observe dissociated HCl.

 

If we want to understand in detail how complex molecules cuddle up together, we have to walk on their interaction energy maps in dozens of dimensions and look for the deepest valleys. Sometimes, that keeps our computers busy for weeks. tun2.gif (4 KB)

 

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Blue haze - organic aerosols
distribution.jpg (150 KB) When a solid or liquid is dispersed up to the point where a significant fraction of its molecules sits on the surface, its material properties change. Also, gravity ceases to play an important role, we are dealing with fine aerosols. We investigate particles with diameters of a few nanometers, as they play a role in cloud formation, air pollution and the green house effect, but also in the design of new materials. For this purpose, we use infrared spectroscopy and light scattering.

 

Our research group is established in Göttingen since 1998. You find us on the first floor of the Institute of Physical Chemistry. Feel free to enquire about participating in our mixed experimental and computational research, e.g. within the framework of a Diploma or Ph.D. thesis. Beyond the qualification requirements of the local university regulations, you just have to share with us an interest in intermolecular forces and spectroscopy, some practical and/or computational skills, and an open mind. ipc20001022.jpg (93 KB)

Selected publications

Philipp Zielke, Martin A. Suhm
Raman jet spectroscopy of formic acid dimers: Low frequency vibrational dynamics and beyond
Phys. Chem. Chem. Phys. 9 (2007) 4528-4534
doi:10.1039/b706094g

Merwe Albrecht, Corey A. Rice and Martin A. Suhm
Elementary Peptide Motifs in the Gas Phase: An FTIR Aggregation Study of Formamide, Acetamide, N-Methylformamide, and N-Methylacetamide
J. Phys. Chem. A 112 (2008) 7530-7542
doi:10.1021/jp8039912

Marija Nedic, Tobias N. Wassermann, Zhifeng Xue, Philipp Zielke, and Martin A. Suhm
Raman spectroscopic evidence for the most stable water/ethanol dimer and for the negative mixing energy in cold water/ethanol trimers
Phys. Chem. Chem. Phys. 10 (2008) 5953-5956
doi:10.1039/b811154e

Anne Zehnacker and Martin A. Suhm
Chirality Recognition between Neutral Molecules in the Gas Phase
Angew. Chem. Int. Ed. 47 (2008) 6970-6992
doi:10.1002/anie.200800957


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Revised 2017-01-27