# Reflection methods in IR spectroscopy

## ATR Spectroscopy - Introduction

ATR spectroscopy enables surface analysis of samples that can be brought into contact with a crystal with a high refractive index. Since the radiation is guided by total reflection at the interface of the ATR crystal, it only comes into contact with the surface of the sample.

principle

Total reflection is observed when the radiation of electromagnetic radiation at the boundary between an optically dense (ATR crystal with the refractive index $n1$) and an optically thin medium (sample with the refractive index $n2$) takes place at an angle of incidence which is greater than the critical angle $αG$ which is total reflection. (If a light beam passes from the optically denser to the optically thinner medium, reflection occurs. The reflected portion of the incident radiation increases with the increasing angle of incidence. The entire radiation is reflected when the limit angle of total reflection has been reached.)

This critical angle is given by:

$sinαG=n1n2n1 - Refractive index of the ATR element (optically dense) n2 - refractive index of the sample (optically thin)$

with $n1>n2$

In total reflection, the light beam is shifted laterally in the order of magnitude of one wavelength (Goos-Hänchen effect). The processes between these two points in the optically thinner medium (sample) are described with the theory of the evanescent field.

use

Quantitative and qualitative analyzes of surfaces with regard to

• chemical composition
• Layer structure
• Diffusion processes
• Monitoring chemical reactions on surfaces

## Thinner, smaller, faster - how IR spectroscopy helps elucidate the functional mechanism of biological and biomimetic systems

Department of Physics - Experimental Molecular Biophysics, Free University of Berlin, Arnimallee 14, 14195 Berlin (Germany), Fax: (+49) 30-838-56510 http://www.physik.fu-berlin.de/einrichtungen/ag/ag -heble / index.html

Faculty of Chemistry - Biophysical Chemistry, Bielefeld University, Universitätsstrasse 25, 33615 Bielefeld (Germany)

Department of Physics - Experimental Molecular Biophysics, Free University of Berlin, Arnimallee 14, 14195 Berlin (Germany), Fax: (+49) 30-838-56510 http://www.physik.fu-berlin.de/einrichtungen/ag/ag -heble / index.html

Department of Physics - Experimental Molecular Biophysics, Free University of Berlin, Arnimallee 14, 14195 Berlin (Germany), Fax: (+49) 30-838-56510 http://www.physik.fu-berlin.de/einrichtungen/ag/ag -heble / index.html

Faculty of Chemistry - Biophysical Chemistry, Bielefeld University, Universitätsstrasse 25, 33615 Bielefeld (Germany)

Department of Physics - Experimental Molecular Biophysics, Free University of Berlin, Arnimallee 14, 14195 Berlin (Germany), Fax: (+49) 30-838-56510 http://www.physik.fu-berlin.de/einrichtungen/ag/ag -heble / index.html

### Abstract

Good vibrations: In this Minireview, new spectroscopic methods for the analysis of biomolecules are presented. Research is currently advancing into ever thinner (down to a single layer), smaller (down to a single molecule) or faster (up to femtosecond) areas. Surface-sensitive vibration spectroscopy techniques such as SEIRAS, SFG and SNIM as well as ultrafast methods of 1D and 2D IR spectroscopy are presented.

### Abstract

New vibrational spectroscopic techniques are very promising methods for the investigation of biological materials, as they enable a very high spatial and temporal resolution with minimal influence on the system to be investigated. This Minireview discusses these techniques and their potential implications for biological and biomimetic systems. Presented are z. B. the tracking of conformational changes in peptides with fs resolution and nm sensitivity with 2D-IR spectroscopy as well as the investigation of the effect of an applied membrane potential on individual proton transfer steps within integral membrane proteins with surface-enhanced infrared difference absorption spectroscopy. Vibration spectra of a molecular monolayer can be registered with the help of the sum frequency generation, while scanning near-field IR microscopy enables chemical images of a surface to be recorded with a lateral resolution of <50 nm.

## Thinner, smaller, faster - how IR spectroscopy helps elucidate the functional mechanism of biological and biomimetic systems

Department of Physics - Experimental Molecular Biophysics, Free University of Berlin, Arnimallee 14, 14195 Berlin (Germany), Fax: (+49) 30-838-56510 http://www.physik.fu-berlin.de/einrichtungen/ag/ag -heble / index.html

Faculty of Chemistry - Biophysical Chemistry, Bielefeld University, Universitätsstrasse 25, 33615 Bielefeld (Germany)

Department of Physics - Experimental Molecular Biophysics, Free University of Berlin, Arnimallee 14, 14195 Berlin (Germany), Fax: (+49) 30-838-56510 http://www.physik.fu-berlin.de/einrichtungen/ag/ag -heble / index.html

Department of Physics - Experimental Molecular Biophysics, Free University of Berlin, Arnimallee 14, 14195 Berlin (Germany), Fax: (+49) 30-838-56510 http://www.physik.fu-berlin.de/einrichtungen/ag/ag -heble / index.html

Faculty of Chemistry - Biophysical Chemistry, Bielefeld University, Universitätsstrasse 25, 33615 Bielefeld (Germany)

Department of Physics - Experimental Molecular Biophysics, Free University of Berlin, Arnimallee 14, 14195 Berlin (Germany), Fax: (+49) 30-838-56510 http://www.physik.fu-berlin.de/einrichtungen/ag/ag -heble / index.html

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### Abstract

New vibrational spectroscopic techniques are very promising methods for the investigation of biological materials, as they enable a very high spatial and temporal resolution with minimal influence on the system to be investigated. This Minireview discusses these techniques and their potential implications for biological and biomimetic systems. Presented are z. B. the tracking of conformational changes in peptides with fs resolution and nm sensitivity with 2D-IR spectroscopy as well as the investigation of the effect of an applied membrane potential on individual proton transfer steps within integral membrane proteins with surface-enhanced infrared difference absorption spectroscopy. Vibration spectra of a molecular monolayer can be registered with the help of the sum frequency generation, while scanning near-field IR microscopy enables chemical images of a surface to be recorded with a lateral resolution of <50 nm.

## Insights into the distribution of CO2-Molecules and their temporal development through micro-imaging using IR spectroscopy and molecular dynamic modeling

CO2- Expressway: The CO2-Concentration in a ZIF-8 @ 6FDA-DAM-MMM during transient adsorption was followed by IR microimaging. The CO2-Molecules move from the ZIF-8 filler, which acts like an “expressway”, into the surrounding polymer. At the filler / polymer interface a layer of high CO2Concentration formed. A microscopic explanation of the first stage of this phenomenon is proposed using molecular modeling.

### Abstract

The development of CO2-Concentration of a ZIF-8 @ 6FDA-DAM-Mixed-Matrix-Membrane recorded spatially and temporally resolved during the adsorption process. By the development of the CO2Concentration is dissolved, it is observed that the CO2-Molecules from the ZIF-8 filler, which acts as an “expressway” of mass transport, enter the surrounding polymer. A layer of high CO is formed2-Concentration at the MOF / polymer interlayer, which occurs at high CO2-Gas pressure trained stronger. A microscopic explanation of the origin of this phenomenon is proposed based on molecular simulation. Using a computational method based on quantum mechanical and force field-based calculations, the formation of micro-voids in the MOF / polymer interlayer is predicted. Grand canonical Monte Carlo simulations also show that the CO2-Molecules tend to prefer to stay in these micro-cavities, from which one can conclude that this is the CO2- Facilitates enrichment in the intermediate layer.

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## Watching Water Migration around a Peptide Bond †

This study was supported by the MEXT (priority area 477 Japan), the Core-to-Core Program of the JSPS, and the DFG (grant number DO 729/4). We thank Kenji Sakota (Kyushu University) and Shun-ichi Ishiuchi (Tokyo Institute of Technology) for stimulating discussions. M. Schmies is grateful for an Elsa Neumann fellowship.

### Abstract

On a winding path: The movement of a single water ligand around a peptide bond in acetanilide was examined in real time with time-resolved IR spectroscopy. Triggered by photoionization, the water ligand is released from the CO side of the peptide bond and captured on the NH side of the same peptide bond after a migration phase of 5 ps (see picture).

Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors.

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## Reflection methods in IR spectroscopy - chemistry and physics

A friendly hello to everyone.

I have to solve the following problem.

Given are the IR spectra of
Potassium permanganate
Potassium manganate
Potassium hypomanganate

. but not assigned. I should do that.

The real problem with this is that I don't know how.
I would somehow have to use the force constant of the Mn-O bond
can explain. but how do I get it out.

or let's ask like that. how does this constant change with increasing
Oxidation number? Is it increasing too?

Hope you can help me

You can't say that in general, but here at least I would
expect for the stretching vibrations of the manganates.

If I remember halfway correctly, it is different
The symmetry of the anions is responsible for the differences in the spectra.
In which area do they absorb? In which matrix are they
been measured?

I don't know the matrix. this is just an exercise.
and I am only supposed to assign the spectra. no longer.

## Physical basics of IR spectroscopy

Authors: pike, Thomas

• Explains the relationships between the molecular structure, binding and bands in the IR spectrum
• Also creates the basis for understanding more complex structures
• With concrete practical examples

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Anyone who sees an IR spectrum for the first time is often stunned by the multitude of bands that appear. This essential gives an overview of infrared spectroscopy and shows that the relationships between molecular structure, binding and bands in the IR spectrum are not that difficult to understand. For this purpose, the analogy between a mechanical spring pendulum builds a bridge to oscillating molecules and finally real IR spectra. Armed in this way, some specific examples are discussed and the basis for understanding more complex spectra is laid.

Dr. Thomas Hecht is a graduate chemist and teaches chemistry and physics as a scientific teacher at the Carl Engler School in Karlsruhe.

## Description IR spectroscopy

IR spectroscopy is based on the principle of absorption of IR radiation. The absorbed energy excites vibrations in the molecules, which is why this type of spectroscopy is also known as vibrational spectroscopy. This video introduces the basics, methodology and applications of IR spectroscopy.

### Transcript IR spectroscopy

Hello and welcome!

The topic of this video is: Infrared Spectroscopy, also called IR Spectroscopy.

After the video you will know: 1. what IR spectroscopy is, 2. how it works and 3. where it is used.

To understand this video, however, you should already know what spectroscopy is in general and how UV-Vis spectroscopy works in particular. It is also important that you know what the electromagnetic spectrum is and also what a dipole is.

First of all, what is IR spectroscopy? At this point it makes sense to recall how UV-Vis spectroscopy works. Do you remember? We had a clear sample container that contained the substance that was to be examined. The container was then irradiated with electromagnetic radiation of a certain wavelength, part of this radiation being absorbed by the sample and part of the radiation being able to penetrate the sample. Behind the sample there was a detector that could measure how much radiation could penetrate the sample, which also made it possible to draw conclusions about how much radiation was absorbed by the sample. The method therefore resulted in the absorption of electromagnetic radiation being measured at different wavelengths, which, however, were all within a certain spectral range, namely the range of ultraviolet light, i.e. UV light and visible light.

IR spectroscopy actually works in exactly the same way, with the only difference that infrared radiation is used instead of UV radiation or visible light. Infrared radiation has a wavelength that is roughly in the μm range and is therefore much lower in energy than UV radiation or visible light. If the UV or visible light had enough energy to excite valence electrons in the atoms, the energy of the IR radiation is only sufficient to trigger vibrations in the molecules. And that is also the reason why one often speaks of vibrational spectroscopy instead of IR spectroscopy.

But how exactly does it work? What exactly happens when a molecule is excited to vibrate? I would like to demonstrate this using the vibrations of the SO2 molecule. The sulfur dioxide molecule consists of 1 sulfur atom and 2 oxygen atoms and has an angled molecular shape due to the lone pair of electrons on the sulfur atom. This molecule can perform 3 different types of vibration. In the first of these oscillations, the two oxygen atoms oscillate in unison along their bonds. It looks something like this. In the 2nd oscillation, the two oxygen atoms also vibrate along their bonds, but no longer in the same mode, e.g. B. so. At the 3rd oscillation the whole molecule is deformed in such a way that the bond angle becomes larger and smaller, something like this. The 1st of these 3 types of vibration is called the symmetrical stretching vibration, the 2nd is the asymmetrical stretching vibration and the 3rd is the flexion vibration. More types of vibration than these 3 are not possible with SO2.

Now it is the case that each of these 3 vibrations needs its own amount of energy to get going. And this energy is provided by the irradiated IR radiation, depending on the wavelength. Exactly 3 wavelengths of the IR range are absorbed by SO2 and consequently our IR spectrum will also contain 3 absorption maxima, 1 for each type of vibration.

Now a very important note: In order for IR radiation to be absorbed at all, a very specific requirement must be met. The dipole moment of the molecule must change in the course of the oscillation. In other words: only vibrations in which the dipole moment of the molecule changes result in an IR signal. I would like to demonstrate that using the stretching vibrations of the CO2 molecule. Here, too, we have a symmetrical and an asymmetrical stretching vibration. If you now compare the two oscillation states of the asymmetrical oscillation, you will find that they have a different dipole moment. This means that the dipole moment changes in the course of the oscillation. If you don't understand this point, please watch the corresponding video "Dipole". In the case of symmetrical oscillation, on the other hand, both oscillation states have a dipole moment of 0. Consequently, one can say: The dipole moment does not change. Therefore, the asymmetrical oscillation would appear as a signal in our IR spectrum, whereas the symmetrical oscillation would not.

Now a few words about the representation of IR spectra. You already know from UV spectroscopy that there the spectrum was entered in a diagram, which consists of a y- and an x-axis, where the x-axis denotes the wavelength and the y-axis denotes the extinction, that is , the absorbed light. In principle, this could be done in the same way in IR spectroscopy. However, it has become common practice to designate the axes differently, although the same information is of course displayed. The y-axis is usually denoted with a large T, the x-axis with a small ν with a wave on top. The capital T stands for the transmission, that is, for the intensity of the transmitted radiation, and specifically stated as a percentage of the radiated radiation. The maximum value is logically 100%, which means nothing else than that 100% of the light was allowed through and 0% was absorbed.

is also called the wave number and it is nothing more than the reciprocal of the wavelength: ν ^

= 1 / λ. Consequently, its unit is also the reciprocal of a segment, and usually cm ^ -1 is taken. Commonly used wave numbers are in the range of 2800-400 per cm. The spectrum of the SO2 shown as an example looks something like this. Each peak stands for a vibration, the left peak for the asymmetrical stretching vibration, the middle peak for the symmetrical stretching vibration and the right peak for the flexion vibration. SO2 is a relatively small, relatively primitive molecule. It consists of a few atoms and can therefore also carry out relatively few types of vibration.

However, IR spectroscopy is mostly used in organic chemistry, which means that we are dealing with much more complicated molecules. The spectra then look correspondingly complicated. This spectrum is now a fantasy spectrum, but it has the typical characteristics. You can typically divide IR spectra into 2 areas. On the one hand, there is the range between 2800 and approximately 1500 per cm, which denotes the range in which individual functional groups perform typical vibrations. The second range covers approximately the wave numbers 1500 to 400 per cm and stands for the range in which a complex organic molecule as a whole oscillates. This area is usually called the fingerprint area, which makes perfect sense because it is something like a typical fingerprint of a certain molecule.

And it is also obvious what information can be drawn from these two areas. The first area can be used to identify individual functional groups present in the molecule. That means you can also predict which class of substance it is, an alcohol, an alkane or whatever. The fingerprint area, on the other hand, is used to identify the exact molecule, and this is usually done using comparison spectra. If 2 compounds in the IR spectrum have an identical fingerprint area, then the molecules are usually identical. In the past, comparing fingerprint areas was an extremely arduous activity. Nowadays computers do that.

And that brings us to the question: Where is IR spectroscopy actually used? Well, the main area of ​​application of IR spectroscopy is obvious: it is the identification of compounds in organic chemistry. Another area is tracking reaction histories. This is done by taking a sample from the reaction mixture at different points in time and then examining it and then determining the point in time at which the desired product was created. For example, this method plays an extremely important role in process control in the chemical industry. Another, more academic field of application of IR spectroscopy is the measurement or calculation of bond strengths. Since the oscillations of the molecule take place around the bonds, one can imagine that the energy at which an oscillation starts also depends on the strength of the bond. A lot of information about the type or strength of a bond can then be derived from this context.

So and that brings us to the end of this video. We learned: 1. what IR spectroscopy is, 2. how it works, namely via the excitation of molecular vibrations, and 3. where it is used.

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## Multi-dimensional infrared spectroscopy

Multi-dimensional infrared spectroscopy: every prospective chemist is confronted with a wide variety of analytical techniques in his academic training. Spectroscopy, i.e. the elucidation of the molecular structure of substances with the help of electromagnetic radiation, plays a central role in this. Infrared (IR) spectroscopy provides insights into the shape of molecules by stimulating their oscillations with infrared light. Knowledge of the vibrational structure of matter is so important that it is now impossible to imagine the basic equipment of chemical research laboratories without modern FTIR (Fourier Transform IR) spectrometers.

With the development more tunable Laser light sources In the infrared spectral range and the possibility of generating high-energy IR pulses with a duration of approx. 100 femtoseconds (10 -13 s), a small revolution in IR spectroscopy has recently been initiated [1,2]. Analogous to multi-dimensional nuclear magnetic resonance spectroscopy (NMR), which was awarded the Nobel Prize in Chemistry to Richard Ernst more than 20 years ago, such ultrashort laser pulses can now also be used to record multi-dimensional vibration spectra coherently, thereby increasing the information content of IR spectroscopy many times over.

Many pulses, multidimensional spectra
Vibrating groups of molecules can interact with each other and the couplings are expressed in quantum mechanical disturbances of the vibrational resonances. However, these cannot be clearly identified in the conventional FTIR spectrum and certainly not be evaluated quantitatively. By irradiating the usually liquid sample with a sequence of IR laser pulses and spectral analysis of the infrared emission coherently emitted by the sample in two frequency dimensions, such couplings within the vibration resonances can be recognized as off-diagonal signals (so-called "cross peaks"). We speak of infrared correlation spectroscopy (or 2DIR-COZY). By varying the time intervals within the pulse sequence, it can be clarified whether, as a result of a dynamic structural change at the molecular level, certain vibrational resonances transform into one another over time. This could be caused, for example, by a spontaneous chemical process or even by an externally forced (e.g. photochemical) reaction. So-called infrared exchange spectroscopy (or 2DIR-EXSY) is used here.

Structure of 2DIR spectroscopy
The instrumental structure of a 2DIR spectrometer is unfortunately still very complex. This is primarily due to the fact that in the liquid phase the lifetimes of excited oscillation states only very rarely exceed several hundred picoseconds (10 -10 s) and that the time scales on which 2DIR spectra can be recorded are due to these very short T1 times are limited. As a result, you first need a femtosecond laser (the so-called “front-end” usually a Ti: sapphire laser that emits around 800 nm), whose light pulses drive non-linear optical frequency converters. Its task is to convert the flashes of light from the “front end” into infrared pulses, the frequencies of which are matched to the vibrational resonances of the sample to be examined. In addition, you need an optical system (the 2DIR interferometer) that is able to synthesize pulse sequences with variable delay times from the tunable and ultrashort infrared pulses. Finally, an infrared detection system is also required, which comprises a grating spectrograph, an IR line sensor and an analog-to-digital converter. All of the devices mentioned are controlled by a computer, which also handles the data processing. An impression of the instrumental complexity of the 2DIR spectrometer built at the University of Bonn is given by a direct look into the laboratory (see opening photo) and the schematic structure shown in Figure 1.

application areas
The fields of application of 2DIR spectroscopy are extremely diverse and range from Structure elucidation in biochemical systems up to the interface analysis of soft condensed matter. At this point, reference is therefore only made briefly to recently published review articles [3 - 5]. However, the potential of the method can be shown particularly impressively on the basis of the elucidation of molecular dynamics in hydrogen-bridged systems. The local geometry of hydrogen bonds can be researched using the stretching vibration of groups of molecules that are involved in the formation of hydrogen bonds. This can be illustrated using the FTIR spectrum of a simple dihydric alcohol such as pinacol (see Fig. 2). In non-polar solvents such as carbon tetrachloride, an intramolecular hydrogen bond is formed between the two hydroxyl groups of the diol. In the relevant spectral range of the OH stretching vibrations around 3500 cm -1, this compound has two clearly separated absorption bands. A first high-frequency resonance can be traced back to the free OH oscillator, which acts as an H-bridge acceptor. A second, low-frequency band is caused by the bound OH oscillator, which acts as an H-bridge donor. This pronounced sensitivity of the stretching vibration frequency of certain groups of molecules to the formation of hydrogen bonds is a very general phenomenon that can be attributed to the charge transfer character of the non-covalent H bond [6].

What information does 2DIR spectroscopy provide that cannot be read from a conventional FTIR spectrum? In short, the answer is: the dynamics of the H-bridge in terms of its service life! To demonstrate this, we consider two 2DIR-EXSY spectra (Fig. 2) recorded at different delay times. Immediately after the sample has interacted with a first resonant laser pulse, the 2DIR spectrum in the region of the fundamental excitation reveals only signals along the diagonal frequency axis (i.e. on the white line along which ν1 = ν3 is). A set of corresponding diagonal signals is in the range of overtone excitation (ν1 = ν3 - Δ and Δ = anharmonicity of the vibrations) can also be recognized. The absence of prompt “cross peaks” shows that the OH stretching vibrations can only be coupled very weakly despite the formation of the H bridge between the two hydroxyl groups. However, if you look at the 2DIR spectrum at later times, you will see that such “cross-peaks” grow slowly, i.e. only occur with a certain delay. This is a clear indication that the two OH oscillators can convert into one another and that the hydrogen bond can reverse its direction. A hydroxyl group that was originally labeled as an H-bond acceptor at the high resonance frequency appears in the 2DIR spectrum at a later point in time at the low resonance frequency of the H-bond donor. Since this chemical exchange is a spontaneous process, the reverse process of converting a bound OH oscillator into a free one is just as likely. Consequently, the delayed “cross peaks” are observed symmetrically around the diagonal. A detailed analysis of the series of 2DIR spectra reveals a lifetime of the pinacol H-bond in CCl4 at room temperature of 2 ps. In an article recently published in Angewandte Chemie, Olschewski describe et al. in detail the nature of the intramolecular movement of the pinacol, which leads to the reversal of direction of the H-bridge and is clearly referred to there as a “flip-flop” [7].

Meaning of the "flip-flop"
Such unique findings on the dynamics of hydrogen bonds are essential for a comprehensive understanding of a large number of biochemically or technologically important processes. Exemplarisch sei an dieser Stelle der Protonentransport genannt, der bei der zellulären Energieerzeugung in Form von ATP durch Membranproteine eine Schlüsselstellung einnimmt. Ebenso spielt der Protonentransport die entscheidende Rolle bei der Umwandlung von chemischer Energie zu elektrischem Strom im Innern von Brennstoffzellen. Dem Chemiestudierenden wird schon frühzeitig in seiner akademischen Ausbildung erläutert, dass Protonen und Hydroxidionen in wässrigen Medien außerordentlich hohe Leitfähigkeiten besitzen. Die hohen Wanderungsgeschwindigkeiten dieser Ionen werden in der Regel in chemischen Lehrbüchern mit dem Grotthus-Mechanismus [5] begründet. Schematisch und stark vereinfacht ist dieser Mechanismus in Abbildung 3 abgebildet.

Ein eindimensionaler Draht von Wasserstoffverbrückten Struktureinheiten, entlang dessen ein Proton transportiert werden kann, ist dadurch gekennzeichnet, dass seine individuellen H-Brücken kollektiv in ein und dieselbe Richtung zeigen. Der Grotthus-Mechanismus ermöglicht dabei die effiziente Wanderung der positiven Ladung über große Abstände unter Vermeidung molekularer Bewegungen mit großer Amplitude. Theoretische und experimentelle Studien belegen, dass in wässrigen Volumenphasen das Brechen und Knüpfen von HBrücken in der zweiten Solvathülle des Protons die eigentliche Migration der positiven Ladung in die Wege leitet [8]. Dank dieser Arbeiten sind wir heute im Besitz einer recht detaillierten Vorstellung des eigentlichen Ladungstransports. Der Durchtritt des Protons über den eindimensionalen Draht hinweg hinterlässt eine Umorientierung der individuellen Wasserstoffbrücken, was den Transport einer nachfolgenden Ladung verhindert. Insbesondere in biochemischen Systemen stellt sich daher die Frage, auf welchem Wege die ursprüngliche Ausrichtung des H-Brückendrahts wieder hergestellt werden kann und wie eine (für die Funktion des Proteins essentielle) unidirektionale Protonenleitfähigkeit wieder sichergestellt werden kann. Wie schnell kann also ein solcher Draht umorientiert und für den Transport des nächsten Protons wieder hergerichtet werden?

Grundsätzlich erfordert die kollektive Umkehr der H-Brücken nach Transmission des Protons die anschließende Wanderung eines Strukturdefekts. Dabei unterscheiden wir zwischen zwei Arten von Strukturdefekten, die bereits vor mehr als 60 Jahren durch Bjerrum vorgeschlagen wurden [6]. Sogenannte L-Defekte sind dadurch gekennzeichnet, dass eine Wasserstoffbrücke im H-Brückennetzwerk frei von H-Atomen ist. Demgegenüber ist im sogenannten D-Defekt eine H-Brücke mit zwei H-Atomen gleichzeitig besetzt. Die jüngsten Arbeiten zur H-Brückenumkehr mit Hilfe der ultraschnellen 2DIR-Spektroskopie liefern nun erstmalig Rückschlüsse auf diesen fundamentalen Prozess der Defektwanderung. Es zeigte sich, dass der D-Defekt-Transport gegenüber dem Transport eines L-Defekts energetisch günstiger ist und dass grundsätzlich die Wanderung der Bjerrum‘schen Fehlstelle für die unidirektionale Protonenleitfähigkeit geschwindigkeitsbestimmend ist. Ob diese Schlussfolgerung von genereller Gültigkeit ist und auch auf mehrdimensionale, ausgedehnte Netzwerke wässriger Systeme übertragen werden kann, steht derzeit im Zentrum intensiver und spannender Forschungsarbeiten unter Verwendung der mehrdimensionalen Infrarotspektroskopie.

literature
[1] Cho M.: CRC Press, Boca Raton (2009)
[2] Hamm P und Zanni M.: Cambridge University Press, Cambridge (2011)
[3] Hunt N.: Chemical Society Reviews 7, 1837-1848 (2009)
[4] Kim Y. S. und Hochstrasser R. M.: Journal of Physical Chemistry B 113, 8231-6323 (2009)
[5] Fayer M. D.: Annual Reviews of Physical Chemistry 60, 21-38 (2009) und dort zitierte Literatur
[6] Jeffrey G. A.: Oxford University Press, New York (1997)
[7] Olschewski M., Lindner J. und Vöhringer P.: Angewandte Chemie 125, 2663-2667 (2013)
[8] Agmon N.: Chemical Physics Letters 244, 456-462 (1995)
[9] Bjerrum N.: Science 115, 385-390 (1952)

## IR-Spektroskopie

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Prof. Dr. Günther Wagner, Leipzig
Prof. Dr. Manfred Weißenfels, Dresden
Dr. Klaus-Peter Wendlandt, Merseburg
Prof. Dr. Otto Wienhaus, Tharandt