Stephan Knop

On the nature of OH-stretching vibrations in hydrogen-bonded chains : Pump frequency dependent vibrational lifetime

Two-dimensional infrared spectroscopy was carried out on stereoselectively synthesized polyalcohols. Depending upon the stereochemical orientation of their hydroxyl groups, the polyols can either feature linear chains of hydrogen bonds that are stable for extended periods of time or they can display ultrafast dynamics of hydrogen-bond breakage and formation. In the former case, the OH-stretching vibrations and their transition dipoles are substantially coupled, hence prior to vibrational relaxation, the initial OH-stretching excitation is rapidly redistributed among the set of hydroxyl-groups constituting the hydrogen-bonded chain. This redistribution is responsible for an ultrafast loss of memory regarding the frequency of initial excitation and as a result, a pump-frequency independent vibrational lifetime is observed. In contrast, in the latter case, the coupling of the OH-groups and their transition dipoles is much weaker. Therefore, the OH-stretching excitation remains localized on the initially excited oscillator for the time scale of vibrational energy relaxation. As a result inhomogeneous relaxation dynamics with a pump-frequency-dependent lifetime are observed.

From Single Hydrogen Bonds to Extended Hydrogen‐Bond Wires: Low‐Dimensional Model Systems for Vibrational Spectroscopy of Associated Liquids

It is fair to say that if we ever wish to understand the anomalous properties of water, we need to study hydrogen bonds. Such a statement is based on statistical mechanics, which tells us how to calculate the structure and the thermodynamic properties of fluids and dense liquids from the forces between the particles. However, in the case of complex associated liquids, such calculations present a formidable--if not even insurmountable--challenge, which largely reflects our still-limited understanding of the hydrogen-bonding phenomenon itself. More experimental research on hydrogen-bonded systems is required to develop a comprehensive, satisfactory theory for associated liquids. This Review gives an introduction to the latest experimental technique currently being used to study the ultrafast structural dynamics of hydrogen bonds, namely two-dimensional infrared spectroscopy, and its applications to hydrogen-bonded systems of systematically increasing complexity, starting from the single hydrogen bond of a diol to low-dimensional extended networks of stereoselectively synthesized polyalcohols.

OH-Stretching in Synthetic Hydrogen-Bonded Chains

We study hydrogen bond dynamics in stereoselectively synthesized polyalcohols by combining linear and two-dimensional (2D) infrared spectroscopy experiments with simulations. We consider two variants of the polyalcohols: the all-syn and all-anti tetrol, which because of their different stereochemistry of the hydroxyl groups form a linear hydrogen-bonded chain that is stable for tens of picoseconds or a system where hydrogen bonds are formed and broken on a picosecond timescale, respectively. The differences in structure and hydrogen bond dynamics gives rise to significant differences in the linear spectra for the two compounds. Furthermore, we show that the stronger hydrogen bonding for the all-syn variant leads to faster fluctuations of the site frequencies than for the all-anti one, which is reflected in the higher degree of homogeneous broadening in the 2D spectra. Because of the different stereochemistry, the coupling in the all-syn molecule is stronger than for the all-anti one, which leads to a faster delocalization of a local excitation. This explains the previously observed pump-frequency independent vibrational lifetime for the all-syn variant, since the excitation loses the memory of the pump frequency before relaxation. For the all-anti form, the coupling is weak and the excitation remains in the initially excited state, maintaining the memory of the pump frequency.

Vibrational relaxation of azide ions in liquid-to-supercritical water

The dynamics of vibrational energy relaxation (VER) of the aqueous azide anion was studied over a wide temperature (300 K ≤ T ≤ 663 K) and density (0.6 g cm(-3) ≤ ρ ≤ 1.0 g cm(-3)) range thereby covering the liquid and the supercritical phase of the water solvent. Femtosecond mid-infrared spectroscopy on the ν(3) band associated with the asymmetric stretching vibration of the azide anion was used to monitor the relaxation dynamics in a time-resolved fashion. The variation of the vibrational relaxation rate constant with temperature and density was found to be rather small. Surprisingly, the simple isolated binary collision model is able to fully reproduce the experimentally observed temperature and density dependence of the relaxation rate provided a local density correction around the vibrationally excited solute based on classical molecular dynamics simulations is used. The simulations further suggest that head-on collisions of the solvent with the terminal nitrogen atoms rather than side-on collisions with the central nitrogen atom of the azide govern the vibrational energy relaxation of this system. Finally, the importance of hydrogen bonding for the VER dynamics in this system is briefly discussed.

Ultrafast internal dynamics of flexible hydrogen-bonded supramolecular complexes.

Supramolecular chemistry is intimately linked to the dynamical interplay between intermolecular forces and intramolecular flexibility. Here, we studied the ultrafast equilibrium dynamics of a supramolecular hydrogen-bonded receptor-substrate complex, 18-crown-6 monohydrate, using Fourier transform infrared (FTIR) and two-dimensional infrared (2DIR) spectroscopy in combination with numerical simulations based on molecular mechanics, density functional theory, and transition state theory. The theoretical calculations suggest that the flexibility of the macrocyclic crown ether receptor is related to an ultrafast crankshaft isomerization occurring on a time scale of several picoseconds and that the OH stretching vibrations of the substrate can serve as internal probes for the receptors flexibility. The importance of population transfer among the vibrational modes of a given binding motif and of chemical exchange between spectroscopically distinguishable binding motifs for shaping the two-dimensional infrared spectrum and its temporal evolution is discussed.

OH and NH Stretching Vibrational Relaxation of Liquid Ethanolamine

Abstract Femtosecond mid-infrared pump-probe spectroscopy was carried out to obtain information about the dynamics of vibrational energy relaxation in liquid ethanolamine at room temperature and ambient pressure. Through partial deuteration it was possible to disentangle the dynamics resulting from the OH and the NH stretching modes that proceed independently and simultaneously in the hydrogen-bonded liquid following an ultrafast vibrational excitation by a resonant mid-infrared pulse. The OH-stretching vibrational lifetime was determined to be 450 fs while the NH-stretching lifetime was found to be 1.2 ps. This large difference in lifetimes highlights the importance of the hydrogen-donating and the hydrogen-accepting character of the vibrating groups that are engaged in hydrogen-bonding.

Two-dimensional Femtosecond Infrared Spectroscopy of Hydrogen-bonded Wires

2DIR reveals frequency-dependent OH-stretching lifetimes and line broadening parameters of synthetic hydrogen-bond wires thereby reflecing uniquely conformational disorder of the supporting scaffold and the resulting wire flexibility.

Template-Substrate Dynamics Studied by 2-DIR: A Random Merry-Go-Round of Water on a Crown

Femtosecond two-dimensional infrared spectroscopy in the OH-stretching spectral region was used to unravel the ultrafast hydrogen-bond recognition dynamics within the prototypical supramolecular template-substrate complex of a water molecule and a crown ether.