Ultrafast Internal Dynamics of Flexible Hydrogen-Bonded Supramolecular Complexes

Olschewski, Martin; Knop, Stephan; Seehusen, Jaane; Lindner, Jörg; Vöhringer, Peter

doi:10.1021/jp110729d

Cited by 13 publications

(9 citation statements)

References 47 publications

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“…It could very well be that the small 5-ps component observed in the fs-MIR pump-probe experiments reported here is brought about by some small remaining population of intramolecular H-bond contacts involving the amino group as donor. More sophisticated experiments including femtosecond 2-dimensional infrared (2DIR) spectroscopy [46][47][48][49][50][51][52][53] in combination with more elaborate molecular dynamics simulations are required to solve this interesting aspect.…”

Section: Resultsmentioning

confidence: 99%

OH and NH Stretching Vibrational Relaxation of Liquid Ethanolamine

Knop

Lindner

Vöhringer³

2011

Zeitschrift Für Physikalische Chemie

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Vibrational Relaxation / Liquid Dynamics / Hydrogen-Bonding / Ultrafast SpectroscopyFemtosecond 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.

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Section: Resultsmentioning

confidence: 99%

OH and NH Stretching Vibrational Relaxation of Liquid Ethanolamine

Knop

Lindner

Vöhringer³

2011

Zeitschrift Für Physikalische Chemie

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“…2−12 Complexation typically involves "linear" H-bonds between RNH 3 + protons and the O (or N) binding sites of the crown ether that tends to adopt a pseudo-D 3d symmetry, 13−15 as in the 18C6/H 3 O + complex. 16,17 In the gas phase, the enthalpy of complexation ΔH c with 18C6 is quite high (−46 kcal/mol for the 18C6/cyclohexylammonium complex), 18 whereas in solution, it is much smaller (−1.5 kcal/mol in water and −12.0 kcal/mol in methanol for the benzylammonium cation 15 ), as generally observed with hydrophilic ions. 19 Furthermore, in solution, strong contributions from entropy terms counteract the enthalpy.…”

Section: ■ Introductionmentioning

confidence: 91%

Ammonium Recognition by 18-Crown-6 in Different Solutions and at an Aqueous Interface: A Simulation Study

Benay

Wipff

2014

J. Phys. Chem. B

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The complexation of alkylammonium RNH3(+) cations by 18-crown-6 (18C6) is studied by molecular dynamics (MD) and potential of mean force (PMF) simulations in different solvents (methanol, chloroform, 90:10 chloroform/methanol mixture, water) and at the chloroform/water interface. The free energies of association ΔGass, obtained with different charge models of 18C6, are compared for PrNH3(+) and K(+) cations yielding, with suitable electrostatic models, a preference for K(+) in the different monophasic solutions, as well as in the gas phase. Furthermore, for a given cation, ΔGass is markedly solvent dependent and decreases in magnitude in the order chloroform ≫ mixture > methanol > water, that is, with the (de)solvation energy of the cation and with the extent of pairing with the counterion (here, picrate, Pic(-)). Despite their macroscopic intermiscibilities at all proportions, chloroform and methanol are found to form, at the microscopic level, an inhomogeneous liquid that displays dual solvation properties toward its solutes. As a result, in the monophasic 90:10 mixture that contains mainly chloroform, the ΔGass energies for PrNH3(+) or K(+) complexation are closer to those in methanol than to those in chloroform. On the other hand, chloroform and water form a biphasic mixture and delineate an interface onto which 18C6 and the tBuNH3(+) and Pic(-) ions, as well as their complex, are found to adsorb, a feature also supported by the different free energy profiles for interface crossing. Interestingly, the complexation energy of tBuNH3(+) is found to be stronger at the interface than in pure water, demonstrating the crucial role of complexation by 18C6 at the interface to promote the cation transfer to the organic phase.

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“…2 with exponential solutions, i.e., a Poissonian process. Two-dimensional exchange spectroscopy measures directly the two-time-point joint probability function, and its realization in the IR spectral range can do so on very fast, picosecond timescales (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). In such an experiment, the frequency of a vibrational mode, which can distinguish the two states C or F of a molecular system, is considered to be the spectroscopic coordinate.…”

Section: Significancementioning

confidence: 99%

Testing for memory-free spectroscopic coordinates by 3D IR exchange spectroscopy

Borek

Perakis

Hamm

2014

Proc. Natl. Acad. Sci. U.S.A.

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Using 3D infrared (IR) exchange spectroscopy, the ultrafast hydrogenbond forming and breaking (i.e., complexation) kinetics of phenol to benzene in a benzene/CCl 4 mixture is investigated. By introducing a third time point at which the hydrogen-bonding state of phenol is measured (in comparison with 2D IR exchange spectroscopy), the spectroscopic method can serve as a critical test of whether the spectroscopic coordinate used to observe the exchange process is a memory-free, or Markovian, coordinate. For the system under investigation, the 3D IR results suggest that this is not the case. This conclusion is reconfirmed by accompanying molecular dynamics simulations, which furthermore reveal that the non-Markovian kinetics is caused by the heterogeneous structure of the mixed solvent.3D IR spectroscopy | solvent dynamics | ultrafast dynamics W henever one writes a chemical reaction equation, such as in the simplest case of an equilibrium between two states of a molecular systemwith the corresponding Master equation for its kineticsone implicitly assumes Markovianity. That is, it is assumed that the probability to react per time unit, e.g., from C to F, scales linearly with concentration [C] with proportionality constant k CF , but does not depend on the history of how C has been formed. When the reaction is slow in comparison with the relaxation of its environment, then this assumption is well justified. One can however think of many situations where this is not the case. For example, proteins are heterogeneous on very long timescales due to their structural complexity, so enzymatic reaction catalyzed by them might not fulfill that condition (1). Other examples are found in glass-forming liquids close to the glass transition, when their relaxation covers many orders of magnitude in time (2). When the speed of a reaction approaches the ultrafast picosecond regime, which is the topic of the present paper, then even the solvation time of normal liquids far away from any glass transition may become time-limiting. For example, it has been shown that the distance between a Na + and a Cl − ion is not a good reaction coordinate to describe ion pair dissociation in aqueous solution (3), in the sense that the distance for which the free energy peaks does not correspond with the transition state of the reaction. Hidden coordinates exist, which must be solvent coordinates because there are no other degrees of freedom. Unless these solvent coordinates have not fully relaxed and lost their memory about a reaction event, the probability for a back reaction must depend on the history. Markovianity is related to a separation of timescales between the speed of a chemical reaction of a molecular system versus that of the relaxation of its solvent. Often, a separation of timescales goes hand-in-hand with a separation of solute and solvent, but such a system-bath approach fails for ultrafast reactions.It is important to recognize that the question of Markovianity is not an intrinsic property of a chemical process, but rather is relate...

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Ultrafast Internal Dynamics of Flexible Hydrogen-Bonded Supramolecular Complexes

Cited by 13 publications

References 47 publications

OH and NH Stretching Vibrational Relaxation of Liquid Ethanolamine

OH and NH Stretching Vibrational Relaxation of Liquid Ethanolamine

Ammonium Recognition by 18-Crown-6 in Different Solutions and at an Aqueous Interface: A Simulation Study

Testing for memory-free spectroscopic coordinates by 3D IR exchange spectroscopy

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