Poul B. Petersen

Observation of a Zundel-like transition state during proton transfer in aqueous hydroxide solutions

It is generally accepted that the anomalous diffusion of the aqueous hydroxide ion results from its ability to accept a proton from a neighboring water molecule; yet, many questions exist concerning the mechanism for this process. What is the solvation structure of the hydroxide ion? In what way do water hydrogen bond dynamics influence the transfer of a proton to the ion? We present the results of femtosecond pump-probe and 2D infrared experiments that probe the O-H stretching vibration of a solution of dilute HOD dissolved in NaOD/D2O. Upon the addition of NaOD, measured pump-probe transients and 2D IR spectra show a new feature that decays with a 110-fs time scale. The calculation of 2D IR spectra from an empirical valence bond molecular dynamics simulation of a single NaOH molecule in a bath of H2O indicates that this fast feature is due to an overtone transition of Zundel-like H3O2− states, wherein a proton is significantly shared between a water molecule and the hydroxide ion. Given the frequency of vibration of shared protons, the observations indicate the shared proton state persists for 2–3 vibrational periods before the proton localizes on a hydroxide. Calculations based on the EVB-MD model argue that the collective electric field in the proton transfer direction is the appropriate coordinate to describe the creation and relaxation of these Zundel-like transition states.

Source for ultrafast continuum infrared and terahertz radiation.

A compact and stable method for generating high-intensity linearly polarized continuum mid-IR and terahertz light using ultrafast femtosecond (fs) laser pulses is demonstrated. Continuous light generation from <400 cm(-1) (12 THz, 25 microm) to >3300 cm(-1) (100 THz, 3 microm) in a sub-100 fs laser pulse is facilitated by nonlinear mixing of the fundamental, second harmonic, and third harmonic of an ultrafast amplified laser source through filamentation in air. Including the third harmonic in the mixing scheme leads to a tenfold increase in the generated IR power. The compact optical configuration utilizing a delay plate in a collinear geometry serves to simplify alignment and increase stability, making it a practical source for transient IR spectroscopy.

Water dimer hydrogen bond stretch, donor torsion overtone, and “in-plane bend” vibrations

We report the measurement and analysis of 64 new Ka=0←0,1 and Ka=1←0,1 transitions of (H2O)2 and 16 new Ka=0←0 transitions of (D2O)2 by terahertz laser vibration–rotation–tunneling spectroscopy of a planar supersonic expansion between 140.5 and 145.5 cm−1. The transitions in both isotopomers correspond to A′ vibrations assigned to the hydrogen bond stretch (translational) and donor torsion overtone vibrations. The interchange splitting is 56.3 GHz in Ka=0 of the excited state of (H2O)2, nearly 3 times the value of the ground state, and the bifurcation tunneling splitting is 1.8 GHz, over 2 times the value of the ground state. We compare the existing experimental spectra with calculations on state-of-the-art intermolecular potential energy surfaces and critically review the vibrational assignments reported in the literature. We show that the discrepancy between theory and experiment regarding the assignment of the feature near 103 cm−1 can be resolved by considering E2→E1 transitions, which had not been co...

Electrolytes induce long-range orientational order and free energy changes in the H-bond network of bulk water

Ions induce changes in the H-bond network of water that extend by >20 nm, vary for H2O and D2O, and lead to surface tension anomalies. Electrolytes interact with water in many ways: changing dipole orientation, inducing charge transfer, and distorting the hydrogen-bond network in the bulk and at interfaces. Numerous experiments and computations have detected short-range perturbations that extend up to three hydration shells around individual ions. We report a multiscale investigation of the bulk and surface of aqueous electrolyte solutions that extends from the atomic scale (using atomistic modeling) to nanoscopic length scales (using bulk and interfacial femtosecond second harmonic measurements) to the macroscopic scale (using surface tension experiments). Electrolytes induce orientational order at concentrations starting at 10 μM that causes nonspecific changes in the surface tension of dilute electrolyte solutions. Aside from ion-dipole interactions, collective hydrogen-bond interactions are crucial and explain the observed difference of a factor of 6 between light water and heavy water.

Ultrafast continuum mid-infrared spectroscopy: probing the entire vibrational spectrum in a single laser shot with femtosecond time resolution

Until now, ultrafast IR spectroscopy has been limited by the bandwidth of optical parametric amplifiers, typically 100-400  cm(-1). Here we present the first example of transient IR spectroscopy using a continuum laser source to probe the entire mid-IR region with ultrafast time resolution. The continuum source is based on focusing the fundamental, second harmonic, and third harmonic of 1 mJ, 25 fs, 800 nm pulses in air, generating ∼150  fs continuum mid-IR pulses that span the frequency range of <400 to >5000  cm(-1) or, conversely, <2 to >25  μm. We characterize the spectral and temporal properties of dicarbonylacetonato rhodium(I) in hexane. We further demonstrate the versatility of the method by measuring the very fast and broad (>1500  cm(-1)) spectral changes following IR excitation associated with the 7-azaindole-acetic acid heterodimer in carbon tetrachloride.

Proton Transfer in Concentrated Aqueous Hydroxide Visualized using Ultrafast Infrared Spectroscopy

While it is generally recognized that the hydroxide ion can rapidly diffuse through aqueous solution due to its ability to accept a proton from a neighboring water molecule, a description of the OH(-) solvation structure and mechanism of proton transfer to the ion remains controversial. In this report, we present the results of femtosecond infrared spectroscopy measurements of the O-H stretching transition of dilute HOD dissolved in NaOD/D(2)O. Pump-probe, photon echo peak shift, and two-dimensional infrared spectroscopy experiments performed as a function of deuteroxide concentration are used to assign spectral signatures that arise from the OH(-) ion and its solvation shell. A spectral feature that decays on a ∼110 fs time scale is assigned to the relaxation of transiently formed configurations wherein a proton is equally shared between a HOD molecule and an OD(-) ion. Over picosecond waiting times, features appear in 2D IR spectra that are indicative of the exchange of population between OH(-) ions and HOD molecules due to deuteron transfer. The construction of a spectral model that includes spectral relaxation, chemical exchange, and thermalization processes, and self-consistently treats all of our data, allows us to qualitatively explain the results of our experiments and gives a lower bound of 3 ps for the deuteron transfer kinetics.

Structure-Function Relations and Rigidity Percolation in the Shear Properties of Articular Cartilage

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Ultrafast N-H Vibrational Dynamics of Cyclic Doubly Hydrogen-Bonded Homo- and Heterodimers

F. N. Keutsch, M. G. Brown, P. B. Petersen, and R. J. Saykally, M. Geleijns, and A. van der Avoird J. Chem. Phys. 114 (9), 3994 February (2001).

Strong intermolecular vibrational coupling through cyclic hydrogen-bonded structures revealed by ultrafast continuum mid-IR spectroscopy.

Among mammalian soft tissues, articular cartilage is particularly interesting because it can endure a lifetime of daily mechanical loading despite having minimal regenerative capacity. This remarkable resilience may be due to the depth-dependent mechanical properties, which have been shown to localize strain and energy dissipation. This paradigm proposes that these properties arise from the depth-dependent collagen fiber orientation. Nevertheless, this structure-function relationship has not yet been quantified. Here, we use confocal elastography, quantitative polarized light microscopy, and Fourier-transform infrared imaging to make same-sample measurements of the depth-dependent shear modulus, collagen fiber organization, and extracellular matrix concentration in neonatal bovine articular cartilage. We find weak correlations between the shear modulus |G(∗)| and both the collagen fiber orientation and polarization. We find a much stronger correlation between |G(∗)| and the concentration of collagen fibers. Interestingly, very small changes in collagen volume fraction vc lead to orders-of-magnitude changes in the modulus with |G(∗)| scaling as (vc - v0)(ξ). Such dependencies are observed in the rheology of other biopolymer networks whose structure exhibits rigidity percolation phase transitions. Along these lines, we propose that the collagen network in articular cartilage is near a percolation threshold that gives rise to these large mechanical variations and localization of strain at the tissues surface.