Katrin Adamczyk

Real-Time Observation of Carbonic Acid Formation in Aqueous Solution

A Glimpse of Wet Carbonic Acid Both carbon dioxide and bicarbonate play extraordinarily widespread roles in biochemical and geochemical reactions. It is surprising therefore that carbonic acid, the intermediate in the water-coupled interconversion of these two compounds, has never been directly characterized in aqueous solution. Adamczyk et al. (p. 1690, published online 12 November) have succeeded in glimpsing the elusive acid by inducing an aqueous photoacid (a compound rendered transiently more acidic upon light absorption) to react with dissolved bicarbonate. Using infrared spectroscopy, they show that the carbonic acid product persists for nanoseconds. Analysis of its formation kinetics affords a direct pKa value of 3.5, substantially lower than the effective value derived from observations of the net bicarbonate/carbon dioxide equilibrium. The use of a photoacid enables the long-sought characterization of the conjugate acid of bicarbonate. Despite the widespread importance of aqueous bicarbonate chemistry, its conjugate acid, carbonic acid, has remained uncharacterized in solution. Here we report the generation of deuterated carbonic acid in deuterium oxide solution by ultrafast protonation of bicarbonate and its persistence for nanoseconds. We follow the reaction dynamics upon photoexcitation of a photoacid by monitoring infrared-active marker modes with femtosecond time resolution. By fitting a kinetic model to the experimental data, we directly obtain the on-contact proton-transfer rate to bicarbonate, previously inaccessible with the use of indirect methods. A Marcus free-energy correlation supports an associated pKa (Ka is the acid dissociation constant) of 3.45 ± 0.15, which is substantially lower than the value of 6.35 that is commonly assumed on the basis of the overall carbon dioxide–to–bicarbonate equilibrium. This result should spur further exploration of acid-base reactivity in carbon dioxide–rich aqueous environments such as those anticipated under sequestration schemes.

Real-Time Observation of the Formation of Excited Radical Ions in Bimolecular Photoinduced Charge Separation: Absence of the Marcus Inverted Region Explained

Unambiguous evidence for the formation of excited ions upon ultrafast bimolecular photoinduced charge separation is found using a combination of femtosecond time-resolved fluorescence up-conversion, infrared and visible transient absorption spectroscopy. The reaction pathways are tracked by monitoring the vibrational energy redistribution in the product after charge separation and subsequent charge recombination. For moderately exergonic reactions, both donor and acceptor are found to be vibrationally hot, pointing to an even redistribution of the energy dissipated upon charge separation and recombination in both reaction partners. For highly exergonic reactions, the donor is very hot, whereas the acceptor is mostly cold. The asymmetric energy redistribution is due to the formation of the donor cation in an electronic excited state upon charge separation, confirming one of the hypotheses for the absence of the Marcus inverted region in photoinduced bimolecular charge separation processes.

Multiple-timescale photoreactivity of a model compound related to the active site of [FeFe]-hydrogenase.

Ultraviolet (UV) photolysis of (mu-S(CH 2) 3S)Fe 2(CO) 6 ( 1), a model compound of the Fe-hydrogenase enzyme system, has been carried out. When ultrafast UV-pump infrared (IR)-probe spectroscopy, steady-state Fourier transform IR spectroscopic methods, and density functional theory simulations are employed, it has been determined that irradiation of 1 in an alkane solution at 350 nm leads to the formation of two isomers of the 16-electron complex (mu-S(CH 2) 3S)Fe 2(CO) 5 within 50 ps with evidence of a weakly associated solvent adduct complex. 1 is subsequently recovered on timescales covering several minutes. These studies constitute the first attempt to study the photochemistry and reactivity of these enzyme active site models in solution following carbonyl ligand photolysis.

On the role of hydrogen bonds in photoinduced electron-transfer dynamics between 9-fluorenone and amine solvents.

Using ultrafast fluorescence upconversion and mid-infrared spectroscopy, we explore the role of hydrogen bonds in the photoinduced electron transfer (ET) between 9-fluorenone (FLU) and the solvents trimethylamine (TEA) and dimethylamine (DEA). FLU shows hydrogen-bond dynamics in the methanol solvent upon photoexcitation, and similar effects may be anticipated when using DEA, whereas no hydrogen bonds can occur in TEA. Photoexcitation of the electron-acceptor dye molecule FLU with a 400 nm pump pulse induces ultrafast ET from the amine solvents, which is followed by 100 fs IR probe pulses as well as fluorescence upconversion, monitoring the time evolution of marker bands of the FLU S(1) state and the FLU radical anion, and an overtone band of the amine solvent, marking the transient generation of the amine radical cation. A comparison of the experimentally determined forward charge-separation and backward charge-recombination rates for the FLU-TEA and FLU-DEA reaction systems with the driving-force dependencies calculated for the forward and backward ET rates reveals that additional degrees of freedom determine the ET reaction dynamics for the FLU-DEA system. We suggest that hydrogen bonding between the DEA molecules plays a key role in this behaviour.

Light-Induced Radical Formation and Isomerization of an Aromatic Thiol in Solution Followed by Time-Resolved X-ray Absorption Spectroscopy at the Sulfur K-Edge

We applied time-resolved sulfur-1s absorption spectroscopy to a model aromatic thiol system as a promising method for tracking chemical reactions in solution. Sulfur-1s absorption spectroscopy allows tracking multiple sulfur species with a time resolution of ∼70 ps at synchrotron radiation facilities. Experimental transient spectra combined with high-level electronic structure theory allow identification of a radical and two thione isomers, which are generated upon illumination with 267 nm radiation. Moreover, the regioselectivity of the thione isomerization is explained by the resulting radical frontier orbitals. This work demonstrates the usefulness and potential of time-resolved sulfur-1s absorption spectroscopy for tracking multiple chemical reaction pathways and transient products of sulfur-containing molecules in solution.

Ultrafast Protonation of Cyanate Anion in Aqueous Solution

We use femtosecond infrared spectroscopy to study the aqueous protonation dynamics of cyanate, OCN−, using a photoacid, 2-naphthol-6,8-disulfonate (2N-6,8S) excited by a 60-fs pulse tuned at 336 nm. The transient response in the spectral range of the cyano-stretching vibrational marker modes of cyanic acid, HOCN, and isocyanic acid, HNCO, reveals how much of both reaction products are formed at early delay times, and whether the on-contact reactive complex between 2N-6,8S and OCN− has a well-defined structure. Using the Szabo-Collins-Kimball approach to describe bimolecular reaction dynamics subject to the Debye-von Smoluchowski diffusional motions, an on-contact proton transfer reaction rate is derived that follows the correlation between free energy and reaction rates found for a large class of aqueous proton transfer of photoacid dissociation and photoacid-base neutralization reactions.

UV-Photochemistry of the Disulfide Bond: Evolution of Early Photoproducts from Picosecond X-ray Absorption Spectroscopy at the Sulfur K-Edge

We have investigated dimethyl disulfide as the basic moiety for understanding the photochemistry of disulfide bonds, which are central to a broad range of biochemical processes. Picosecond time-resolved X-ray absorption spectroscopy at the sulfur K-edge provides unique element-specific insight into the photochemistry of the disulfide bond initiated by 267 nm femtosecond pulses. We observe a broad but distinct transient induced absorption spectrum which recovers on at least two time scales in the nanosecond range. We employed RASSCF electronic structure calculations to simulate the sulfur-1s transitions of multiple possible chemical species, and identified the methylthiyl and methylperthiyl radicals as the primary reaction products. In addition, we identify disulfur and the CH2S thione as the secondary reaction products of the perthiyl radical that are most likely to explain the observed spectral and kinetic signatures of our experiment. Our study underscores the importance of elemental specificity and the potential of time-resolved X-ray spectroscopy to identify short-lived reaction products in complex reaction schemes that underlie the rich photochemistry of disulfide systems.

Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates

We present a setup combining a liquid flatjet sample delivery and a MHz laser system for time-resolved soft X-ray absorption measurements of liquid samples at the high brilliance undulator beamline UE52-SGM at Bessy II yielding unprecedented statistics in this spectral range. We demonstrate that the efficient detection of transient absorption changes in transmission mode enables the identification of photoexcited species in dilute samples. With iron(II)-trisbipyridine in aqueous solution as a benchmark system, we present absorption measurements at various edges in the soft X-ray regime. In combination with the wavelength tunability of the laser system, the set-up opens up opportunities to study the photochemistry of many systems at low concentrations, relevant to materials sciences, chemistry, and biology.

Aqueous Proton Transfer Pathways in Bimolecular Acid-Base Neutralization

We expand the classic Eigen-Weller reaction model with solvent-switch pathways, mediating proton transfer between acids and bases, having one or several water molecules, activated by the solvent and controlled by the base strength.