Spectroscopy of cluster aerosol models: IR and UV spectra of hydrated glyoxylate with and without sea salt.

Glyoxylic acid is formed in the troposphere by oxidation of organic molecules. In sea salt aerosols, it is expected to be present as glyoxylate, integrated into the salt environment and strongly interacting with water molecules. In water, glyoxylate is in equilibrium with its gem-diol form. To understand the influence of water and salt on the photophysics and photochemistry of glyoxylate, we generate small model clusters containing glyoxylate by electrospray ionization and study them by Fourier-Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. We used infrared multiple photon dissociation spectroscopy and UV/vis photodissociation spectroscopy for structural characterization as well as quantum chemical calculations to model the spectra and dissociation patterns. Resonant absorption of infrared radiation leads to water evaporation, which indicates that water and glyoxylate are separate molecular entities in a significant fraction of the clusters, in line with the observed absorption of UV light in the actinic region. Hydration of glyoxylate leads to a change of the dihedral angle in the CHOCOO-·H2O complex, causing a slight redshift of the S1 ← S0 transition. However, the barriers for internal rotation are below 5 kJ mol-1, which explains the broad S1 ← S0 absorption extending from about 320 to 380 nm. Most importantly, hydration hinders dissociation in the S1 state, thus enhancing the quantum yield of fluorescence combined with water evaporation. No C-C bond photolysis is observed, but due to the limited signal-to-noise ratio, it cannot be ruled out. The quantum yield, however, will be relatively low. Fluorescence dominates the photophysics of glyoxylate embedded in the dry salt cluster, but the quantum yield shifts towards internal conversion upon addition of one or two water molecules.

Toward Detection of FeH+ in the Interstellar Medium: Infrared Multiple Photon Dissociation Spectroscopy of Ar2FeH.

The iron hydride molecular cation FeH+ is expected to be present in the interstellar medium. Because of the lack of laboratory data, it is yet to be identified in spectrally resolved astronomic observations. As a benchmark for computational predictions and to guide an experimental search for the ro-vibrational lines of FeH+, we performed infrared multiple photon dissociation (IRMPD) spectroscopy of FeH+ tagged with two argon atoms. The Fe-H stretching mode in Ar2FeH+ is observed at 1860 cm-1. Combination bands of the Fe-H stretch with the two Fe-H bending and the asymmetric Fe-Ar stretching modes are observed at 2012 cm-1, 2054 cm-1, and 2078 cm-1. Quantum chemical calculations show that the molecule has C2v symmetry. The Ar-Fe-Ar bending mode at 46 cm-1 is significantly populated at the temperature of the experiment, causing thermal broadening of the Fe-H stretch and its redshift with increasing internal energy.

Size-dependent H and H2 formation by infrared multiple photon dissociation spectroscopy of hydrated vanadium cations, V+(H2O)n, n = 3-51.

Infrared spectra of the hydrated vanadium cation (V+(H2O)n; n = 3-51) were measured in the O-H stretching region employing infrared multiple photon dissociation (IRMPD) spectroscopy. Spectral fingerprints, along with size-dependent fragmentation channels, were observed and rationalized by comparing to spectra simulated using density functional theory. Photodissociation leading to water loss was found for cluster sizes n = 3-7, consistent with isomers featuring intact water ligands. Loss of molecular hydrogen was observed as a weak channel starting at n = 8, indicating the advent of inserted isomers, HVOH+(H2O)n-1. The majority of ions for n = 8, however, are composed of two-dimensional intact isomers, concordant with previous infrared studies on hydrated vanadium. A third channel, loss of atomic hydrogen, is observed weakly for n = 9-11, coinciding with the point at which the H and H2O calculated binding energies become energetically competitive for intact isomers. A clear and sudden spectral pattern and fragmentation channel intensity at n = 12 suggest a structural change to inserted isomers. The H2 channel intensity decreases sharply and is not observed for n = 20 and 25-51. IRMPD spectra for clusters sizes n = 15-51 are qualitatively similar indicating no significant structural changes, and are thought to be composed of inserted isomers, consistent with recent electronic spectroscopy experiments.

Infrared Multiple Photon Dissociation Spectroscopy Confirms Reversible Water Activation in Mn+(H2O)n, n ≤ 8.

Controlled activation of water molecules is the key to efficient water splitting. Hydrated singly charged manganese ions Mn+(H2O)n exhibit a size-dependent insertion reaction, which is probed by infrared multiple photon dissociation spectroscopy (IRMPD) and FT-ICR mass spectrometry. The noninserted isomer of Mn+(H2O)4 is formed directly in the laser vaporization ion source, while its inserted counterpart HMnOH+(H2O)3 is selectively prepared by gentle removal of water molecules from larger clusters. The IRMPD spectra in the O-H stretch region of both systems are markedly different, and correlate very well with quantum chemical calculations of the respective species at the CCSD(T)/aug-cc-pVDZ//BHandHLYP/aug-cc-pVDZ level of theory. The calculated potential energy surface for water loss from HMnOH+(H2O)3 shows that this cluster ion is metastable. During IRMPD, the system rearranges back to the noninserted Mn+(H2O)3 structure, indicating that the inserted structure requires stabilization by hydration. The studied system serves as an atomically defined single-atom redox-center for reversible metal insertion into the O-H bond, a key step in metal-centered water activation.

Photochemical Hydrogen Evolution at Metal Centers Probed with Hydrated Aluminium Cations, Al+ (H2 O)n , n=1-10.

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s-3p excitations of Al+ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S1 to T2 takes place, and photochemical hydrogen formation starts on the T2 surface, which passes through a conical intersection, changing to T1 . On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH+ (H2 O)n-1 and are transparent in the investigated energy range. For n=4-8, a mixture of Al+ (H2 O)n and HAlOH+ (H2 O)n-1 is present in the experiment.

The software defined implantable modular platform (STELLA) for preclinical deep brain stimulation research in rodents.

Context.Long-term deep brain stimulation (DBS) studies in rodents are of crucial importance for research progress in this field. However, most stimulation devices require jackets or large head-mounted systems which severely affect mobility and general welfare influencing animals' behavior.Objective.To develop a preclinical neurostimulation implant system for long-term DBS research in small animal models.Approach.We propose a low-cost dual-channel DBS implant called software defined implantable platform (STELLA) with a printed circuit board size of Ø13 × 3.3 mm, weight of 0.6 g and current consumption of 7.6µA/3.1 V combined with an epoxy resin-based encapsulation method.Main results.STELLA delivers charge-balanced and configurable current pulses with widely used commercial electrodes. Whilein vitrostudies demonstrate at least 12 weeks of error-free stimulation using a CR1225 battery, our calculations predict a battery lifetime of up to 3 years using a CR2032. Exemplary application for DBS of the subthalamic nucleus in adult rats demonstrates that fully-implanted STELLA neurostimulators are very well-tolerated over 42 days without relevant stress after the early postoperative phase resulting in normal animal behavior. Encapsulation, external control and monitoring of function proved to be feasible. Stimulation with standard parameters elicited c-Fos expression by subthalamic neurons demonstrating biologically active function of STELLA.Significance.We developed a fully implantable, scalable and reliable DBS device that meets the urgent need for reverse translational research on DBS in freely moving rodent disease models including sensitive behavioral experiments. We thus add an important technology for animal research according to 'The Principle of Humane Experimental Technique'-replacement, reduction and refinement (3R). All hardware, software and additional materials are available under an open source license.

Photochemistry and UV/vis spectroscopy of hydrated vanadium cations, V+(H2O)n, n = 1-41, a model system for photochemical hydrogen evolution.

Photochemical hydrogen evolution provides fascinating perspectives for light harvesting. Hydrated metal ions in the gas phase are ideal model systems to study elementary steps of this reaction on a molecular level. Here we investigate mass-selected hydrated monovalent vanadium ions, with a hydration shell ranging from 1 to 41 water molecules, by photodissociation spectroscopy. The most intense absorption bands correspond to 3d-4p transitions, which shift to the red from n = 1 to n = 4, corresponding to the evolution of a square-planar complex. Additional water molecules no longer interact directly with the metal center, and no strong systematic shift is observed in larger clusters. Evolution of atomic and molecular hydrogen competes with loss of water molecules for all V+(H2O)n, n ≤ 12. For n ≥ 15, no absorptions are observed, which indicates that the cluster ensemble is fully converted to HVOH+(H2O)n-1. For the smallest clusters, the electronic transitions are modeled using multireference methods with spin-orbit coupling. A large number of quintet and triplet states is accessible, which explains the broad features observed in the experiment. Water loss most likely occurs after a series of intersystem crossings and internal conversions to the electronic ground state or a low-lying quintet state, while hydrogen evolution is favored in low lying triplet states.

Asymmetric Solvation of the Zinc Dimer Cation Revealed by Infrared Multiple Photon Dissociation Spectroscopy of Zn2+(H2O)n (n = 1-20).

Investigating metal-ion solvation-in particular, the fundamental binding interactions-enhances the understanding of many processes, including hydrogen production via catalysis at metal centers and metal corrosion. Infrared spectra of the hydrated zinc dimer (Zn2+(H2O)n; n = 1-20) were measured in the O-H stretching region, using infrared multiple photon dissociation (IRMPD) spectroscopy. These spectra were then compared with those calculated by using density functional theory. For all cluster sizes, calculated structures adopting asymmetric solvation to one Zn atom in the dimer were found to lie lower in energy than structures adopting symmetric solvation to both Zn atoms. Combining experiment and theory, the spectra show that water molecules preferentially bind to one Zn atom, adopting water binding motifs similar to the Zn+(H2O)n complexes studied previously. A lower coordination number of 2 was observed for Zn2+(H2O)3, evident from the highly red-shifted band in the hydrogen bonding region. Photodissociation leading to loss of a neutral Zn atom was observed only for n = 3, attributed to a particularly low calculated Zn binding energy for this cluster size.

Getting Ready for the Hydrogen Evolution Reaction: The Infrared Spectrum of Hydrated Aluminum Hydride-Hydroxide HAlOH+ (H2 O)n-1 , n=9-14.

Hydrated singly charged aluminum ions eliminate molecular hydrogen in a size regime from 11 to 24 water molecules. Here we probe the structure of HAlOH+ (H2 O)n-1 , n=9-14, by infrared multiple photon spectroscopy in the region of 1400-2250 cm-1 . Based on quantum chemical calculations, we assign the features at 1940 cm-1 and 1850 cm-1 to the Al-H stretch in five- and six-coordinate aluminum(III) complexes, respectively. Hydrogen bonding towards the hydride is observed, starting at n=12. The frequency of the Al-H stretch is very sensitive to the structure of the hydrogen bonding network, and the large number of isomers leads to significant broadening and red-shifting of the absorption of the hydrogen-bonded Al-H stretch. The hydride can even act as a double hydrogen bond acceptor, shifting the Al-H stretch to frequencies below those of the water bending mode. The onset of hydrogen bonding and disappearance of the free Al-H stretch coincides with the onset of hydrogen evolution.

Microsolvation of Zn cations: infrared multiple photon dissociation spectroscopy of Zn+(H2O)n (n = 2-35).

The structures, along with solvation evolution, of size-selected Zn+(H2O)n (n = 2-35) complexes have been determined by combining infrared multiple photon photodissociation (IRMPD) spectroscopy and density functional theory. The infrared spectra were recorded in the O-H stretching region, revealing varying shifts in band position due to different water binding motifs. Concordant with previous studies, a coordination number of 3 is observed, determined by the sudden appearance of a broad, red-shifted band in the hydrogen bonding region for clusters n > 3. The coordination number of 3 seems to be retained even for the larger clusters, due to incoming ligands experiencing significant repulsion from the Zn+ valence 4s electron. Evidence of spectrally distinct single- and double-acceptor sites are presented for medium-sized clusters, 4 ≤n≤ 7, however for larger clusters, n≥ 8, the hydrogen bonding region is dominated by a broad, unresolved band, indicative of the increased number of second and third coordination sphere ligands. No evidence of a solvated, six-fold coordinated Zn2+ ion/solvated electron pair is present in the spectra.

IR multiple photon dissociation spectroscopy of MO2 + (M = V, Nb, Ta).

A laser vaporization cluster source is coupled to the Fourier-transform ion cyclotron resonance mass spectrometer beamline of the free-electron laser for intracavity experiments. Gas phase metal ions and their oxides (VO2 +, NbO2 +, and TaO2 +) are formed and spectroscopically characterized using IR multiple-photon dissociation spectroscopy via loss of atomic oxygen and overcoming fragmentation energies of 3 eV-6 eV. The signal is observed for all MO2 + fundamental modes: the symmetric and anti-symmetric ν1 and ν3 stretch modes in the 900 cm-1-1000 cm-1 range and the ν2 bending mode in the 300 cm-1-450 cm-1 range. A remarkable substructure is observed for the bending vibration, which is at least partly due to the rovibrational substructure.

Evidence for lactone formation during infrared multiple photon dissociation spectroscopy of bromoalkanoate doped salt clusters.

Reaction mechanisms of organic molecules in a salt environment are of fundamental interest and are potentially relevant for atmospheric chemistry, in particular sea-salt aerosols. Here, we found evidence for lactone formation upon infrared multiple photon dissociation (IRMPD) of non-covalent bromoalkanoate complexes as well as bromoalkanoate embedded in sodium iodide clusters. The mechanism of lactone formation from bromoalkanoates of different chain lengths is studied in the gas phase with and without salt environment by a combination of IRMPD and quantum chemical calculations. IRMPD spectra are recorded in the 833-3846 cm-1 range by irradiating the clusters with tunable laser systems while they are stored in the cell of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The measurements of the binary complex Br(CH2)mCOOH·Br(CH2)mCOO- for m = 4 indicate valerolactone formation without salt environment while lactone formation is hindered for longer chain lengths. When embedded in sodium iodide clusters, butyrolactone formation from 4-bromobutyrate seems to take place already during formation of the doped clusters in the electrospray process, evidenced by the infrared (IR) signature of the lactone. In contrast, IRMPD spectra of sodium iodide clusters containing 5-bromovalerate contain signatures for both valerate as well as valerolactone. In both cases, however, a neutral fragment corresponding to the mass of valerolactone is eliminated, indicating that ring formation can be activated by IR light in the salt cluster. Quantum chemical calculations show that already complexation with one sodium ion significantly increases the barrier for lactone formation for all chain lengths. IRMPD of sodium iodide clusters doped with neutral bromoalkanoic acid molecules proceeds by elimination of HI or desorption of the intact acid molecule from the cluster.

Probing the Structural Evolution of the Hydrated Electron in Water Cluster Anions (H2O)n-, n ≤ 200, by Electronic Absorption Spectroscopy.

Electronic absorption spectra of water cluster anions (H2O)n-, n ≤ 200, at T = 80 K are obtained by photodissociation spectroscopy and compared with simulations from literature and experimental data for bulk hydrated electrons. Two almost isoenergetic electron binding motifs are seen for cluster sizes 20 ≤ n ≤ 40, which are assigned to surface and partially embedded isomers. With increasing cluster size, the surface isomer becomes less populated, and for n ≥ 50, the partially embedded isomer prevails. The absorption shifts to the blue, reaching a plateau at n ≈ 100. In this size range, the absorption spectrum is similar to that of the bulk hydrated electron but is slightly red-shifted; spectral moment analysis indicates that these clusters are reasonable model systems for hydrated electrons near the liquid-vacuum interface.

Infrared Spectroscopy of Size-Selected Hydrated Carbon Dioxide Radical Anions CO2 .- (H2 O)n (n=2-61) in the C-O Stretch Region.

Understanding the intrinsic properties of the hydrated carbon dioxide radical anions CO2 .- (H2 O)n is relevant for electrochemical carbon dioxide functionalization. CO2 .- (H2 O)n (n=2-61) is investigated by using infrared action spectroscopy in the 1150-2220 cm-1 region in an ICR (ion cyclotron resonance) cell cooled to T=80 K. The spectra show an absorption band around 1280 cm-1 , which is assigned to the symmetric C-O stretching vibration νs . It blueshifts with increasing cluster size, reaching the bulk value, within the experimental linewidth, for n=20. The antisymmetric C-O vibration νas is strongly coupled with the water bending mode ν2 , causing a broad feature at approximately 1650 cm-1 . For larger clusters, an additional broad and weak band appears above 1900 cm-1 similar to bulk water, which is assigned to a combination band of water bending and libration modes. Quantum chemical calculations provide insight into the interaction of CO2 .- with the hydrogen-bonding network.

Infrared multiple photon dissociation of cesium iodide clusters doped with mono-, di- and triglycine.

Charged cesium iodide clusters doped with mono-, di- and triglycine serve as a model system for sea salt aerosols containing biological molecules. Here, we investigate reactions of these complexes under infrared irradiation, with spectra obtained by infrared multiple photon dissociation. The cluster ions are generated via electrospray ionization and analyzed in the cell of a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Depending on the cluster size and peptide length, loss of HI or loss of a glycine unit is observed. The experimental measurements are supported by quantum chemical calculations. We show that N-H and O-H stretching modes dominate the spectrum, with large shifts depending on local interactions, namely due to interaction with iodide anions or intramolecular hydrogen bonding. Both experiment and theory indicate that several isomers are present in the experimental mixture, with different infrared fingerprints as well as dissociation pathways.

Photodissociation of Sodium Iodide Clusters Doped with Small Hydrocarbons.

Marine aerosols consist of a variety of compounds and play an important role in many atmospheric processes. In the present study, sodium iodide clusters with their simple isotope pattern serve as model systems for laboratory studies to investigate the role of iodide in the photochemical processing of sea-salt aerosols. Salt clusters doped with camphor, formate and pyruvate are studied in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) coupled to a tunable laser system in both UV and IR range. The analysis is supported by ab initio calculations of absorption spectra and energetics of dissociative channels. We provide quantitative analysis of IRMPD measurements by reconstructing one-photon spectra and comparing them with the calculated ones. While neutral camphor is adsorbed on the cluster surface, the formate and pyruvate ions replace an iodide ion. The photodissociation spectra revealed several wavelength-specific fragmentation pathways, including the carbon dioxide radical anion formed by photolysis of pyruvate. Camphor and pyruvate doped clusters absorb in the spectral region above 290 nm, which is relevant for tropospheric photochemistry, leading to internal conversion followed by intramolecular vibrational redistribution, which leads to decomposition of the cluster. Potential photodissociation products of pyruvate in the actinic region may be formed with a cross section of <2×10-20  cm2 , determined by the experimental noise level.