Deuterium and Hydrogen Tunneling in the Hydrogenation of 4‐Oxocyclohexa‐2,5‐dienylidene

4‐Oxocyclohexa‐2,5‐dienylidene is a highly reactive triplet ground state carbene that is hydrogenated in solid H₂, HD, and D₂ at temperatures as low as 3 K. The mechanism of the insertion of the carbene into dihydrogen was investigated by IR and EPR spectroscopy and by kinetic studies. H or D atoms were observed as products of the reaction with H₂ and D₂, respectively, whereas HD produces exclusively D atoms. The hydrogenation shows a very large kinetic isotope effect and remarkable isotope selectivity, as was expected for a tunneling reaction. The experiments, therefore, provide clear evidence for both hydrogen tunneling and the rare deuterium tunneling in an intermolecular reaction.     

Hydrogen Generation by Water Reduction with [Cp*2Ti(OTf)]: Identifying Elemental Mechanistic Steps by Combined In Situ FTIR and In Situ EPR Spectroscopy Supported by DFT Calculations

A detailed mechanism of hydrogen production by reduction of water with decamethyltitanocene triflate [Cp*₂Ti^III(OTf)] has been derived for the first time, based on a comprehensive in situ spectroscopic study including EPR and ATR‐FTIR spectroscopy supported by DFT calculations. It is demonstrated that two H₂O molecules coordinate to [Cp*₂Ti^III(OTf)] subsequently forming [Cp₂*Ti^III(H₂O)(OTf)] and [Cp*Ti^III(H₂O)₂(OTf)]. Triflate stabilizes the water ligands by hydrogen bonding. Liberation of hydrogen proceeds only from the diaqua complex [Cp*Ti^III(H₂O)₂(OTf)] and involves, most probably, abstraction and recombination of two H atoms from two molecules of [Cp*Ti^III(H₂O)₂(OTf)] in close vicinity, which is driven by the formation of a strong covalent Ti? OH bond in the resulting final product [Cp*₂Ti^IV(OTf)(OH)].     

Ultrafast Structural Fluctuations of Myoglobin‐Bound Thiocyanate and Selenocyanate Ions Measured with Two‐Dimensional Infrared Photon Echo Spectroscopy

Structural dynamics within the distal cavity of myoglobin protein is investigated using 2D‐IR and IR pump–probe spectroscopy of the N≡C stretch modes of heme‐bound thiocyanate and selenocyanate ions. Although myoglobin‐bound thiocyanate group shows a doublet in its IR absorption spectrum, no cross peaks originating from chemical exchange between the two components are observed in the time‐resolved 2D IR spectra within the experimental time window. Frequency–frequency correlation functions of the two studied anionic ligands are obtained by means of a few different analysis approaches; these functions were then used to elucidate the differences in structural fluctuation around ligand, ligand–protein interactions, and the degree of structural heterogeneity within the hydrophobic pocket of these myoglobin complexes.     

Control of the Transition between Ni‐C and Ni‐SIa States by the Redox State of the Proximal FeS Cluster in the Catalytic Cycle of [NiFe] Hydrogenase

[NiFe] hydrogenase catalyzes the reversible cleavage of H₂. The electrons produced by the H₂ cleavage pass through three Fe–S clusters in [NiFe] hydrogenase to its redox partner. It has been reported that the Ni‐SI_a, Ni‐C, and Ni‐R states of [NiFe] hydrogenase are involved in the catalytic cycle, although the mechanism and regulation of the transition between the Ni‐C and Ni‐SI_a states remain unrevealed. In this study, the FT‐IR spectra under light irradiation at 138–198 K show that the Ni‐L state of [NiFe] hydrogenase is an intermediate between the transition of the Ni‐C and Ni‐SI_a states. The transition of the Ni‐C state to the Ni‐SI_a state occurred when the proximal [Fe₄S₄]_p^2+/+ cluster was oxidized, but not when it was reduced. These results show that the catalytic cycle of [NiFe] hydrogenase is controlled by the redox state of its [Fe₄S₄]_p^2+/+ cluster, which may function as a gate for the electron flow from the NiFe active site to the redox partner.     

When Like Charged Ions Attract in Ionic Liquids: Controlling the Formation of Cationic Clusters by the Interaction Strength of the Counterions

The properties of ionic liquids are described by a subtle balance between Coulomb interaction, hydrogen bonding, and dispersion forces. We show that lowering the attractive Coulomb interaction by choosing weakly coordinating anions leads to the formation of cationic clusters. These clusters of like‐charged ions are stabilized by cooperative hydrogen bonding and controlled by the interaction potential of the anion. IR and NMR spectroscopy combined with computational methods are used to detect and characterize these unusual, counter‐intuitively formed clusters. They can be only observed for weakly coordinating anions. When cationic clusters are formed, cyclic tetramers are particularly stable. Therein, cooperative hydrogen‐bond attraction can compete with like‐charge repulsion. We present a simple but effective spectroscopic scale for the possibility of like‐charge attraction in ionic liquids, based on IR and NMR signatures.     

Spectroscopic and Computational Insight into the Intermolecular Interactions between Zwitter‐Type Ionic Liquids and Water Molecules

Geometric and conformational changes of zwitter‐type ionic liquids (ZILs) due to hydrogen‐bonding interactions with water molecules are investigated by density functional theory (DFT), two‐dimensional IR correlation spectroscopy (2D IR COS), and pulsed‐gradient spin‐echo NMR (PGSE NMR). Simulation results indicate that molecular structures in the optimized states are strongly influenced by hydrogen bonding of water molecules with the sulfonate group or imidazolium and pyrrolidinium rings of 3‐(1‐methyl‐3‐imidazolio)propanesulfonate ( 1 ) and 3‐(1‐methyl‐1‐pyrrolidinio)propanesulfonate ( 2 ), respectively. Concentration‐dependent 2D IR COS reveals kinetic conformational changes of the two ZIL–H₂O systems attributable to intermolecular interactions, as well as the interactions of sulfonate groups and imidazolium or pyrrolidinium rings with water molecules. The dramatic changes in the ¹H self‐diffusion coefficients elucidate the formation of proton‐conduction pathways consisting of ZIL networks. In ZIL domains, protons are transferred by a Grotthuss‐type mechanism through formation, breaking, and restructuring of bonds between ZILs and H₂O, leading to an energetically favorable state. The simulation and experimental investigations delineated herein provide a perspective to understanding the interactions with water from an academic point of view as well as to designing ILs with desired properties from the viewpoint of applications.     

Dissecting Noncovalent Interactions in Carboxyl-Functionalized Ionic Liquids Exhibiting Double and Single Hydrogens Bonds Between Ions of Like Charge

We show that the carboxyl-functionalized ionic liquid 1-(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC-CH₂-py][NTf₂] exhibits three types of hydrogen bonding: the expected single hydrogen bonds between cation and anion, and, surprisingly, single and double hydrogen bonds between the cations, despite the repulsive Coulomb forces between the ions of like charge. Combining X-ray crystallography, differential scanning calorimetry, IR spectroscopy, thermodynamic methods and DFT calculations allows the analysis and characterization of all types of hydrogen bonding present in the solid, liquid and gaseous states of the ionic liquid (IL). We find doubly hydrogen bonded cationic dimers (c⁺=c⁺) in the crystalline phase. With increasing temperature, this binding motif opens in the liquid and is replaced by (c⁺−c⁺−a⁻ species, with a remaining single cationic hydrogen bond and an additional hydrogen bond between cation and anion. We provide clear evidence that the IL evaporates as hydrogen-bonded ion pairs (c⁺−a⁻) into the gas phase. The measured transition enthalpies allow the noncovalent interactions to be dissected and the hydrogen bond strength between ions of like charge to be determined.     

Photocatalytic Removal of Soot: Unravelling of the Reaction Mechanism by EPR and in situ FTIR Spectroscopy

Photocatalytic soot oxidation is studied on P25 TiO₂ as an important model reaction for self‐cleaning processes by means of electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopy. Contacting of carbon black with P25 leads on the one hand to a reduction of the local dioxygen concentration in the powder. On the other hand, the weakly adsorbed radicals on the carbon particles are likely to act as alternative traps for the photogenerated conduction‐band electrons. We find furthermore that the presence of dioxygen and oxygen‐related radicals is vital for the photocatalytic soot degradation. The complete oxidation of soot to CO₂ is evidenced by in situ FTIR spectroscopy, no intermediate CO is detected during the photocatalytic process.     

Hydrogen-Bond Structure and Dynamics of TADDOL Asymmetric Organocatalysts Correlate with Catalytic Activity

The catalytic efficiency of diol-based organocatalysts has been shown to strongly depend on the diols molecular structure including hydrogen-bonding, yet, the underlying molecular-level origins have remained elusive. Herein a study on the inter- and intramolecular hydrogen-bonding of two isomeric diol-based catalysts (TADDOLs) in solution is presented: 1-Naphthyl substituted TADDOL (1nTADDOL), which exhibits high catalytic efficiency, and 2-naphthyl substituted TADDOL (2nTADDOL), which is a poor catalyst. Using nuclear magnetic resonance and infrared spectroscopy, comparable hydrogen-bond strengths for both TADDOLs in solution were found, however, significantly slower bonding dynamics for 1nTADDOL. In aromatic solvents, 1nTADDOL forms less, but longer-lived, intermolecular OH⋅⋅⋅π bonds to solvent molecules, as compared to 2nTADDOL. Thus, rather than previously suggested differences in intermolecular hydrogen-bonding strengths, the results suggest that the hydrogen-bonding kinetics and entropies differ for both TADDOLs, which also explains their vastly different catalytic activities.     

Ions with a strong symmetrical H bond in the HCl−BuiOH system by IR spectroscopy data

Ion-molecular interactions in the HCl−BuⁱOH system with different compositions (from neat isobutyl alcohol to 37 mol.% HCl) were studied by Multiple Attenuated Total Reflectance (MATR) IR spectroscopy at 30 °C. Proton disolvates (Buⁱ(H)O…H…O(H)Buⁱ)⁺ with strong symmetrical H bonds are formed upon the addition of HCl to BuⁱOH. At high concentrations of HCl (C ⁰ _HCl>33 mol.%), (Cl…H…Cl)⁻ ions are formed along with (BuⁱOH)₂H⁺. The spectra of positively and negatively charged proton disolvates were compared to those of similar ions in the HCl−PrⁱOH and HCl−MeOH systems. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2496–2500, December, 1998.     

Spectroscopic Evidence for Clusters of Like‐Charged Ions in Ionic Liquids Stabilized by Cooperative Hydrogen Bonding

Direct spectroscopic evidence for hydrogen‐bonded clusters of like‐charged ions is reported for ionic liquids. The measured infrared O?H vibrational bands of the hydroxyethyl groups in the cations can be assigned to the dispersion‐corrected DFT calculated frequencies of linear and cyclic clusters. Compensating the like‐charge Coulomb repulsion, these cationic clusters can range up to cyclic tetramers resembling molecular clusters of water and alcohols. These ionic clusters are mainly present at low temperature and show strong cooperative effects in hydrogen bonding. DFT‐D3 calculations of the pure multiply charged clusters suggest that the attractive hydrogen bonds can compete with repulsive Coulomb forces.     

Ions with a strong symmetric H-bond in solutions of sodium acetate in acetic acid

The multiple attenuated total reflection IR spectra of solutions of sodium acetate in acetic acid have been recorded in the range from 9000 to 4000 cm^–1. The CH₃COO^– anion and an acid molecule form be complex (CH₃COO...H...OOCCH₃) with a strong symmetric H-bond.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp, 1854–1857, July, 1996.     

Molecular Vibrations of an Oxygen-Evolving Complex and Its Synthetic Mimic

Bio-inspired catalysis for artificial photosynthesis has been widely studied for decades, in particular, with the purpose of using bio-disposable and non-toxic metals as building blocks. The characterisation of such catalysts has been achieved by using different kinds of spectroscopic methods, from X-ray crystallography to NMR spectroscopy. An artificial Mn₄CaO₄ cubane cluster with dangling Mn₄ was synthesised in 2015 [Zhang et al. Science 2015 , 348, 690–693]; this cluster showed many structural similarities to that of the natural oxygen-evolving complex. An accurate structural and spectroscopic comparison between the natural and artificial systems is highly relevant to understand the catalytic mechanism. Among data from different techniques, the differential FTIR spectra (S_n+1−S_n) of photosystem II are still lacking a complete interpretation. The availability of IR data of the artificial cluster offers a unique opportunity to assign absolute absorption spectra on a well-defined and easier to interpret analogous moiety. The present work aims to investigate the novel inorganic compound as a model system for an oxygen-evolving complex through measurement of its spectroscopic properties. The experimental results are compared with calculations by using a variety of theoretical methods (normal mode analysis, effective normal mode analysis) in the S₁ state. We underline the similarities and the differences in the computational spectra based on atomistic models of Mn₄CaO₅ and Mn₄CaO₄ complexes.     

Generation and Identification of the Linear OCBNO and OBNCO Molecules with 24 Valence Electrons

Two structural isomers containing five second-row element atoms with 24 valence electrons were generated and identified by matrix-isolation IR spectroscopy and quantum chemical calculations. The OCBNO complex, which is produced by the reaction of boron atoms with mixtures of carbon monoxide and nitric oxide in solid neon, rearranges to the more stable OBNCO isomer on UV excitation. Bonding analysis indicates that the OCBNO complex is best described by the bonding interactions between a triplet-state boron cation with an electron configuration of (2s)⁰(2p_σ)⁰(2p_π)² and the CO/NO⁻ ligands in the triplet state forming two degenerate electron-sharing π bonds and two ligand-to-boron dative σ bonds.