Solvent‐Dependent Structure of the I3− Ion Derived from Photoelectron Spectroscopy and Ab Initio Molecular Dynamics Simulations

Ab initio molecular dynamics (MD) simulations of the solvation of LiI₃ in four different solvents (water, methanol, ethanol, and acetonitrile) are employed to investigate the molecular and electronic structure of the I₃^? ion in relation to X‐ray photoelectron spectroscopy (XPS). Simulations show that hydrogen‐bond rearrangement in the solvation shell is coupled to intramolecular bond‐length asymmetry in the I₃^? ion. By a combination of charge analysis and I 4 d core‐level XPS measurements, the mechanism of the solvent‐induced distortions has been studied, and it has been concluded that charge localization mediates intermolecular interactions and intramolecular distortion. The approach involving a synergistic combination of theory and experiment probes the solvent‐dependent structure of the I₃^? ion, and the geometric structure has been correlated with the electronic structure.     

From Single Hydrogen Bonds to Extended Hydrogen‐Bond Wires: Low‐Dimensional Model Systems for Vibrational Spectroscopy of Associated Liquids

It is fair to say that if we ever wish to understand the anomalous properties of water, we need to study hydrogen bonds. Such a statement is based on statistical mechanics, which tells us how to calculate the structure and the thermodynamic properties of fluids and dense liquids from the forces between the particles. However, in the case of complex associated liquids, such calculations present a formidable—if not even insurmountable—challenge, which largely reflects our still‐limited understanding of the hydrogen‐bonding phenomenon itself. More experimental research on hydrogen‐bonded systems is required to develop a comprehensive, satisfactory theory for associated liquids. This Review gives an introduction to the latest experimental technique currently being used to study the ultrafast structural dynamics of hydrogen bonds, namely two‐dimensional infrared spectroscopy, and its applications to hydrogen‐bonded systems of systematically increasing complexity, starting from the single hydrogen bond of a diol to low‐dimensional extended networks of stereoselectively synthesized polyalcohols.     

The Temperature‐Dependent Hydrogen‐Bonding Signature of Lipids Monitored in the Far‐Infrared Domain

Phospholipids are studied by means of Fourier transform infrared (FTIR) spectroscopy in the mid‐ and far‐infrared spectral ranges, thereby establishing the hydrogen‐bonding continuum as a function of the temperature. The well‐known mid‐infrared spectrum of the phospholipid layer clearly shows a temperature‐dependent phase transition. In the far‐infrared region (from 300 to 50 cm^?1), an alternation of the interaction between the phospholipids and water molecules is found. The hydrogen‐bonding network ensemble and bound water molecules can be monitored in this spectral region. The lipid structure is found to strongly influence the intermolecular hydrogen‐bonding interplay. Thus, studies in the far‐infrared region provide significant information—at the molecular level—about the intermolecular hydrogen‐bonding signature of self‐assembled phospholipids.     

Water Confined in the Local Field of Ions

Interionic distances are shorter in concentrated ionic solutions, thus instigating the interaction and overlap of hydration shells, as ions become separated by only one or two layers of water molecules. The simultaneous interaction of water with two oppositely charged ions has, so far, only been investigated by computer simulation studies, because the isolated vibrational spectroscopic signature of these molecules remains undetected. Our combined near‐infrared spectroscopic and molecular dynamics simulation studies of alkali halide solutions present a distinct spectral feature, which is highly responsive to depletion of bulk water and merging of hydration shells. The analysis of this spectral feature demonstrates that absorption trends are in good agreement with the law of matching affinities, thus providing the first successful vibrational spectroscopic treatment of this topic. Combined with commonly observed near‐infrared bands, this feature provides a spectral pattern that describes some relevant aspects of ionic hydration.     

Ester Carbonyl Vibration as a Sensitive Probe of Protein Local Electric Field

The ability to quantify the local electrostatic environment of proteins and protein/peptide assemblies is key to gaining a microscopic understanding of many biological interactions and processes. Herein, we show that the ester carbonyl stretching vibration of two non‐natural amino acids, L ‐aspartic acid 4‐methyl ester and L ‐glutamic acid 5‐methyl ester, is a convenient and sensitive probe in this regard, since its frequency correlates linearly with the local electrostatic field for both hydrogen‐bonding and non‐hydrogen‐bonding environments. We expect that the resultant frequency–electric‐field map will find use in various applications. Furthermore, we show that, when situated in a non‐hydrogen‐bonding environment, this probe can also be used to measure the local dielectric constant (ε). For example, its application to amyloid fibrils formed by Aβ_16–22 revealed that the interior of such β‐sheet assemblies has an ε value of approximately 5.6.     

Numerical Simulation of the Effect of Solvent Viscosity on the Motions of a β‐Peptide Heptamer

This report examines the effect of a decrease in solvent viscosity on the simulated folding behaviour of a β‐peptide heptamer in methanol. Simulations of the molecular dynamics of the heptamer H‐β³‐HVal‐β³‐HAla‐β³‐HLeu‐(S,S)‐β³‐HAla(αMe)‐β³‐HVal‐β³‐HAla‐β³‐HLeu‐OH in methanol, with an explicit representation of the methanol molecules, were performed for 80 ns at various solvent viscosities. The simulations indicate that at a solvent viscosity of one third of that of methanol, only the dynamic aspects of the folding process are altered, and that the rate of folding is increased. At a viscosity of one tenth of that of methanol, insufficient statistics are obtained within the 80 ns period. We suggest that 80 ns is an insufficient time to reach conformational equilibrium at very low viscosity because the dependence of the folding rate of a β‐peptide on solvent viscosity has two regimes; a result that was observed in another computational study for α‐peptides.     

What Far‐Infrared Spectra Can Contribute to the Development of Force Fields for Ionic Liquids Used in Molecular Dynamics Simulations

Symbiosis: Far‐infrared spectra can be used to check the quality of force fields for molecular dynamics simulations of ionic liquids. On the other hand, MD simulations can explain the molecular basis of measured properties for this new liquid material (see picture).

     

Molecular Reorientational Dynamics of the Neat Ionic Liquid 1‐Butyl‐3‐methylimidazolium Hexafluorophosphate by Measurement of 13C Nuclear Magnetic Relaxation Data

The reorientational dynamics of the ionic liquid 1butyl‐3‐methylimidazolium hexafluorophosphate ([BMIM]PF₆) were studied over a wide range of temperatures by measurement of ¹³C spin–lattice relaxation rates and NOE factors. The reorientational dynamics were evaluated by performing fits to the experimental relaxation data. Thus, the overall reorientational motion was described by a Cole–Davidson spectral density with a Vogel–Fulcher–Tammann temperature dependence of the correlation times. The reorientational motion of the butyl chain was modelled by a combination of the latter model for the overall motion with a Bloembergen–Purcell–Pound spectral density and an Arrhenius temperature dependence for the internal motion. Except for C2 in the aromatic ring, an additional reduction of the spectral density by the Lipari–Szabo model had to be employed. This reduction is a consequence of fast molecular motions before the rotational diffusion process becomes effective. The C2 atom did not exhibit this reduction, because the librational motion of the corresponding C2? H vector is severely hindered due to hydrogen bonding with the hexafluorophosphate anion. The observed dynamic features of the [BMIM]⁺ cation confirm quantum‐chemical structures obtained in a former study.     

The Strained Sesquiterpene β‐Caryophyllene as a Probe for the Solvent‐Assisted Epoxidation Mechanism

In our attempt to synthesize β‐caryophyllene oxide in food‐compatible conditions, we observed the uncatalyzed and highly selective epoxidation of β‐caryophyllene, a strained bicyclic sesquiterpene, in ethanol with aqueous H₂O₂ under radical‐suppressing conditions without the addition of a catalyst. The unusual reactivity of β‐caryophyllene allowed us to use it as a probe for the mechanism of the solvent‐assisted epoxidation in a wide range of organic solvents. A kinetic study was performed to investigate the epoxidation mechanism; an excellent correlation was found between the observed epoxidation rates in different solvents and the Abraham’s hydrogen bond formation parameters of these solvents. By means of computational analysis, it was found that the main role of the solvent consists of the stabilization of the elongated O?O bond of H₂O₂ in the transition state through hydrogen‐bond donation to the leaving OH moiety of H₂O₂. α‐Humulene was found to possess similar reactivity as β‐caryophyllene whereas isocaryophyllene—the unstrained isomer of β‐caryophyllene—was unreactive.     

Local Disorder in Hydrogen Storage Compounds: The Case of Lithium Amide/Imide

Amides and imides of alkali metals are a very promising class of materials for use as a hydrogen‐storage system, as they are able to store and release hydrogen via a chemical route at controllable temperatures and pressures. We critically revise the present picture of the atomic structure of the lightest member (LiNH₂/Li₂NH) by using a combined computational and experimental approach. Specifically, ab initio path integral molecular dynamics simulations and solid‐state ¹H NMR techniques are combined. The results show that the presently assumed local structure might be inconsistent or at least incomplete and needs considerable revision. In particular, the Li atoms turn out to be more mobile and more disordered than suggested by structural data obtained from X‐ray scattering. Also, the configuration of the hydrogen atoms, which is accessible via the NMR experiment and the corresponding first‐principles calculations, is different from the previously assumed data. The computed and experimentally observed ¹H NMR parameters are in very good mutual agreement and illustrate the unusual chemical environment of the hydrogen atoms in this system. Incorporating our results on the new lithium data, we show that the effect of nuclear quantum delocalization for the hydrogen atoms is considerably reduced compared to the perfect crystal structure.     

Molecular Recognition by a Short Partial Peptide of the Adrenergic Receptor: A Bottom‐Up Approach

Receptor–neurotransmitter molecular recognition is key for neurotransmission. Although crystal structures of the receptors are known, the mechanism for recognition is not clear. Reported here is the ultraviolet (UV) and infrared (IR) spectra of complexes between a partial peptide (SIVSF), mimicking the binding motif of a catechol ring in the adrenergic receptor, and various ligands. The UV spectra show that two isomers coexist in the complex of SIVSF with properly recognized ligands, such as protonated adrenaline (adrenalineH⁺). From IR spectra, they are assigned to catechol‐ and amino‐bound structures. The catechol‐bound structure is not observed when the ligand is replaced by nonproper molecules, such as noradrenalineH⁺. The results suggest that SIVSF not only recognizes the catechol ring but can distinguish differences in the amine side chain. The method provides a new possibility for screening molecules as potential therapeutics for activating the receptor.     

Solvent Migration in Microhydrated Aromatic Aggregates: Ionization‐Induced Site Switching in the 4‐Aminobenzonitrile–Water Cluster

The dependence of the preferred microhydration sites of 4‐aminobenzonitrile (4ABN) on electronic excitation and ionization is determined through IR spectroscopy of its clusters with water (W) in a supersonic expansion and through quantum chemical calculations. IR spectra of neutral 4ABN and two isomers of its hydrogen‐bonded (H‐bonded) 4ABN–W complexes are obtained in the ground and first excited singlet states (S₀, S₁) through IR depletion spectroscopy associated with resonance‐enhanced multiphoton ionization. Spectral analysis reveals that electronic excitation does not change the H‐bonding motif of each isomer, that is, H₂O binding either to the CN or the NH site of 4ABN, denoted as 4ABN–W(CN) and 4ABN–W(NH), respectively. The IR spectra of 4ABN⁺–W in the doublet cation ground electronic state (D₀) are measured by generating them either in an electron ionization source (EI‐IR) or through resonant multiphoton ionization (REMPI‐IR). The EI‐IR spectrum shows only transitions of the most stable isomer of the cation, which is assigned to 4ABN⁺–W(NH). The REMPI‐IR spectrum obtained through isomer‐selective resonant photoionization of 4ABN–W(NH) is essentially the same as the EI‐IR spectrum. The REMPI‐IR spectrum obtained by ionizing 4ABN–W(CN) is also similar to that of the 4ABN⁺–W(NH) isomer, but differs from that calculated for 4ABN⁺–W(CN), indicating that the H₂O ligand migrates from the CN to the NH site upon ionization with a yield of 100 %. The mechanism of this CN→NH site‐switching reaction is discussed in the light of the calculated potential energy surface and the role of intracluster vibrational energy redistribution.     

Proton/Hydrogen‐Transfer Coordinate of 2,5‐Dihydroxybenzoic Acid Investigated in a Supersonic Beam: Combined IR/UV Spectroscopy in the S0, S1, and D0 States

As a model system for intramolecular proton/hydrogen‐transfer coordinates, the structure of 2,5‐dihydroxybenzoic acid is investigated for the ground, first electronically excited and also the ionic state. Combined IR/UV spectroscopy in molecular‐beam experiments is applied and the experimental results are interpreted by the application of DFT and CASPT2 methods. No proton or hydrogen transfer is observed, but evidence is given for a hydrogen dislocation of the intramolecular hydrogen bond in the S₁ state and to lesser extent in the D₀ state. To obtain direct information on the proton/hydrogen‐transfer coordinate, IR spectra are recorded both in the region of the OH and especially the CO stretching vibrations by also applying two new variants of combined IR/UV spectroscopy for the S₁ and D₀ states. The CO groups are directly involved in the hydrogen bond and, in contrast to the hydrogen‐bonded OH groups, the CO stretching frequencies can be observed in all electronic states.     

Defective Continuous Hydrogen‐Bond Networks: An Alternative Interpretation of IR Spectroscopy

We apply our previously developed deconvolution method and interpretation to analyze changes in the OH stretching band [ν(OH) band] of low‐concentration (≤0.2 m) aqueous solutions of NaCl and KCl. We treat these simple, monovalent ions as defects in the hydrogen‐bond network of pure water and quantify the changes in the spectra at low defect concentration with an “order parameter”. Order‐parameter analysis of difference spectra of the two solutions leads to hydration numbers of 7.0±1.0 and 5.9±0.3 for K⁺ and Na⁺, respectively. Additionally, we find that changes in the ν(OH) band due to low concentrations of ions result from changes in the topology of the hydrogen‐bond network.     

The Role of Hydrogen‐Bonding Interactions in the Ultrafast Relaxation Dynamics of the Excited States of 3‐ and 4‐Aminofluoren‐9‐ones

The dynamics of the excited states of 3‐ and 4‐aminofluoren‐9‐ones (3AF and 4AF, respectively) are investigated in different kinds of solvents by using a subpicosecond time‐resolved absorption spectroscopic technique. They undergo hydrogen‐bonding interaction with protic solvents in both the ground and excited states. However, this interaction is more significant in the lowest excited singlet (S₁) state because of its substantial intramolecular charge‐transfer character. Significant differences in the spectroscopic characteristics and temporal dynamics of the S₁ states of 3AF and 4AF in aprotic and protic solvents reveal that the intermolecular hydrogen‐bonding interaction between the S₁ state and protic solvents plays an important role in its relaxation process. Perfect linear correlation between the relaxation times of the S₁ state and the longitudinal relaxation times (τ_L) of alcoholic solvents confirms the prediction regarding the solvation process via hydrogen‐bond reorganization. In the case of weakly interacting systems, the relaxation process can be well described by a dipolar solvation‐like process involving rotation of the OH groups of the alcoholic solvents, whereas in solvents having a strong hydrogen‐bond‐donating ability, for example, methanol and trifluoroethanol, it involves the conversion of the non‐hydrogen‐bonded form to the hydrogen‐bonded complex of the S₁ state. Efficient radiationless deactivation of the S₁ state of the aminofluorenones by protic solvents is successfully explained by the energy‐gap law, by using the energy of the fully solvated S₁ state determined from the time‐resolved spectroscopic data.