Indirect Nuclear Spin-Spin Exchange Coupling through Solvated Electron in Small Sizes of Cyclic and Cage-Shaped Perfluoroalkanes

Small perfluorocycloalkanes (hexafluorocyclopropane (c-C₃F₆), octafluorocyclobutane (c-C₄F₈) and decafluorocyclopentane (c-C₅F₁₀) and cage-shaped perfluoroalkanes (perfluoro tetrahedral alkane (C₄F₄), perfluoro prismane (C₆F₆) and perfluoro cubane (C₈F₈)) are better electron scavengers. The captured excess electrons are weakly bound inside their backbone voids or over their backbones, forming the solvated electron ( ) systems (e@c-C_nF_2ns (n=3, 4, 5) and e@C_nF_n (n=4, 6, 8)). There have been many studies on the structures and properties of such systems. However, the effect of on the indirect nuclear spin-spin coupling (J-coupling) is unknown. In this work, we explore how affects ^NeJ-coupling between two coupled F nuclei (^NeJ_FF-coupling) in perfluoroalkane systems through density functional theory calculations. We find unusual trans-^NeJ_FF-couplings (two coupled F nuclei in trans-position) in e@c-C_nF_2n (n=3, 4, 5) and ^NeJ_FF-couplings in e@C_nF_n (n=4, 6, 8). One excess electron not only changes the molecular structures, but also enforces unique distributions and properties, depending on the structural characteristics. We also confirm that such unusual ^NeJ_FF-couplings are realized by through- (T-SE) transmission mechanism, rather than the conventional through-bonds (T−B)/through-space (T−S) ones. The novel transmission mechanism consists of the T-SE coupling path (path 1) and -enhanced T−B T−S coupling path (path 2), and the two paths jointly control ^NeJ_FF through cooperation and competition. Interestingly, the former plays a dominant role for long-range ^NeJ_FF-coupling (N=5), while the latter plays a role in the short-range ^NeJ_FF-coupling (N=3, 4). Path bending angle mainly influences path 1, while path 2 is mainly influenced by the path length. This work not only provides novel insights into the mediating role of in the coupling information exchange, but also proposes a new -based coupling mechanism, possibly opening up potential applications for the -based indirect nuclear spin couplings.     

The Threshold Photoelectron Spectrum of Fulvenone: A Reactive Ketene Derivative in Lignin Valorization

Unveiling reaction mechanisms by isomer-selective detection of reactive intermediates requires advanced spectroscopic knowledge. We study the photoionization of fulvenone (c-C₅H₄=C=O), a reactive ketene species relevant in catalytic pyrolysis of lignin, which was generated by pyrolysis of 2-methoxy acetophenone. The high-resolution threshold photoelectron spectrum (TPES) with vacuum ultraviolet synchrotron radiation revealed well-resolved vibrational transitions, assigned to ring deformation modes of the cyclopentadiene moiety. The adiabatic ionization energy was determined to be 8.25±0.01 eV and is assigned to the ²A₂← ¹A₁ transition. A broad and featureless band arising at 9 eV is associated with the ²B₁← ¹A₁ excitation. A conical intersection is responsible for the ultrafast relaxation of the fulvenone cation from the into the state resulting in a featureless and lifetime broadened band. These insights will increase the detection capabilities for fulvenone and thereby help to elucidate reaction mechanisms in lignin catalytic pyrolysis.     

Vibrationally Resolved Inner-Shell Photoexcitation of the Molecular Anion C2−

Carbon 1s core-hole excitation of the molecular anion C₂⁻ has been experimentally studied at high resolution by employing the photon-ion merged-beams technique at a synchrotron light source. The experimental cross section for photo–double-detachment shows a pronounced vibrational structure associated with and core excitations of the C₂⁻ ground level and first excited level, respectively. A detailed Franck-Condon analysis reveals a strong contraction of the C₂⁻ molecular anion by 0.2 Å upon this core photoexcitation. The associated change of the molecule's moment of inertia leads to a noticeable rotational broadening of the observed vibrational spectral features. This broadening is accounted for in the present analysis which provides the spectroscopic parameters of the C₂⁻ and core-excited levels.     

The Reaction of Muonium with Hydrogen Peroxide in Aqueous Solution

Rate constants for the reactions of muonium (Mu) (the ultralight isotope of the hydrogen atom) with H₂O₂ in H₂O and D₂O₂ in D₂O have been determined at various temperatures and pH (pD) values. The data are consistent with the three reactions: , , and the equivalent for the deuterated entities. A significant positive H/D isotope effect was found for the undissociated peroxide, while for the anions the effect was negligible or slightly in the opposite direction. In addition, for concentrated solutions of peroxide a study of the muon spin polarization as a function of applied transverse magnetic field yielded results consistent with the rate constants determined from the direct decay measurements, and indicated that the reaction products are diamagnetic, most likely MuH and MuOH, i. e., no muoniated radical products are formed. These results are potentially relevant for management of the radiolysis products in nuclear industry.     

Cover Feature: How Different are the Diamagnetic and Paramagnetic Contributions to Off-Nucleus Shielding in Aromatic and Antiaromatic Rings? (ChemPhysChem 9/2023)

The spatial variations in the diamagnetic and paramagnetic contributions to the off-nucleus isotropic shielding, , and to the zz component of the off-nucleus shielding tensor, , around benzene (C₆H₆) and cyclobutadiene (C₄H₄) are investigated using complete-active-space self-consistent field wavefunctions. Despite the substantial differences between and around the aromatic C₆H₆ and the antiaromatic C₄H₄, the diamagnetic and paramagnetic contributions to these quantities, and , and and , are found to behave similarly in the two molecules, shielding and deshielding, respectively, each ring and its surroundings. The different signs of the most popular aromaticity criterion, the nucleus-independent chemical shift (NICS), in C₆H₆ and C₄H₄ are shown to follow from a change in the balance between the respective diamagnetic and paramagnetic contributions. Thus, the different NICS values for antiaromatic and antiaromatic molecules cannot be attributed to differences in the ease of access to excited states only; differences in the electron density, which determines the overall bonding picture, also play an important role.     

Combined Modelling of Triply Paired Electronic States of HO2+ ion with 3 Symmetry using Coupled Eigen Model

The multi state triply paired coupled adiabatic electronic states of , symmetries for the title ion have been modelled using the triply paired coupled eigen valued functions with the principal aim of getting a set of well optimized triply paired coupled functions, capable of explaining all the microscopic topograhical features present in the chosen coupled electronic states. The high quality size truncated dynamically correlated points at the limit of complete basis set (CBS) were utilized for this purpose. The proposed model function are presented in an “easy to understand” linear form with respect to the pre-defined variables which are directly dependent on the diabatic potentials and coupling strengths found in the diabatic matrix. Its most important feature (an essential criterion to be selected for modelling the triply degenerate seam pathway) is the ability to predict the existence of an uniquely defined triply degenerate iso-energetic geometry among the chosen coupled states in addition to the prediction of the existence of many doubly degenerate iso-energetic geometries in those states and hence can be adaptable to any general molecular system where such triply or doubly degenerate geometries are believed to occur. The title ion is one among them where many doubly iso-energetic geometries were located but not the triply degenerate one as it is a three atomic system unlike the polyatomic case where the triply degenerate geometry will most likely to be found. To do this work, one requires the knowledge of four asymptotic diatomic curves upon atom+diatom dissociation whose correct correlated spectroscopic states are O₂⁺ , O₂ , O₂⁺ , OH⁺ . The quality and the characteristics of analytical surface of the ion is discussed well in detail.     

Reversible Metal-Hydride Transformation in Mg-Ti-H Nanoparticles at Remarkably Low Temperatures

We study the kinetics of hydrogen sorption in Mg-Ti-H nanoparticles prepared by gas phase condensation of mixed Mg-Ti vapors under a H₂-containing atmosphere. Four samples with different Ti contents from 14 to 63 at.% Ti are examined in the 100–150 °C range. The hydrogen absorption kinetics coupled with the formation of MgH₂ can be described by a nucleation and growth model. The activation energy is in the range kJ/mol and the rate constant (at 150 °C) increases from s⁻¹ to s⁻¹ with increasing Ti content. Hydrogen desorption is well modeled by a sequence of surface-limited and contracting-volume kinetics, except at the highest Ti content where nucleation and growth is observed. The activation energy of surface-limited kinetics is /mol. The rate constant (at 150 °C) increases from s⁻¹ to s⁻¹ with the Ti content. These results open an unexplored kinetic window for Mg-based reversible hydrogen storage close to ambient temperature.     

A Multiconformational Transition State Theory Approach to OH Tropospheric Degradation of Fluorotelomer Aldehydes

Experimental work on the OH-initiated oxidation reactions of fluorotelomer aldehydes (FTALs) strongly suggests that the respective rate coefficients do not depend on the size of the C_xF_2x+1 fluoroalkyl chain. FTALs hence represent a challenging test to our multiconformer transition state theory (MC-TST) protocol based on constrained transition state randomization (CTSR), since the calculated rate coefficients should not show significant variations with increasing values of . In this work we apply the MC-TST/CTSR protocol to the cases and calculate both rate coefficients at 298.15 K with a value of cm³ molecule⁻¹ s⁻¹, practically coincident with the recommended experimental value of k_exp= cm³ molecule⁻¹ s⁻¹. We also show that the use of tunneling corrections based on improved semiclassical TST is critical in obtaining Arrhenius-Kooij curves with a correct behavior at lower temperatures.     

NMR Self-Diffusion and Transverse Relaxation Time in Bitumen: The Effect of Aging

In this investigation the dynamics of two types of bitumens with different penetration grade were tested by using dynamic shear rheometry (DSR) and Nuclear Magnetic Resonance (NMR) at unaged conditions, and upon both short- and long-term artificial aging. The gel-sol transition temperature was found to increase with increasing the time of aging treatment. Arrhenius parameters of the viscosity were found, unexpectedly, to be correlated with those of simple liquids, suggesting that the two kinds of systems, although chemically and physically quite different, share the same basic process at the molecular level. The molecular dynamics has been then investigated by NMR Pulsed Field Gradient Stimulated-Echo (PFGSE) and relaxometry (Carr-Purcell-Meiboom-Gill, CPMG, spin-echo pulse sequence) to capture the effect of aging upon dynamics variables such as self-diffusion coefficients D and transverse relaxation times T₂. The translational diffusion at T> of the light molecular components of both types of bitumens was characterized by broad distributions of D which were found independent of the experimental time scale up to 0.2 s. Similarly, T₂ data could be described as a continuous unimodal distributions of relaxation times determined both at T< and T> .     

The Reaction of Sulfur Dioxide Radical Cation with Hydrogen and its Relevance in Solar Geoengineering Models

SO₂ has been proposed in solar geoengineering as a precursor of H₂SO₄ aerosol, a cooling agent active in the stratosphere to contrast climate change. Atmospheric ionization sources can ionize SO₂ into excited states of , which quickly reacts with trace gases in the stratosphere. In this work we explore the reaction of with excited by tunable synchrotron radiation, leading to ( ), where H contributes to O₃ depletion and OH formation. Density Functional Theory and Variational Transition State Theory have been used to investigate the dynamics of the title barrierless and exothermic reaction. The present results suggest that solar geoengineering models should test the reactivity of with major trace gases in the stratosphere, such as H₂ since this is a relevant channel for the OH formation during the nighttime when there is not OH production by sunlight. OH oxides SO₂, triggering the chemical reactions leading to H₂SO₄ aerosol.     

Front Cover: Critical Assessment of pH-Dependent Lipophilicity Profiles of Small Molecules: Which One Should We Use and In Which Cases? (ChemPhysChem 24/2023)

Lipophilicity is a physicochemical property with wide relevance in drug design, computational biology, food, environmental and medicinal chemistry. Lipophilicity is commonly expressed as the partition coefficient for neutral molecules, whereas for molecules with ionizable groups, the distribution coefficient (D) at a given pH is used. The logD_pH is usually predicted using a pH correction over the logP_N using the pK_a of ionizable molecules, while often ignoring the apparent ion pair partitioning . In this work, we studied the impact of on the prediction of both the experimental lipophilicity of small molecules and experimental lipophilicity-based applications and metrics such as lipophilic efficiency (LipE), distribution of spiked drugs in milk products, and pH-dependent partition of water contaminants in synthetic passive samples such as silicones. Our findings show that better predictions are obtained by considering the apparent ion pair partitioning. In this context, we developed machine learning algorithms to determine the cases that should be considered. The results indicate that small, rigid, and unsaturated molecules with logP_N close to zero, which present a significant proportion of ionic species in the aqueous phase, were better modeled using the apparent ion pair partitioning . Finally, our findings can serve as guidance to the scientific community working in early-stage drug design, food, and environmental chemistry.     

Resonant Fragmentation of the Water Cation by Electron Impact: a Wave-Packet Study

We have investigated the dissociation of a resonant state that can be formed in low energy electron scattering from H₂O⁺. We have chosen the second triplet resonance above the state of H₂O⁺ whose autoionization mainly produces H₂O⁺ ( ). We have considered both dissociation of the resonant state itself, dissociative recombination (DR), or the dissociation of the H₂O⁺ cation after autodetachment, dissociative excitation (DE). The time-evolution of a wave packet on the potential energy surfaces of the resonance and cationic states shows, for the initial conditions studied, that the probability for DR is about 38 % while the probability for DE is negligible.     

Atomic-Scale Studies of Fe3O4(001) and TiO2(110) Surfaces Following Immersion in CO2-Acidified Water

Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water and studied its interaction with TiO₂ and Fe₃O₄ surfaces. Herein, we describe a simple approach to vary the pH through the partial pressure of CO₂ ( ) in the surrounding vacuum chamber and use this to study how these surfaces react to an acidic solution. The TiO₂(110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe₃O₄(001)-( × )R45° surface begins to dissolve at a pH 4.0–3.9 ( =0.8–1 bar) and, although it is significantly roughened, the atomic-scale structure of the Fe₃O₄(001) surface layer remains visible in scanning tunneling microscopy (STM) images. X-ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced and contains a significant accumulation of bicarbonate (HCO₃⁻) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO₃)₂), which precipitate onto the surface when the water evaporates.     

Development and Verification of a Diagnostic Technology for Waste Battery Deterioration Factors

We defined four major deterioration factors (electrolyte loss (EL), lithium loss (LL), lithium precipitation (LP), and compound deterioration (CD)). Then, we derived eleven key performance indicators (KPIs) for comparative analysis. After that, we fabricated three deteriorated cells for each of three deterioration factors (EL, LL, and LP) and one cell with CD (for verification) with four individual (dis)charging experiment manuals. The two major contributions of this study are the performance of 1) trend analysis to determine a suitable diagnostic metric by inspecting the eleven KPIs and 2) comparison analysis of and to verify the effectiveness of utilizing as a real-time deterioration diagnostic factor using a concept of model-in-the-loop simulation. The results show that 1) has the most conspicuous trendline tendency among the eleven comparison targets for all four major deterioration factors, and 2) the angle difference between the two trends of and lies within a minimum of 9° and a maximum of 43° (with a sscale on the x-axis and a scale on the y-axis for a clear trend line analysis). From this, we can conclude that the trendline-based real-time deterioration analysis employing may be practically applicable to a limited extent.     

Photoionization Bands of Cyanogen: Multi-Mode Vibronic Coupling and Renner-Teller Effects

Multi-mode vibronic coupling in the , , and electronic states of Cyanogen radical cation (C N ) is investigated with the aid of ab initio quantum chemistry and first principles quantum dynamics methods. The electronic degenerate states of Π symmetry of C N undergo Renner-Teller (RT) splitting along degenerate vibrational modes of π symmetry. The RT split components form symmetry allowed conical intersections with those from nearby RT split states or with non-degenerate electronic states of Σ symmetry. A parameterized vibronic Hamiltonian is constructed using standard vibronic coupling theory in a diabatic electronic basis and symmetry rules. The parameters of the Hamiltonian are derived from ab initio calculated adiabatic electronic energies. The vibronic spectrum is calculated, assigned and compared with the available experimental data. The impact of various electronic coupling on the vibronic structure of the spectrum is discussed.     

Odd-Even Effect on Luminescence Properties of Europium Aliphatic Dicarboxylate Complexes

The odd–even effect in luminescent [Eu₂(L)₃(H₂O)_x]⋅y(H₂O) complexes with aliphatic dicarboxylate ligands (L: OXA, MAL, SUC, GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x=2–6 and y=0–4), prepared by the precipitation method, was observed for the first time in lanthanide compounds. The final dehydration temperatures of the Eu³⁺ complexes show a zigzag pattern as a function of the carbon chain length of the dicarboxylate ligands, leading to the so-called odd-even effect. The FTIR data confirm the ligand–metal coordination via the mixed mode of bridge–chelate coordination, except for the Eu³⁺-oxalate complex. XRD results indicate that the highly crystalline materials belong to the monoclinic system. The odd–even effect on the 4 f–4 f luminescence intensity parameters (Ω₂ and Ω₄) is explained by using an extension of the dynamic coupling mechanism, herein named the ghost-atom model. In this method, the long-range polarizabilities ( ) were simulated by a ghost atom located at the middle of each ligand chain. The values of were estimated using the localized molecular orbital approach. The emission intrinsic quantum yield ( ) of the Eu³⁺ complexes also presented an the odd-even effect, successfully explained in terms of the zigzag behavior shown by the Ω₂ and Ω₄ intensity parameters. Luminescence quenching due to water molecules in the first coordination sphere is also discussed and rationalized.     

On the Mechanical Properties of Popgraphene-Based Nanotubes: a Reactive Molecular Dynamics Study

Carbon-based tubular materials have sparked a great interest in future electronics and optoelectronics device applications. In this work, we computationally studied the mechanical properties of nanotubes generated from popgraphene (PopNTs). Popgraphene is a 2D carbon allotrope composed of 5-8-5 rings. We carried out fully atomistic reactive (ReaxFF) molecular dynamics for PopNTs of different chiralities ( and ) and/or diameters and at different temperatures (from 300 up to 1200 K). Results showed that the tubes are thermally stable (at least up to 1200 K). All tubes presented stress/strain curves with a quasi-linear behavior followed by an abrupt drop of stress values. Interestingly, armchair-like PopNTs ( ) can stand a higher strain load before fracturing when contrasted to the zigzag-like ones ( ). Moreover, it was obtained that Young's modulus (Y_Mod) (750–900 GPa) and ultimate strength (σ_US) (120–150 GPa) values are similar to the ones reported for conventional armchair and zigzag carbon nanotubes. Y_Mod values obtained for PopNTs are not significantly temperature-dependent. While the σ_US values for the showed a quasi-linear dependence with the temperature, the exhibited no clear trends.     

Electronic Structure of the Low-Lying States of the Triatomic MoS2 Molecule: The Building Block of 2D MoS2

Molybdenum disulfide (MoS₂) is the building component of 1D-monolayer, 2D-layered nanosheets and nanotubes having many applications in industry, and it is detected in various molecular systems observed in nature. Here, the electronic structure and the chemical bonding of sixteen low-lying states of the triatomic MoS₂ molecule are investigated, while the connection of the chemical bonding of the isolated MoS₂ molecule to the relevant 2D-MoS₂, is emphasized. The MoS₂ molecule is studied via DFT and multireference methodologies, i. e., MRCISD(+Q)/aug-cc-pVQZ(−PP)_Mo. The ground state, ³B₁, is bent (Mo−S=2.133 Å and ϕ(SMoS)=115.9°) with a dissociation energy to atomic products of 194.7 kcal/mol at MRCISD+Q. In the ground and in the first excited state a double bond is formed between Mo and each S atom, i. e., . These two states differ in which d electrons of Mo are unpaired. The Mo−S bond distances of the calculated states range from 2.108 to 2.505 Å, the SMoS angles range from 104.1 to 180.0°, and the Mo−S bonds are single or double. Potential energy curves and surfaces have been plotted for the ³B₁, ⁵A₁ and ⁵B₁ states. Finally, the low-lying septet states of the triatomic molecule are involved in the material as a building block, explaining the variety of its morphologies.     

Molecular Mobility of Tert-butyl Alcohol Confined in a Breathing MIL-53 (Al) Metal-Organic Framework

We present a detailed solid-state NMR characterization of the molecular dynamics of tert-butyl alcohol (TBA) confined inside breathing metal-organic framework (MOF) MIL-53(Al). ²⁷Al MAS NMR has demonstrated that TBA adsorption induces the iX phase of MIL-53 material with partially shrunk channels. ²H solid-state NMR has shown that the adsorbed alcohol exhibits anisotropic rotations of the methyl groups around two axes and librations of the molecule as a whole about the axis passing through the TBA C−O bond. These librations are realized by two distinct ways: fast molecule orientation change during the translational jump diffusion along the channel with characteristic time τ_D of about 10⁻⁹ s at 300 K; slow local librations at a single coordination site, representing framework hydroxyl groups, with τ_l≈10⁻⁶ s at 300 K. Self-diffusion coefficient of the alcohol in the MOF has been estimated: D=3.4×10⁻¹⁰ m² s⁻¹ at 300 K. It has been inferred that both the framework flexibility and the interaction with framework hydroxyl groups define the dynamics of TBA confined in the channels of MIL-53 (Al).     

Fluorine Substitution of TCNQ Alters the Redox-Driven Catalytic Pathway for the Ferricyanide-Thiosulfate Reaction

Mechanistic variation in catalysis through substituent-based redox tuning is well established. Fluorination of TCNQ (TCNQ=tetracyanoquinodimethane) provides ~850 mV variation in the redox potentials of the and (n=0, 2, 4) processes. With , catalysis of the kinetically very slow ferrocyanide-thiosulfate redox reaction in aqueous solution occurs via a mechanism in which the catalyst is reduced to when reacting with which is oxidised to . Subsequently, reacts with to form and reform the catalyst, in another thermodynamically favoured process. An analogous mechanism applies with as a catalyst. In contrast, since the reaction of with is thermodynamically unfavourable, an alternative mechanism is required to explain the catalytic activity observed in this non-fluorinated system. Here, upon addition of , reduction of to occurs with concomitant oxidation of to , which then acts as the catalyst for oxidation. Thermodynamic data explain the observed differences in the catalytic mechanisms. (n=0, 4) also act as catalysts for the ferricyanide-thiosulfate reaction in aqueous solution. The present study shows that homogeneous pathways are available following addition of these dissolved materials. Previously, these (n=0, 4) coordination polymers have been regarded as insoluble in water and proposed as heterogeneous catalysts for the ferricyanide-thiosulfate reaction. Details and mechanistic differences were established using UV-visible spectrophotometry and cyclic voltammetry.