Dimer Cation Research Articles

Theoretical study on the spectroscopic and transition properties of phosphorus dimer cation

The potential energy curves of 18 Λ-S states of the P2 + cation have been constructed using the internally contracted multi-reference configuration interaction approach combined with the Davidson correction. The basis-set extrapolation to the complete basis set limit is performed. The relativistic effect is considered in calculations. Treatment of scalar relativistic effect at the third-order Dkroll-Hess approximate and spin-orbit coupling with the state-average Breit-Pauli operator is made. Based on the obtained potential energy curves, the spectroscopic parameters for all bound Λ-S and Ω states are determined. These results are in agreement with experiment where the spectroscopic data are available. The transition dipole moments are calculated. The properties of the dipole-allowed transitions among doublet or quartet Λ-S states are evaluated, from which Einstein coefficients, Franck-Condon factors and radiative lifetimes are estimated. The spin-orbit coupling effect on the transition properties of the 12Πg→X2Πu system is also studied. The spectroscopic and transition properties reported in this study can be used to guide the detection of phosphorus dimer cation in spectroscopy experiments.

Molecular Association and Reactivity of the Pyridine Dimer Cation.

A recent experimental report has identified the formation of the C-N hemibonded pyridine dimer cation following vacuum ultraviolet near-threshold photoionization [J. Phys. Chem. Lett., 2021, 12, 4936-4943]. Herein, the dynamics and consequent reactivity of the pyridine dimer cation were investigated employing Born-Oppenheimer molecular dynamics (BOMD) simulations. An antiparallel π-stacked pyridine dimer in the neutral ground state is transformed into a noncovalently interacting C-H···N hydrogen-bonded structure which can lead to proton transfer in the cationic state. Additionally, C-N- and N-N-bonded adducts were formed in the cationic state. Further, metastable C-H···H-C-bonded cationic species was observed, which rearranged to an N-N bonded adduct. In contrast to the experimental observation, migration of the proton to the α position was not observed in the C-N bonded adduct owing to a high barrier of about 2 eV. The observed trends in the molecular association, proton transfer, and the formation of C-N and N-N bonded adducts are a consequence of the roaming dynamics of one pyridine moiety over the other in the cationic state.

Crystal structure of hexa-glycinium dodeca-iodo-triplumbate.

The crystal structure of hexa-glycinium tetra-μ-iodido-octa-iodido-triplumbate, (C2H6NO2)6[Pb3I12] or (GlyH)6[Pb3I12], is reported. The compound crystallizes in the triclinic space group P. The [Pb3I12]6- anion is discrete and located around a special position: the central Pb ion located on the inversion center is holodirected, while the other two are hemidirected. The supra-molecular nature is mainly based on C-H⋯I, N-H⋯I, O-H⋯I and N-H⋯O hydrogen bonds. Dimeric cations of type (A +⋯A +) for the amino acid glycine are observed for the first time.

Proton Transfer Processes in 2-Butenenitrile Dimer Cation Studied by Mass-Selective Infrared Spectroscopy.

2-Butenenitrile (2-Bu) is a recently discovered crucial interstellar molecule. Herein, an abnormal NH band was observed in the infrared spectrum of the 2-Bu dimer cation, suggestive of a proton transfer reaction within the cluster. Through a comprehensive theoretical analysis of the IR spectrum of (2-Bu)2+, we discovered not only the formation of a new C-N bond through the attachment of one 2-Bu to another but also the occurrence of a proton transfer reaction in the cluster. This proton was identified as originating from the methyl group of the attaching 2-Bu in the cluster based on the analysis of IR spectra of (2-Bu)+ and [2-Bu-acrylonitrile (AN)]+. Furthermore, the detailed reaction process of this ion-molecule reaction is examined with theoretical calculation. This finding contributes significantly to our deeper understanding of ion-molecule reactions in the gas phase and the formation of nitrogen-containing prebiotic molecules in the interstellar medium.

Spectroscopic Evidence for Doubly Hydrogen-Bonded Cationic Dimers in the Solid, Liquid, and Gaseous Phases of Carboxyl-Functionalized Ionic Liquids.

Intermolecular interactions determine whether matter sticks together, gases condense into liquids, or liquids freeze into solids. The most prominent example is hydrogen bonding in water, responsible for the anomalous properties in the liquid phase and polymorphism in ice. The physical properties are also exceptional for ionic liquids (ILs), wherein a delicate balance of Coulomb interactions, hydrogen bonds, and dispersion interactions results in a broad liquid range and the vaporization of ILs as ion pairs. In this study, we show that strong, local, and directional hydrogen bonds govern the structures and arrangements in the solid, liquid, and gaseous phases of carboxyl-functionalized ILs. For that purpose, we explored the H-bonded motifs by X-ray diffraction and attenuated total reflection (ATR) infrared (IR) spectroscopy in the solid state, by ATR and transmission IR spectroscopy in the liquid phase, and by cryogenic ion vibrational predissociation spectroscopy (CIVPS) in the gaseous phase at low temperature. The analysis of the CO stretching bands reveals doubly hydrogen-bonded cationic dimers (c═c), resembling the archetype H-bond motif known for carboxylic acids. The like-charge doubly hydrogen-bonded ion pairs are present in the crystal structure of the IL, survive phase transition into the liquid state, and are still present in the gaseous phase even in (2,1) complexes wherein one counterion is removed and repulsive Coulomb interaction increased. The interpretation of the vibrational spectra is supported by quantum chemical methods. These observations have implications for the fundamental nature of the hydrogen bond between ions of like charge.

Internal Energy Dependence of the Pyrrole Dimer Cation Structures Formed in a Supersonic Plasma Expansion: Charge-Resonance and Hydrogen-Bonded Isomers.

The structures of the pyrrole dimer cation (Py2+) formed in an electron-ionization-driven supersonic plasma expansion of Py seeded in Ar or N2 are probed as a function of its internal energy by infrared photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. The IRPD spectra recorded in the CH and NH stretch ranges are analyzed by dispersion-corrected density functional theory (DFT) calculations at the B3LYP-D3/aug-cc-pVTZ level. The spectra of the cold Ar/N2-tagged Py2+ clusters, Py2+Ln (n = 1-5 for Ar, n = 1 for N2), indicate the exclusive formation of the most stable antiparallel π-stacked Py2+ structure under cold conditions, which is stabilized by charge-resonance interaction. The bare Py2+ dimers produced in the ion source have higher internal energy, and the observation of additional transitions in their IRPD spectra suggests a minor population of less stable hydrogen-bonded isomers composed of heterocyclic Py/Py+ structures formed after intramolecular H atom transfer and ring opening. These intermolecular isomers differ from the chemically bonded structures proposed earlier in the analysis of IRPD spectra of Py2+ generated by VUV ionization of neutral Pyn clusters.

Self-Aggregation, Enhanced Guest-Binding Behavior, and Anion-Induced Agglutination of Cyclophane Dimers Linked with Florescent Perylene Diimide.

Cationic, anionic, and nonionic cyclophane dimers, 1a, 1b, and 1c, covalently linked with fluorescent perylene diimide were synthesized, respectively, as a host for aggregation-induced multivalent effects on guest-binding. These PDI-linked cyclophane dimers 1a and 1b, especially 1c, formed self-aggregates in H2O-rich solvents through π-stacking interactions. Critical aggregation concentrations of 1a, 1b, and 1c in H2O were 4.0 × 10-6, 4.0 × 10-6, and less than 0.1 × 10-6 M, respectively. The self-aggregates of the PDI-linked cyclophane dimers in H2O were several hundred nanometers in size by DLS and TEM measurements. In an aggregated state, 1a exhibited guest-binding abilities toward hydrophobic guests, such as 6-p-toluidinonaphthalene-2-sulfonate and 6-anilino-naphthalene-2-sulfonate. Moreover, anion-induced agglutination of 1a was observed upon the addition of phosphate ions, and its effectiveness was as follows: ATP > ADP > H2P2O72- > AMP ≈ PO43- > HPO42- > H2PO4-. The contribution of the adenine base of the nucleotides as a guest moiety used for effective agglutination was confirmed by 1H NMR spectroscopy.

Exciton Dynamics of TiOPc/WSe2 Heterostructure.

The van der Waals (vdW) heterostructures composed of two-dimensional (2D) transition metal dichalcogenides (TMDs) and organic semiconductors demonstrate numerous compelling optoelectronic properties. However, the influence of the vdW epitaxial effect and temperature on the optoelectronic properties and interface exciton dynamics of heterostructures remains unclear. This study systematically investigates the fluorescence properties of TiOPc/WSe2 heterostructure. Comprehensive spectral characterization elucidates that the emission behavior of the TiOPc/WSe2 heterostructure arises from charge/energy transfer at the heterostructure interfaces and the structural ordering of the organic layer on the 2D monolayer WSe2 induced by vdW epitaxy. The interface exciton dynamic features probed by ultrafast transient spectroscopy reveal that the face-to-face molecular stacking configuration of TiOPc exhibits ultrafast exciton dynamics. In particular, we observe picosecond-scale absorption of organic molecular dimer cations, providing direct evidence of interface charge transfer at room temperature. Moreover, energy transfer from the TiOPc to WSe2 may exist based on the tunability in the fluorescence emission of the TiOPc/WSe2 heterostructure as the temperature changes. This study unveils the critical role of vdW epitaxy and temperature in the exciton dynamics of organic/2D TMDs hybrid systems and provides guidance for studying interlayer charge and energy transfer in organic/inorganic heterostructures.

Stability of monoradical cation dimer of viologen derivatives in aqueous redox flow battery

Stability of monoradical cation dimer of viologen derivatives in aqueous redox flow battery

Correlation of the Charge Resonance Interaction with Cluster Conformations Probed by Electronic Spectroscopy of Dimer Radical Cations of CO2 and CS2 in a Cryogenic Ion Trap.

Radical cations of dimeric clusters of carbon dioxide/disulfide, [(CX2)2]+• (X = O and S), form strong intracluster bonds through charge resonance (CR) interactions. We herein performed electronic photodissociation spectroscopy of [(CX2)2]+• while regulating the temperature under ambient and cryogenic conditions using a quadrupole ion trap. Both ions exhibited broad band absorption in the near-infrared-visible light region; it is called the "CR band", as a measure of the strength of the CR interaction. Strikingly, this band underwent a noticeable blue shift upon cryogenic cooling for [(CS2)2]+• while not for [(CO2)2]+•. On the basis of quantum chemical calculations with a coupled cluster method, the band shift was attributed to the variations in the relative population of two energetically close conformers found for [(CS2)2]+•. This study highlights a strong correlation between CR interactions and conformation of the radical dimer cations, demonstrating the exceptional significance of cryogenic cooling in the chemistry of ionic molecular clusters.

L-Cysteinium…l-cysteine phosphite

In the system l-cysteine-H3PO3-H2O, a new salt with a dimeric cation, (l-CysH···l-Cys)(H2PO3), has been obtained. The compound exhibits incongruent solubility and is synthesized using an equimolar ratio of l-cysteine and phosphorous acid. It crystallizes in the orthorhombic space group P212121. The dimeric (l-CysH···l-Cys) cation is formed due to a strong O-H···O hydrogen bond with an O···O distance of 2.498 (2) Å. Phosphite anions in the structure are linked to infinite chains. An IR spectrum is recorded and discussed based on its structure.

L-Cysteine oxalates with dimeric and trimeric cations

Single crystals of two new compounds, (l-CysH···l-Cys)(HC2O4) (I) and (l-CysH···l-Cys···l-CysH)(HC2O4)2·0.5H2O (II), were obtained. Crystal structures were solved and refined using single-crystal X-ray diffraction, and compared with the previously known orthorhombic and monoclinic modifications of the simple salt (l-CysH)(HC2O4). Compound (I) crystallizes in the triclinic crystal system (space group P1, Z=2) and contains (l-CysH···l-Cys) dimeric cations with very short O···O interatomic distance of 2.434(2) Å. Hydrogen oxalate anions form head-to-tail type chains with very short hydrogen bonds, characterized by O···O distances of 2.422(2) Å and 2.417(3) Å. The crystal lattice of (II) is monoclinic with the space group C2, Z=4. It contains a new type of cation, that has never been observed before for cysteine salts, i.e. trimeric (l-CysH···l-Cys···l-CysH) cation. Protonated l-cysteinium cations are connected to the zwitterionic l-cysteine moiety through the same oxygen atom of the carboxylate group via hydrogen bonds, with O···O distances of 2.6410(18) Å and 2.4888(19) Å. Similar to the structure (I), the hydrogen oxalate anions form head-to-tail type chains linked by short hydrogen bonds with O···O distances of 2.4369(17) Å and 2.4330(17) Å, respectively. The crystals were characterized by IR and Raman spectroscopy. The batches obtained on crystallization were also characterized by X-ray powder diffraction, in order to check the phase purity of all the sample.

Near-infrared absorption and radiative cooling of naphthalene dimers (C10H8)2.

The radiative cooling of naphthalene dimer cations, (C10H8)2+ was studied experimentally through action spectroscopy using two different electrostatic ion-beam storage rings, DESIREE in Stockholm and Mini-Ring in Lyon. The spectral characteristics of the charge resonance (CR) band were observed to vary significantly with a storage time of up to 30 seconds in DESIREE. In particular, the position of the CR band shifts to the blue, with specific times (inverse of rates) of 0.64 s and 8.0 s in the 0-5 s and 5-30 s storage time ranges, respectively. These long-time scales indicate that the internal energy distribution of the stored ions evolves by vibrational radiative cooling, which is consistent with the absence of fast radiative cooling via recurrent fluorescence for (C10H8)2+. Density functional based tight binding calculations with local excitations and configuration interactions (DFTB-EXCI) were used to simulate the absorption spectrum for ion temperatures between 10 and 500 K. The evolution of the bandwidth and position with temperature is in qualitative agreement with the experimental findings. Furthermore, these calculations yielded linear temperature dependencies for both the shift and the broadening. Combining the relationship between the CR band position and the ion temperature with the results of the statistical model, we demonstrate that the observed blue shift can be used to determine the radiative cooling rate of (C10H8)2+.

Polyiodides of amino acids. Betainium triiodide

A new salt of betaine, betainium triiodide, (BetH)(I3), was synthesized and characterized using structural analysis and infrared spectroscopy. It crystallizes in the monoclinic space group C2/m with the asymmetric part containing half of the formula unit. The structure determination revealed that the betainium cation forms an (A+···A+) type centrosymmetric betainium···betainium dimeric cation with an O···O distance of 2.662(6) Å. The presence of the triiodide anion in the structure provides semiconductor properties of the resulting crystal. The bandgap for (BetH)(I3) was calculated from the electronic structure based on the crystal structure and measured from the diffuse reflectance spectrum. The results are compared with two previously known structurally characterized salts of amino acids that also contain the triiodide anion: (l-AlaH···l-Ala)(I3) and (GlyH)(I3). Thermal properties of the crystal were studied by TG and DSC methods.

Salts of amino acids with polar symmetry exhibiting a dimeric cation of the [A(1)+···A(2)] type. New type of polymorphism

A new class of amino acids salts containing different amino acids has recently been discovered: the present paper reports on synthesis and crystal structures of thirteen salts with a dimeric cation of the [A(1)+···A(2)] type. All have polar symmetry, thus being potential pyroelectrics or ferroelectrics: (l-ProH···Sar)BF4 (I), (l-ProH···Sar)ClO4 (II), (l-ProH···Sar)NO3 (III), (l-ProH···Sar)Br (IV), (l-ProH···Sar)I (V), (all with space group C2), (β-AlaH···DMG)I (VI), (β-AlaH···DMG)BF4 (VII), (β-AlaH···DMG)NH2SO3 (VIII) and (DMGH···Sar)ClO4 (XIII) (all with space group Pna21), (β-AlaH···Bet)Br (IX), (β-AlaH···Bet)I (X) (both with space group Cc), as well as (SarH···DMG)BF4 (XI) and (SarH···DMG)ClO4 (XII) (both with space group Pc). These compounds except (XIII) are formed by slow evaporation at room temperature from aqueous solutions containing stoichiometric ratios of components. Protonation in a pair of amino acids may depend on the anion, as in the cases of (SarH···Bet)ClO4 and (BetH···Sar)NH2SO3·0.5H2O, (DMGH···Bet)ClO4·H2O and (BetH···DMG)I·H2O or, for the same anion, on the conditions of crystallization, as in the case of (SarH···DMG)ClO4 (obtained from aqueous solution) and (DMGH···Sar)ClO4 (crystallized from acetic acid). The phases (SarH⋅⋅⋅DMG)(ClO4) (s.g. Pc) and (DMGH⋅⋅⋅Sar)(ClO4), (s.g. Pna21) represent a peculiar type of polymorphism, where the difference is evident in the position of the proton in the dimeric cation, too.

Electrochemical oxidation of dicarbonylcyclopentadienylcobalt(I) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ionic liquid

The electrochemical oxidation of the 18-electron half-sandwich organometallic complex [Co(η5-C5H5)(CO)2], 1, has been studied in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, [bmpyr][NTf2], and in dichloromethane, CH2Cl2, containing [bmpyr][NTf2] as the supporting electrolyte. In the ionic liquid, the 17-electron cation 1+ is generated under voltammetric conditions via a process that has a reversible mid-point potential (Em) of 0.22 V vs. Fc0/+ (Em = 0.23 V vs. Fc0/+ in CH2Cl2/[bmpyr][NTf2]). 1+ reacts with 1 to form the cationic dimer [Co2(η5-C5H5)2(CO)4]+, 2+, which reacts with [NTf2]− to form cobaltocenium, Cc+, and [Co(NTf2)n](n−2)− (with n = 2, 4, or 6) complexes as the products of bulk electrolysis. A second voltammetric oxidation process at more positive potential is associated with the oxidation of 2+ (Em = 0.62 V vs. Fc0/+ in [bmpyr][NTf2] and 0.72 V vs. Fc0/+ in CH2Cl2/[bmpyr][NTf2]) to form 22+, which accelerates the formation of Cc+ and [Co(NTf2)n](n−2)− complexes, which are reduced to cobaltocene and cobalt metal under conditions of cyclic voltammetry and bulk electrolysis. Mechanistic details and the reaction products have been characterised by cyclic voltammetry, chronocoulometry, bulk electrolysis, and FT-IR, UV–Vis and ESR spectroelectrochemical measurements. This investigation reveals how ionic liquid media may influence the electrochemical oxidation mechanism of half-sandwich metal and other complexes.

Time-Resolved EPR Spectroscopy of Channelrhodopsin-2 Helix B Movements

The light-gated dimeric cation channel channelrhodopsin-2 (ChR2) is one of the most important optogenetic tools. Upon light activation ChR2 undergoes conformational changes, the most prominent ones include a movement of transmembrane helix B. In the present work, we apply time resolved continuous wave EPR spectroscopy to follow spectral changes of a spin label bound to position C79 located in helix B. We observed an increase of the motional freedom of the spin label side chain in illuminated ChR2. The recovery of the underlying light-induced conformational change in the dark is correlated with the recovery of the P480 state of ChR2. The observed conformational changes might be thus key elements responsible for desensitizing the channel for cation conduction.

Isolation and Infrared Spectroscopic Characterization of Hemibonded Water Dimer Cation in Superfluid Helium Nanodroplets.

The structure of the minimum unit of the radical cationic water clusters, the (H2O)2+ dimer, has attracted much attention because of its importance for the radiation chemistry of water. Previous spectroscopic studies indicated that the dimers have a proton-transferred structure (H3O+·OH), though the alternate metastable hemibonded structure (H2O·OH2)+ was also predicted based on theoretical calculations. Here, we produce (H2O)2+ dimers in superfluid helium nanodroplets and study their infrared spectra in the range of OH stretching vibrations. The observed spectra indicate the coexistence of the two structures in the droplets, supported by density functional theory calculations. This is the first spectroscopic identification of the hemibonded isomer of water radical cation dimers. The observation of the higher-energy isomer reveals efficient kinetic trapping for metastable ionic clusters due to the rapid cooling in helium droplets.

Partial Oxidation of Methane to Methanol by Using Molecular O2 on Pd2+ Catalyst: An Insight from Theory

AbstractThe partial oxidation of methane to methanol using cationic Pd2 dimers is investigated by employing density functional theory (DFT) method. We used B3PW91 functional for geometry optimization, and frequency calculations of all species involved in [Pd2]++O2+2CH4 reaction. Furthermore, a density‐fitting triple zeta valence with single‐polarization (def2TZVP) is used in the calculation to determine the atomic orbitals of the atoms. We oxidized Pd2+ to [Pd2O2]+ using O2 and performed possible partial oxidation of methane to methanol on [Pd2O2]+ and [Pd2O]+ and explored various intermediates and transition states on the potential energy surface (PES) diagram. From Potential Energy Surface (PES) analysis, it is found that the [Pd2O2]+ in doublet spin state multiplicity (SM=2) following radical mechanism is the more preferred pathway for methane to methanol conversion.

Studies of monomer–dimer equilibria of methylene blue as a probe to investigate the solute–solvent and hydrophobic interactions in aqueous solutions of polyethylene glycols at 298 K

Electronic absorption spectrums of methylene blue (MB) in the concentration range wherein monomer–dimer equilibria exist have been systematically investigated in aqueous solutions of polyethylene glycols (PEG's) having molar mass 400, 1000 and 4000 g∙mol–1, at 298 K. The data on the absorption spectrums (550–700 nm) of MB are resolved to obtain monomer and dimer spectrums as well the geometrical details of the species (such as oscillatory strength, dipole moment etc.) are calculated. The molar absorption extinction coefficients for monomer and dimer cations of MB, the dimer dissociation constant values are calculated as a function of PEG concentrations. It has been noted that, absorption maximum occurs at 664 ± 1 and 605 ± 1 nm, respectively, for monomer and dimers which do not get affected much by presence of polymeric molecules and the data show perfect obedience of Beer–Lambert's law. The changes in dimer geometry from sandwich type to end–on–end type have been studied by applying exciton theory of Kasha et al. The dimer dissociation constant values at finite as well for infinitely dilute concentration of PEG's exhibit orderly amendment with respect to molar mass of PEG's and alteration in chain length of molecules. We report the free energy changes of transfer of dimeric cations of different concentrations from water to aqueous solutions of PEG's of fixed concentration, that is, to probe the interactions of water and PEG molecules with a hope of getting information about salting–in/salting–out effects, as well of binding of dye molecules with PEG's. We have suggested complexation of end hydroxylic groups of PEG's with dimer cations. The results are further interpreted as salting–in of PEG complex with MB–dimers only in the limit of infinite dilution (solute–solvent interactions) while salting–out of these complexed species is observed as a function of MB–dimer concentration.