Dimer Cation Research Articles

Electronic transitions in Rb2+ dimers solvated in helium

We have measured depletion spectra of the heteronuclear (85Rb87Rb+) dimer cation complexed with up to 10 He atoms. Two absorption bands are observed between 920 and 250 nm. The transition into the repulsive 12Σu+ state of HeRb2+ gives rise to a broad feature at 790 nm (12,650 cm−1); it exhibits a blueshift of 98 cm−1 per added He atom. The transition into the bound 12Πu state of HeRb2+ reveals vibrational structure with a band head at ≤ 15,522 cm−1, a harmonic constant of 26 cm−1, and a spin–orbit splitting of ≤ 183 cm−1. The band experiences an average redshift of − 38 cm−1 per added He atom. Ab initio calculations rationalize the shape of the spectra and spectral shifts with respect to the number of helium atoms attached. For a higher number of solvating helium atoms, symmetric solvation on both ends of the Rb2+ ion is predicted.

Ion-neutral photofragment coincidence imaging of photodissociation dynamics of ionic species

The recently constructed cryogenic cylindrical ion trap velocity map imaging spectrometer (CIT-VMI) has been upgraded for coincidence imaging of both ionic and neutral photofragments from photodissociation of ionic species. The prepared ions are cooled down in a home-made cryogenic cylindrical ion trap and then extracted for photodissociation experiments. With the newly designed electric fields for extraction and acceleration, the ion beam can be accelerated to more than 4500 eV, which is necessary for velocity imaging of the neutral photofragments by using the position-sensitive imaging detector. The setup has been tested by the 355 nm photodissociation dynamics of the argon dimer cation (Ar2+). From the recorded experimental images of both neutral Ar and ionic Ar+ fragments, we interpret velocity resolutions of Δv/v≈4.6% for neutral fragments, and Δv/v≈1.5% for ionic fragments, respectively.

Dynamical simulation of collision-induced dissociation of pyrene dimer cation

We report a theoretical investigation of the collision-induced dissociation of pyrene dimer cation, as recently investigated in the experimental work by Zamith et al. (J. Chem. Phys. 153, 054311 (2020)). Molecular dynamics simulations using potential energies and forces computed at the self-consistent charge density functional-based tight binding level were conducted for different collision energies between 2.5 and 30 eV. It appears that most of the dissociation occurs on a short timescale (less than 3 ps). The dynamical simulations allow to visualize the dissociation processes. At low collision energies, the dissociation cross section increases with collision energies, whereas it remains almost constant for collision energies greater than 10-15 eV. The analysis of the kinetic energy partition is used to get insights into the collision/dissociation processes at the atomic scale. The simulated time-of-flight mass spectra of parent and dissociation products are obtained from the combination of molecular dynamics simulations and phase space theory to address the short and long timescales dissociation, respectively. The agreement between the simulated and measured mass spectra suggests that the main processes are captured by this approach.

Surface electrostatic force in presence of dimer counter-ion

Combining classical density functional theory with modified interfacial statistical associating fluid theory, we investigate the surface electrostatic force (SEF) between two negatively and similarly charged planar surfaces immersed in electrolyte solution consisting of monomer anion and dimer cation. The dimer cation is comprised of two hard spheres (HSs) tangentially bonded together and freely rotating on the top of each other; one of the two HS sites is positively charged and the other is neutral. New findings about novel properties of the SEF caused by the dimer counter-ion are summarized below. (i) In the presence of only univalent counter-ion, the SEF always becomes more and more repulsive with increasing of the dimer counter-ion neutral site size, whether the bulk electrolyte solution is dense or not, and the surface charge strength is small, moderate, or large; repulsion strength of the SEF always reduces with the bulk concentration, and at equal surface distance rapidly increases with the surface charge strength. (ii) In the presence of bivalent counter-ion of monomer or dimer, rather complex SEF behavior emerges. Specifically, (a) at low surface charge strength, the dimer counter-ion with small neutral site size induces one obvious like-charge attraction (LCA) at lower bulk concentrations; whereas at higher bulk concentrations, a repulsive SEF emerges instead, whose strength monotonously decreases with the neutral site size. (b) For high surface charge strengths, the SEF curves exhibit oscillatory structure with the LCA feature; moreover, the dimer counter-ion always induces larger LCA position and strength than the monomer counter-ion does; the LCA strength decreases with the counter-ion neutral site size, and the LCA position always reduces with increase of the surface charge strength. (c) Dependence of the LCA strength on the bulk concentration is conditional on the surface charge strengths considered; to speak specifically, for very high surface charge strength, the LCA strength reduces with the bulk concentration, whereas for not so high value, the opposite is true. The above observations can be analyzed from fluctuation and balance of the three factors: osmotic pressure difference between inside and outside the plates; hard sphere repulsion in congested space, and “hydrogen bond” formation.

Anti-electrostatic hydrogen bonding between anions of ionic liquids: a density functional theory study.

Hydrogen bonds (HBs) play a crucial role in the physicochemical properties of ionic liquids (ILs). To date, HBs between cations and anions (Ca-An) or between cations (Ca-Ca) in ILs have been reported extensively. Here, we provided DFT evidence for the existence of HBs between anions (An-An) in the IL 1-(2-hydroxyethyl)-3-methylimidazolium 4-(2-hydroxyethyl)imidazolide [HEMIm][HEIm]. The thermodynamic stabilities of anionic, cationic, and H2O dimers together with ionic pairs were studied using potential energy scans. The results show that the cation-anion pair is the most stable one, while the HB in the anionic dimer possesses similar thermodynamic stability to the water dimer. The further geometric, spectral and electronic structure analyses demonstrate that the inter-anionic HB meets the general theoretical criteria of traditional HBs. The strength order of four HBs in complexes is cation-anion pair > H2O dimer ≈ cationic dimer > anionic dimer. The energy decomposition analysis indicates that induction and dispersion interactions are the crucial factors to overcome strong Coulomb repulsions, forming inter-anionic HBs. Finally, the presence of inter-anionic HBs in the ionic cluster has been confirmed by a global minimum search for a system containing two ionic pairs. Even though hydroxyl-functionalized cations are more likely to form HBs with anions, there are still inter-anionic HBs between hydroxyl groups in the low-lying structures. Our studies broaden the understanding of HBs in ionic liquids and support the recently proposed concept of anti-electrostatic HBs.

CRASY: correlated rotational alignment spectroscopy of pyridine. The rotational Raman spectrum of pyridine and asymmetric fragmentation of pyridine dimer cations.

We investigated the rotational Raman spectrum of pyridine monomers and pyridine dimers with mass-correlated rotational alignment spectroscopy (mass-CRASY) and ab initio calculations. The mass spectrum showed a strong signal for the protonated pyridine cation, which we assigned to asymmetric fragmentation of the dimer: ab initio calculations revealed facile proton transfer in the dimer cation and thermodynamically favorable asymmetric fragmentation. In the rotational spectrum correlated to the monomer mass channel, we assigned up to 40 lines for rotational states J ≤ 8. No spectrum could be assigned for the dimer, possibly due to the theoretically predicted presence of multiple dimer structures.

Mechanistic insights into the α-branched amine formation with pivalic acid assisted C-H bond activation catalysed by Cp*Rh complexes.

Density functional theory computations revealed a pivalic acid assisted C-H bond activation mechanism for rhodium catalyzed formation of α-branched amines with C-C and C-N bond couplings. The reaction energies of the [Cp*RhCl2]2 dimer and silver cations indicate that the Cp*RhCl+ cation is the active catalyst. The essential role of pivalic acid is a co-catalyst for the activation of the ortho-C(sp2)-H bond in phenyl(pyrrolidin-1-yl)methanone, while the reaction of NaHCO3 and HCl reduces the overall barrier of the catalytic cycle. In the presence of both pivalic acid and NaHCO3 in the reaction, the C(sp2)-H bond is activated through a concerted metallation deprotonation process, and the C-C bond coupling is the rate-determining step with a total free energy barrier of 23.9 kcal mol-1. Without pivalic acid and NaHCO3, the C(sp2)-H bond can only be activated through a σ-bond metathesis process and the free energy barrier increases to 32.2 kcal mol-1. We also investigated the mechanisms of a side reaction for β-branched amine formation and the reaction without styrene and found that their free energy barriers are 33.4 and 30.5 kcal mol-1, respectively.

Experimental and theoretical study of photo-dissociation spectroscopy of pyrene dimer radical cations stored in a compact electrostatic ion storage ring.

In this paper, we present an experimental and theoretical study of the photo-dissociation of free-flying dimer radical cations of pyrene (C16H10)2+. Experimentally, the dimers were produced in the plasma of an electron cyclotron resonance ion source and stored in an electrostatic ion storage ring, the Mini-Ring for times up to 10 ms and the photo-dissociation spectrum was recorded in the 400 to 2000 nm range. Two broad absorption bands were observed at 550 (2.25 eV) and 1560 nm (0.79 eV), respectively. Theoretical simulations of the absorption spectrum as a function of the temperature were performed using the Density Functional based Tight Binding approach within the Extended Configuration Interaction scheme (DFTB-EXCI) to determine the electronic structure. The simulation involved all excited electronic states correlated asymptotically with the five lowest excited states D1-D5 of the monomer cation and a Monte Carlo exploration of the electronic ground state potential energy surface. The simulations exhibit three major bands at 1.0, 2.1 and 2.8 eV respectively. They allow assigning the experimental band at 1560 nm to absorption by the charge resonance (CR) excited state correlated with the ground state of the monomer D0. The band at 550 nm is tentatively attributed to dimer states correlated with excited states D2-D4, in the monomer cation. Simulations also show that the CR band broadens and shifts towards longer wavelength with increasing temperature. It results from the dependence on the geometry of the energy gap between the ground state and the lowest excited state. The comparison of the experimental spectrum with theoretical spectra at various temperatures allows us to estimate the temperature of the stored (C16H10)2+ in the 300-400 K range, which is also in line with the expected temperatures of the ions deduced from the analysis of the natural decay curve.

Modelling interactions of cationic dimers in He droplets: microsolvation trends in HenK2+ clusters.

We report the results of a detailed theoretical investigation of small K2+-doped He clusters. The structural characteristics and stabilities of such cations are determined from ab initio electronic structure calculations at the MRCI+Q level of theory. The underlying interactions show a multireference character and such effects are analyzed. The interaction potentials are constructed employing an interpolation technique within the inverse problem theory method, while the nuclear quantum effects are computed for the trimers, their spatial arrangements are discussed, and information was extracted on the orientational anisotropy of the forces. We found that energetically the most stable conformer corresponds to linear arrangements that are taking place under large amplitude vibrations, with high zero-point energy. We have further looked into the behavior of higher-order species with various He atoms surrounding the cationic dopant. By using a sum of potentials approach and an evolutionary programming method, we analyzed the structural stability of clusters with up to six He atoms in comparison with interactions energies obtained from MRCI+Q quantum chemistry computations. Structures containing Hen motifs that characterize pure rare gas clusters, appear for the larger K2+-doped He clusters, showing selective growth during the microsolvation process of the alkali-dimer cation surrounded by He atoms. Such results indicate the existence of local solvation microstructures in these aggregates, where the cationic impurity could get trapped for a short time, contributing to the slow ionic mobility observed experimentally in ultra-cold He-droplets.

Enhancement of deformation of redox-active hydrogel as an actuator by increasing pendant viologens and adding filler or counter-charged polymer

Strategies for enhancement of redox-driven deformation of a hydrogel, viologen pendant poly-L-Lysine based glutaraldehyde cross-linked hydrogel (PLLV-GA-gel), were pursued. To surpass the previously reported fastest reductive contraction, a 93 % volume decrease within 100 s on dithionite reduction, and to accelerate expansion on oxidation, we examined the pendant rate dependence and the effect of the addition of conductive fillers or an anionic polymer, poly(styrene sulfonate) (PSS). In testing deformation of water-floated PLLV-GA-gels, the contraction of a 25 % pendant rate gel was greater and faster than others. The initial rate of reductive contraction of the gels with fillers was in the order of graphene nano-platelet (GNP) > gold nanoparticle > carbon nanotube (CNT) > no-filler. The initial rates of swelling of GNP and CNT gels on oxidation were faster than the no-filler gel. PSS incorporation enhanced the swelling rate and amplitude but slowed down the contraction. All the PLLV-based samples on a Au electrode showed largely diffusion-controlled redox reactions of viologen in voltammograms and radical cation dimer rich light-absorption features in electroreflectance spectra. The guide for the choice of conductive filler and coexistence of anionic polymer described herein should be applicable to enhance the deformation of many redox-driven hydrogel actuators.

Cu(I) Arylsulfonate π-Complexes with 3-Allyl-2-thiohydantoin: The Role of the Weak Interactions in Structural Organization

The present work is directed toward preparation and structural characterization of two novel Cu(I) arylsulfonate π-complexes with 3-allyl-2-thiohydantoin, namely [Cu2(Hath)4](C6H5SO3)2 (1) and [Cu2(Hath)4](p-CH3C6H4SO3)2 · 2H2O (2) (Hath = 3-allyl-2-thiohydantoin), obtained by the means of alternating current electrochemical synthesis and studied with X-ray diffraction method. In both structures, the inner coordination sphere is represented by the cationic dimer [Cu2(Hath)4]2+ with one crystallographically independent copper(I) atom which has a trigonal pyramidal coordination environment formed by three Hath thiogroup S atoms and double C=C bond of its allyl group. [Cu2(Hath)4]2+ fragments in both coordination compounds are very similar, despite some divergences such as a big difference in Cu−S distance to the apical S atom (3.0374(8) Å in 1 and 2.7205(9) Å in 2). This difference was explained by the impact of the system of weak interactions, which are quite different.

Salts containing different amino acids: Three types salts of l-arginine containing glycine, dimethylglycine or β-alanine

Recently we have discovered a new class of amino acids salts containing different amino acids. In the present paper we report synthesis, crystal structures and infrared spectra of four such salts: l-Argininium(+) dimethylglycine iodide (l-ArgH)(DMG)I (I), di-l-argininium(+) glycinium glycine triiodide monohydrate (l-ArgH)2(GlyH⋅⋅⋅Gly)I3⋅H2O (II), di-l-argininium(+) β-alaninium β-alanine triiodide (l-ArgH)2(β-AlaH⋅⋅⋅β-Ala)I3 (III), and l-argininium(2+) bis-dimethylglycine diiodide (l-ArgH2⋅⋅⋅DMG)(DMG)I2 (IV). The salts (I-III) crystallize in P212121, P1, P21 space groups, respectively, each with one formula unit in the asymmetric unit, while the salt (IV) forms in space group P21 with two formula units in the asymmetric unit and is pseudo-centrosymmetric. The dimeric cations (GlyH⋅⋅⋅Gly), (β-AlaH⋅⋅⋅β-Ala) and (l-ArgH2⋅⋅⋅DMG) are built by O-H⋅⋅⋅O strong hydrogen bonds with very short O⋅⋅⋅O distances. The (l-ArgH2⋅⋅⋅DMG) dimeric cation of the [A(1)2+⋅⋅⋅A(2)] type was encountered for the first time. The obtained iodide salts (I, II, III, IV) containing different amino acids can be used in medicine as regulators of thyroxine synthesis in the body.

Structure of Mono‐Hydrated Aniline Dimer Cation

Photodissociation spectroscopy of mono‐hydrated aniline dimer (An2W+) cation was performed to determine its most stable structure and the charge resonance (CR) interaction therein. In its photodissociation action spectrum, two local excitation (LE) bands at ∼1.9 eV (656 nm) and >3.0 eV (<420 nm), previously assigned to 2 ← 1 and 1 ← 1 transition, respectively, for aniline dimer (An2+), were also observed for An2W+. On the other hand, the absorption band at ∼1.12 eV (1110 nm) from the CR interaction reported for An2+ disappeared in case of An2W+. Based on this infrared photodissociation action spectra supported by quantum chemical calculations, we suggest that the most stable structure of An2W+ maintains the stacked structure of An2+ but H2O molecule connected to the amino group of aniline molecule via hydrogen bonding plays a role to suppress the CR interaction.

Breaking the Barrier of Polynucleotide Size, Type, and Topology in Smad2 Antisense Therapy Using a Cationic Cholesterol Dimer with Flexible Spacer.

A liposomal formulation comprising a dicationic cholesterol based lipid, Chol+-(CH2)5-Chol+, and a helper zwitterionic lipid, DOPE (1:4), was prepared to deliver polynucleotides of different topologies, molecular weights, and backbones. This formulation was used to transfect HeLa cells with circular and linearized plasmid pEGFP-C3. The transfection efficiency of the dicationic cholesterol based coliposomal formulation Chol+-(CH2)5-Chol+/DOPE (1:4) was observed to be better when compared against different commercial delivery agents, Lipofectamine2000, Effectene, and a known oligonucleotide delivery agent, Oligofectamine. The efficacy was also compared with the respective monocationic cholesterol based liposomal formulations. Western blot analysis for Smad2 protein detection showed almost 100% downregulation of the Smad2 protein by polynucleotides delivered by Chol+-(CH2)5-Chol+/DOPE (1:4), which was better than that with Oligofectamine and Effectene. Similarly, semiquantitative RT-PCR showed the downregulation of Smad2 RNA along with that of a downstream target of Smad2, Id2. The higher efficiency of different types of nucleic acid delivery was also evident with Chol+-(CH2)5-Chol+/DOPE (1:4) in A549 cells. As an added benefit, the formulation Chol+-(CH2)5-Chol+/DOPE (1:4) was found to be highly biocompatible at all the compositions investigated herein.

Halogenides of β-alanine

The following halogenides of β-alanine: β-Ala·HF (I), β-Ala·HCl (II), β-Ala·HBr (III), β-Ala·HI (IV), 2β-Ala·HBr (V) and 2β-Ala·HI (VI) have been synthesized and characterized structurally and by vibrational spectroscopy. The salts (I–III) are simple salts A+X− but are not isostructural. The salt (IV) of the type of (A+⋯A+)(X–)2 comprises a centrosymmetric dimeric dication. The isotypic salts (V), (VI) belong to the type (A+⋯A)X– with centrosymmetric dimeric cations and strong hydrogen bonds.

The Role of the Solvent in the Condensed-Phase Dynamics and Identity of Chemical Bonds: The Case of the Sodium Dimer Cation in THF.

When a solute molecule is placed in solution, is it acceptable to presume that its electronic structure is essentially the same as that in the gas phase? In this paper, we address this question from a simulation perspective for the case of the sodium dimer cation (Na2+) molecule in both liquid Ar and liquid tetrahydrofuran (THF). In previous work, we showed that, when local specific interactions between a solute and solvent are energetically on the order of a hydrogen bond, the solvent can become part of the chemical identity of the solute. Here, using mixed quantum/classical molecular dynamics simulations, we see that, for the Na2+ molecule, solute-solvent interactions lead to two stable, chemically distinct coordination states (Na(THF)4-Na(THF)5+ and Na(THF)5-Na(THF)5+) that are not only stable themselves as gas-phase molecules but that also have a completely new electronic structure with important implications for the excited-state photodissociation of this molecule in the condensed phase. Furthermore, we show through a set of comparative classical simulations that treating the solute's bonding electron explicitly quantum mechanically is necessary to understand both the ground-state dynamics and chemical identity of this simple diatomic molecule; even use of the quantum-derived potential of mean force is insufficient to describe the behavior of the molecule classically. Finally, we calculate the results of a proposed transient hole-burning experiment that could be used to spectroscopically disentangle the presence of the different coordination states.

Chiral Lanthanide Complexes with l- and d-Alanine: An X-ray and Vibrational Circular Dichroism Study.

A whole series of [Ln(H2O)4(Ala)2]26+ dimeric cationic lanthanide complexes with both l- and d-alanine enantiomers was synthesized. The single-crystal X-ray diffraction at 100 and 292 K shows the formation of two types of dimers (I and II) in crystals. Between the dimer centers, the alanine molecules behave as bridging (μ2-O,O’-) and chelating bridging (μ2-O,O,O’-) ligands. The first type of bridge is present in dimers I, while both bridge forms can be observed in dimers II. The IR and vibrational circular dichroism (VCD) spectra of all l- and d-alanine complexes were registered in the 1750–1250 cm−1 range as KBr pellets. Despite all the studied complexes are exhibiting similar crystal structures, the spectra reveal correlations or trends with the Ln–O1 distances which exemplify the lanthanide contraction effect in the IR spectra. This is especially true for the positions and intensities of some IR bands. Unexpectedly, the ν(C=O) VCD bands are quite intense and their composed shapes reveal the inequivalence of the C=O vibrators in the unit cell which vary with the lanthanide. Unlike in the IR spectra, the ν(C=O) VCD band positions are only weakly correlated with the change of Ln and the VCD intensities at most show some trends. Nevertheless, this is the first observation of the lanthanide contraction effect in the VCD spectra. Generally, for the heavier lanthanides (Ln: Dy–Lu), the VCD band maxima are very close to each other and the mirror reflection of the band of two enantiomers is usually better than that of the lighter Lns. DFT calculations show that the higher the multiplicity the higher the stability of the system. Actually, the molecular geometry in crystals (at 100 K) is well predicted based on the highest-spin structures. Also, the simulated IR and VCD spectra strongly depend on the Ln electron configuration but the best overall agreement was reached for the Lu complex, which is the only system with a fully filled f-shell.

Reversible M-M Bonding by Alkaline Earth Metals (Mg, Ca, Sr, Ba) in Graphite Intercalation Compounds.

The alkaline earth metals (M=Mg, Ca, Sr, and Ba) exhibit a +2 oxidation state in nearly all known stable compounds, but MI dimeric complexes with M-M bonding, [M2 (en)2 ]2+ , (en=ethylenediamine) of all these metals can be stabilized within the galleries of donor-type graphite intercalation compounds (GICs). These metals can also form GICs with more conventional metal (II) ion complexes, [M(en)2 ]2+ . Here, the facile interconversion between dimeric-MI and monomeric-MII intercalates upon the addition/removal of en are reported. Thermogravimetry, powder X-ray diffraction, and pair distribution function analysis of total scattering data support the presence of either [M2 (en)2 ]2+ or [M(en)2 ]2+ guests. This phase conversion requires coupling graphene and metal redox centers, with associated reversible M-M bond formation within graphene galleries. This chemistry allows the facile isolation of unusual oxidation states, reveals M0 →M2+ reaction pathways, and present new opportunities in the design of hybrid conversion/intercalation materials for applications such as charge storage.

Balancing Density Functional Theory Interaction Energies in Charged Dimers Precursors to Organic Semiconductors.

The pursuit of an increasingly accurate description of intermolecular interactions within the framework of Kohn-Sham density functional theory (KS-DFT) has motivated the construction of numerous benchmark databases over the past two decades. By far, the largest efforts have been spent on closed-shell, neutral dimers for which today the interaction energies and geometries can be accurately reproduced by various combinations of dispersion-corrected density functional approximations (DFAs). In sharp contrast, charged, open-shell dimers remain a challenge as illustrated by the analysis of the OREL26rad benchmark set, composed of π-dimer radical cations. Aside from the methodological aspect, achieving a proper description of radical cationic complexes is appealing due to their role as models for charge carriers in organic semiconductors. In the interest of providing an assessment of more realistic dimer systems, we construct a data set of large radical cationic dimers (CryOrel9) and jointly train the 19 parameters of a dispersion corrected, range-separated hybrid density functional (ωB97X-dDsC). The main objective of ωB97X-dDsC is to provide the maximum balance between the treatment of long-range London dispersion and reduction of the delocalization error, which are essential conditions to obtain accurate energy profiles and binding energies of charged, open-shell dimers. The performance of ωB97X-dDsC, its parent ωB97X functional series, and a selection of wave function-based methods is reported for the CryOrel9 data set. The robustness of the reoptimized variant (ωB97X-dDsC) is also tested on other GMTKN30 data sets.

L-Tryptophanium iodide and l-tryptophanium l-tryptophan iodide monohydrate

The new salts l-tryptophanium iodide (I) and l-tryptophanium l-tryptophan iodide monohydrate (II) have been obtained. The salt (I) crystallizes in the orthorhombic system (space group P212121, Z = 4) and is not isostructural with l-tryptophanium chloride and l-tryptophanium bromide, which have P21 symmetry. Compound (II) is a salt with (l-TrpH···l-Trp) type dimeric cation. It crystallizes in the orthorhombic system (space group P212121, Z = 4) and is isostructural with l-tryptophanium l-tryptophan bromide monohydrate. The dimeric cation in (II) is formed by an O–H⋯O hydrogen bond with an O⋯O distance of 2.482(2) Å. The infrared and Raman spectra of the crystals are studied and discussed.