Dimer Cation Research Articles

Transport and dissociation of neon dimer cations in neon gas: a joint dynamical and Monte Carlo modeling

A hybrid dynamical method based on the classical treatment of nuclei and the quantum treatment of electrons was used to calculate momentum transfer and dissociation cross-sections for collisions of neon dimer cations with neon atoms. For the inclusion of nuclear quantum effects, a semi-empirical factor was introduced to correct the hybrid momentum transfer cross-sections at low collision energies. Both uncorrected and quantum corrected hybrid cross-sections were used to calculate the mobility, and longitudinal and transverse characteristic diffusion energies over a wide range of the reduced electric field. Furthermore, the dissociation rate constant was calculated and compared to measured data. In addition, an approximate inverse method based on an effective isotropic interaction potential was also used to calculate the momentum transfer cross-sections and related transport data.

First Experimental Evidence for the Diverse Requirements of Excimer vs Hole Stabilization in π-Stacked Assemblies

Exciton formation and charge separation and transport are key dynamical events in a variety of functional polymeric materials and biological systems, including DNA. Beyond the necessary cofacial approach of a pair of aromatic molecules at van der Waals contact, the extent of overlap and necessary geometrical reorganization for optimal stabilization of an excimer vs dimer cation radical remain unresolved. Here, we compare experimentally the dynamics of excimer formation (via emission) and charge stabilization (via threshold ionization) of a novel covalently linked, cofacially stacked fluorene dimer (F2) with the unlinked van der Waals dimer of fluorene, that is, (F)2. Although the measured ionization potentials are identical, the excimeric state is stabilized by up to ∼30 kJ/mol in covalently linked F2. Supported by theory, this work demonstrates for the first time experimentally that optimal stabilization of an excimer requires a perfect sandwich-like geometry with maximal overlap, whereas hole stabilization in π-stacked aggregates is less geometrically restrictive.

Calculation of Vibrational Spectra for Coordinated Thiocyanate Ion in Acetonitrile

The impact of the association of lithium cation with NCS– ion in acetonitrile on the vibrational spectrum was studied by the density-functional method in the B3LYP/6-31+G(d,p) approximation. The best agreement between experimental and calculated ionic association data was achieved taking into account the nonspecific solvation, oversolvation, and solubility of ionic complexes within the discrete-continuum model. The microstructures of the thiocyanate ion in a contact ion pair with lithium cation and ion-pair dimer and trimer in acetonitrile were established.

Study of mechanism of cyclohexane dehydrogenation by gas-phase Ti2+ cationic dimer in mixed PESs

The mechanism of Ti2+ catalyzed cyclohexane dehydrogenation had been studied at the B3LYP level of density functional theory (DFT). The first molecular dehydrogenation products were [Ti2(C6H10)]+ and H2, this process proceeded quite facile. After the elimination of the first molecular H2, there were two competitive mechanisms, one was the same-face dehydrogenation, the other was the H/D scrambling mechanism. The activation barrier for rate determining step of same-face dehydrogenation was 15.34kcal/mol, while 14.49kcal/mol for the different-face dehydrogenation which was more favorable than same-face one. Whole reaction proceeded in mixed potential energy surfaces (PESs) and the reaction products were mixture of the doublet and the quartet states. We calculated also all minimum energy crossing points (MECPs) between potential energy surfaces and analyzed bonding orbital of MECPs.

L-Cysteine halogenides: A new family of salts with an l-cysteine⋯l-cysteinium dimeric cation

Two l-cysteinium-halogenides with (l-cysteine···l-cysteinium) dimeric cations have been obtained, (l-Cys⋯l-Cys+)·Cl−, and (l-Cys⋯l-Cys+)·Br−. Both salts crystallize in monoclinic space group P21. Although these salts have the same dimeric cations and isotypical halogen anions, crystal packing is different. The main difference between the two salts rests in the conformation of (l-Cys⋯l-Cys+) dimeric cation, which also differs from that of the dimeric cation in the previously reported compound l-Cys+(l-Cys⋯l-Cys+)·F−·(F−⋯HF). The dimeric cation is formed by a very short O–H⋯O hydrogen bond with d(O···O) of 2.449(2) Å and 2.435(11) Å in the chloride and bromide, respectively. In addition to crystal structure analysis, Infrared and Raman spectra have been registered and discussed with a particular focus on intermolecular interactions. The l-Cys+·Br−·H2O salt with a simple l-cysteinium cation was also obtained and the crystal structure solved. It resembles its chloride analogue, l-Cys+·Cl−·H2O.

Conformational flexibility of xanthene‐based covalently linked dimers

The conformational flexibility of three covalently linked dimers consisting of two xanthene‐based moieties connected by a diphenyl ether linker was studied using NMR spectroscopy, X‐ray crystallography, and density functional theory (DFT) calculations. The three dimers interconvert as a function of pH: the doubly cationic dimer (Xan+)2 exists in acidic solutions (pH < 0.5), the mono‐alcohol monocation Xan+–Xan‐OH at intermediate pH values (pH = 1–3), and the neutral diol at the highest pH‐values (pH > 3). Each dimer exhibits conformational degrees of freedom associated with rotations of either the xanthene moiety or of the diphenyl ether (DPE) linker. The barriers for rotation of the xanthylium moiety were evaluated using DFT calculations, yielding values of 23 kcal/mol for (Xan+)2 and 11 kcal/mol for (Xan‐OH)2, respectively. The rotational barrier for the diphenyl ether linker in Xan+–Xan‐OH (15 kcal/mol) was experimentally determined using variable temperature NMR measurements. The relative orientation of the two –OH groups in (Xan‐OH)2 diol was investigated in solution and the solid state using NMR spectroscopy and X‐ray crystallography. The conformer observed in the solid state was found to be the In–Out conformer, while free rotation of the xanthenol units is thought to occur on the NMR timescale at room temperature. These studies are relevant for the design of linkers for efficient water oxidation catalysts. Copyright © 2016 John Wiley & Sons, Ltd.

Epoxide Formation Via Radical Cations and Biradicals

Epoxide is an important class of compounds, and is frequently formed in alkene oxygenation reactions. Detailed mechanism for the epoxide formation in reactions of alkene oxygenation via electron transfer is not known, however. In order to understand mechanism for the epoxide formation, we have performed a mechanistic study with DFT B3LYP/6-31G(d) level of theory for aliphatic alkenes. We have found that dimerization of peroxy radical cation, which gives the corresponding tetraoxide dication, followed by elimination of molecular oxygen, may be a major pathway for epoxide formation. We have also found that diradical that may be formed from peroxy radical cation via one electron reduction undergoes similar reaction to afford epoxide. Comparison of these results with those of electrochemical oxygenation of an alkene would also be presented.

Charge resonance structure of aniline dimer cation

The molecular structure of aniline dimer cation, (PhNH2)2+, was investigated by photodissociation spectroscopy coupled with tandem time of flight mass spectrometry. Mass-selected (PhNH2)2+ was dissociated by the irradiation of photons ranging from visible to near infrared region (420–1500 and 2600–3600nm). An intense absorption band was observed at ∼1.12eV (1110nm) in the photodissociation action spectrum and was assigned as the charge resonance (CR) interaction band of (PhNH2)2+ cation. Two other bands observed at ∼1.9eV (650nm) and >3.0eV (<420nm) were assigned as the local excitation (LE) bands, A2A2←X2B1 and B2B1←X2B1 of PhNH2+ in (PhNH2)2+, respectively (Honda et al., 2002). In the infrared spectrum, two broad and intense absorption bands were observed at ∼3190 and 3420cm−1 and were assigned as the symmetric and antisymmetric NH2 stretching of PhNH2+, respectively. The density functional theory calculations at the level of M06-2X/6-311++G(2d) and B3LYP/cc-pVTZ suggest a “head to tail” type stacked CR structure for (PhNH2)2+, stabilized by at least 800 and 250cm−1 compared to other possible isomers in the two theory levels, respectively. Besides, the basis set superposition error corrected binding energy derived from counterpoise calculation also indicated that the interaction energy was peaked in the CR structure.

Neutral, Anionic, and Cationic Manganese Dimers through Density Functional Theory.

The manganese dimer is the only first row transition metal dimer that presents an antiferromagnetic 1Σg + ground state bonded by a van der Waals interaction. The various density functional theory (DFT) investigations devoted to the study of the ground state of this molecule and of its anionic and cationic states, are usually based on the generalized gradient approximation, giving results which contradict the experimental observations. In this work, we describe the overall spectroscopic properties of the neutral, cationic and anionic manganese dimers with DFT, focusing on understanding the effects of the percentage of Hartree-Fock exchange and of the different exchange-correlation functionals, on the relative stability of the various potential energy curves. For each of the three species we classify the ferromagnetic and antiferromagnetic states, studying the vertical detachment energies, the ionization energies and the electron affinities. In this way, we locate a hybrid exchange-correlation functional able to give for all the three species, results comparable with the experimental measurements and with previous accurate multiconfigurational calculations, defining a more accurate density functional theory approach to study larger charged or neutral manganese clusters.

Sarcosine sarcosinium trifluoroacetate

We report the synthesis of a new salt in the system sarcosine – trifluoroacetic acid – water, namely sarcosine sarcosinium trifluoroacetate, whose structure contains a dimeric cation formed by an O–H···O hydrogen bond with an O···O distance equal to 2.528(2) Å. This is the first example among salts of amino acids with dimeric cation that contains the trifluoroacetate anion. Vibrational spectra of this crystal and previously known sarcosinium trifluoroacetate crystal have been studied. Peculiarities of the dimeric cation have been compared with other known salts of sarcosine with dimeric cations.

Synthesis and effect of linker length on guest-binding affinity of water-soluble tetraazacyclophane dimers

Cationic cyclophane dimers which are constructed with two tetraaza[6.1.6.1]para-cyclophanes and a short or long hydrophilic linker interposed between them were synthesized as a water-soluble host (1a and 1b, respectively). The binding constant (K) values of 1a having a short linkage with fluorescence guests such as 6-p-toluidinonaphthalene-2-sulfonate and 6-anilinonaphthalene-2-sulfonate were about 2-fold larger than those of 1b having a long one with the identical guests. Local concentration in the macrocycles of 1a was more effective than that of 1b for such enhancements of the guest-binding affinity. Moreover, the K values of cationic cyclophane dimer having a short hydrophobic linkage 1c were comparable to those by 1a, indicating that guest-binding affinity was less affected on the hydrophilic/hydrophobic characteristics of these short linkers.

Effects of single water molecule on proton transfer reaction in uracil dimer cation

Ionizing radiation to DNA induces sometimes the DNA damage. In this report, the ionization dynamics of uracil dimer (U)2 and its water complex (U)2–H2O have been investigated by means of direct ab initio molecular dynamics (AIMD) method in order to elucidate the effects of single water molecule on the reaction rate of proton transfer (PT) in DNA model base pair. The (U)2 dimer is widely used as a simplified mimetic model of Watson–Crick base pair. The static ab initio calculation showed that two conformers exist as neutral complex of (U)2–H2O. The direct AIMD calculation of ionization process of (U)2–H2O showed that the rate of PT is affected even by a single water molecule, while it was dependent on the position of H2O around (U)2. The interaction of water molecule with (U)2 affected the potential energy curve for PT. Especially, the activation barrier along the PT coordinate was significantly changed by the interaction with one H2O molecule. The effects of one H2O molecule on the PT process were discussed on the basis of theoretical results.

Insertion Reactions of Neutral Phosphidozirconocene Complexes as a Convenient Entry into Frustrated Lewis Pair Territory.

Neutral phosphidozirconocene complexes [Cp2Zr(PR2)Me] (Cp=cyclopentadienyl; 1a: R=cyclohexyl (Cy); 1b: R=mesityl (Mes); 1c: R=tBu) undergo insertion into the Zr-P bond by non-enolisable carbonyl building blocks (O=CR'R''), such as benzophenone, aldehydes, paraformaldehyde or CO2, to give [Cp2Zr(OCR'R''PR2)Me] (3-7). Depending on the steric bulk around P, complexes 3-7 react with B(C6F5)3 to give O-bridged cationic zirconocene dimers that display typical frustrated Lewis pair (FLP)/ambiphilic ligand behaviour. Thus, the reaction of {[Cp2Zr(μ-OCHPhPCy2)][MeB(C6F5)3]}2 (10a) with chalcone results in 1,4 addition of the Zr(+)/P FLP, whereas the reaction of {[Cp2Zr(μ-OCHFcPCy2)][MeB(C6F5)3]}2 (11a; Fc=(C5H4)CpFe) with [Pd(η(3)C3H5)Cl]2 yields the unique Zr-Fe-Pd trimetallic complex 13a, which has been characterised by XRD analysis.

Double layer for hard spheres with an off-center charge

Simulations for the density and potential profiles of the ions in the planar electrical double layer of a model electrolyte or an ionic liquid are reported. The ions of a real electrolyte or an ionic liquid are usually not spheres; in ionic liquids, the cations are molecular ions. In the past, this asymmetry has been modelled by considering spheres that are asymmetric in size and/or valence (viz., the primitive model) or by dimer cations that are formed by tangentially touching spheres. In this paper we consider spherical ions that are asymmetric in size and mimic the asymmetrical shape through an off-center charge that is located away from the center of the cation spheres, while the anion charge is at the center of anion spheres. The various singlet density and potential profiles are compared to (i) the dimer situation, that is, the constituent spheres of the dimer cation are tangentially tethered, and (ii) the standard primitive model. The results reveal the double layer structure to be substantially impacted especially when the cation is the counterion. As well as being of intrinsic interest, this off-center charge model may be useful for theories that consider spherical models and introduce the off-center charge as a perturbation.

Pentane transformations over sulfated alumina catalyst

The transformation of n-pentane at 295 K in the presence of catalysts of defined acid strength from −9.6 to −15 HO units were studied. The results were compared with those for $${{\text {Al}}_2}{{\text O}_{3}-}{\text{SO}}_{4}^{2 - }$$ (8 %) of acid strength equal to −11.5 < HO ≤ −10.8. It was found that: (a) for superacids of acid strength HO < −13, the reaction begins with the protonation of the C–H σ bond of a pentane and proceeds according to the bimolecular mechanism; (b) for less acidic catalysts such as sulfated alumina, pentane is oxidized to pentene, which is converted to the pentyl cation after protonation. The latter reacts in the presence of catalysts of acid strength of at least HO equal to −9.6 according to two pathways: (a) unimolecular at an excess of H− (in the presence of additional hydride donor pentane co-substrate) with selective formation of 2-methylbutane; (b) bimolecular in the absence of H− (pentane alone is the substrate). In the latter case, the transformation proceeds through the formation of a ten-carbon dimer cation state, which upon decomposition leads to 2-methylpropane, 2-methylbutane and unsaturated coke precursors.

The effect of counteranions on the molecular structures of phosphanegold(i) cluster cations formed by polyoxometalate (POM)-mediated clusterization.

The effect of counteranions on the molecular structures of phosphanegold(i) cluster cations formed by polyoxometalate (POM)-mediated clusterization was investigated. A novel intercluster compound, [{(AuLCl)2(μ-OH)}2]3[α-PMo12O40]2·3EtOH (1-PMo12), was obtained as orange-yellow plate crystals in 12.0% yield from a 6 : 1 molar ratio reaction of the monomeric phosphanegold(i) carboxylato complex [Au(RS-pyrrld)(LCl)] (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid; LCl = tris(4-chlorophenyl)phosphane) in CH2Cl2 with the free acid-form of Keggin polyoxometalate (POM), H3[α-PMo12O40]·14H2O. An EtOH/H2O (5 : 1, v/v) solvent mixture was used. The dimeric cation [{(AuLCl)2(μ-OH)}2](2+) in 1-PMo12 was in a parallel-edge arrangement that was formed by self-assembly through the inter-cationic aurophilic interactions of the μ-OH-bridged dinuclear phosphanegold(i) cation. The POM anion in 1-PMo12 was successfully exchanged with a smaller PF6(-) anion by the use of an anion-exchange resin. POM-free, colorless block crystals of [{(AuLCl)3(μ3-O)}2](PF6)2·4CH2Cl2 (2-PF6) were obtained by vapor diffusion in 14.1% yield. During the synthesis of 2-PF6, a compound with mixed counteranions (one POM and one PF6(-) anion), i.e. [{(AuLCl)4(μ4-O)}]2[α-PMo12O40]PF6 (3-PMo12PF6), was obtained in 66.4% yield. Both products were characterized by elemental analysis, TG/DTA, FT-IR, (31)P{(1)H} NMR, (1)H NMR, and X-ray crystallography. X-ray crystallography revealed that the countercation in 2-PF6 was the dimeric cation of the μ3-O-bridged tris{phosphanegold(i)} species, whereas that in 3-PMo12PF6 consisted of an unusual μ4-O-bridged tetragonal-pyramidal tetrakis{phosphanegold(i)} cation. Therefore, we concluded that the POM anion significantly contributed to the stabilization of these countercations (parallel-edged arrangement in 1-PMo12 and μ4-O-bridged tetragonal-pyramid in 3-PMo12PF6). Moreover, the previously reported yellow crystals of [{(AuLF)2(μ-OH)}2]3[PMo12O40]2·3EtOH (4-PMo12: LF = tris(4-fluoro phenyl)phosphane) were successfully converted to the POM-free crystalline OTf(-) salt [{(AuLF)2(μ-OH)}2](OTf)2·0.5Et2O (4-OTf) by the use of an anion-exchange resin. X-ray crystallography also revealed that the parallel-edge arrangement of the dimeric cation in 4-PMo12 was converted to the crossed-edge arrangement of that in 4-OTf. These results illustrate that the AuOPOM and hydrogen-bonding (C-HOPOM and O-HOPOM) interactions between the phosphanegold(i) cluster cation and the Keggin POM anion in the solid state significantly contribute to the structure, composition, and stability of the phosphane gold(i) cluster cations in 4-PMo12.

Photoionization-induced π↔ H site switching dynamics in phenol(+)-Rg (Rg = Ar, Kr) dimers probed by picosecond time-resolved infrared spectroscopy.

The ionization-induced π↔ H site switching reaction in phenol(+)-Rg (PhOH(+)-Rg) dimers with Rg = Ar and Kr is traced in real time by picosecond time-resolved infrared (ps-TRIR) spectroscopy. The ps-TRIR spectra show the prompt appearance of the non-vanishing free OH stretching band upon resonant photoionization of the π-bound neutral clusters, and the delayed appearance of the hydrogen-bonded (H-bonded) OH stretching band. This result directly proves that the Rg ligand switches from the π-bound site on the aromatic ring to the H-bonded site at the OH group by ionization. The subsequent H →π back reaction converges the dimer to a π↔ H equilibrium. This result is in sharp contrast to the single-step π→ H forward reaction in the PhOH(+)-Ar2 trimer with 100% yield. The reaction mechanism and yield strongly depend on intracluster vibrational energy redistribution. A classical rate equation analysis for the time evolutions of the band intensities of the two vibrations results in similar estimates for the time constants of the π→ H forward reaction of τ+ = 122 and 73 ps and the H →π back reaction of τ- = 155 and 188 ps for PhOH(+)-Ar and PhOH(+)-Kr, respectively. The one order of magnitude slower time constant in comparison to the PhOH(+)-Ar2 trimer (τ+ = 7 ps) is attributed to the decrease in density of states due to the absence of the second Ar in the dimer. The similar time constants for both PhOH(+)-Rg dimers are well rationalized by a classical interpretation based on the comparable potential energy surfaces, reaction pathways, and density of states arising from their similar intermolecular vibrational frequencies.

Quantum molecular motion in the mixed ion-radical complex, [(H2O)(H2S)

The cation dimer of water and hydrogen sulfide, [(H2O)(H2S)]+, serves as a fundamental model for the oxidation chemistry of H2S. The known oxidative metabolism of H2S by biological species in sulfur-rich environments has motivated the study of the inherent properties of this benchmark complex, with possible mechanistic implications for modern water oxidation chemistry. The low-energy isomer of this open-shell ion is a proton-transferred (PT) structure, H3O+SH˙. An alternative PT structure, H3S+OH˙, and a hemibonded (HB) isomer, [H2O·SH2]+, are also stable isomers, placing this complex intermediate to known (H2O)2+ (PT) and (H2S)2+ (HB) limiting regimes. This intermediate character suggested the possibility of unique molecular motion, even in the vibrational ground state. Path integral molecular dynamics and anharmonic vibrational spectroscopy simulations have been performed in this study, in order to understand the inherent quantum molecular motion of this complex. The resulting structural distributions were found to deviate significantly from both classical and harmonic analyses, including the observation of large-amplitude anharmonic motion of the central proton and nearly free rotation of the terminal hydrogens. The predicted vibrational spectra are particularly unique and suggest characteristic signatures of the strong electronic interactions and anharmonic vibrational mode couplings in this radical cation.

Use of [Re6Q8(CN)6]4− (Q = S, Se, Te) cluster anions and Cu(I) cationic complexes with 2,2′-bipyridine for the construction of new cyano-bridged coordination compounds

A series of four new coordination compounds [{Cu(bpy)2(μ-CN)Cu(bpy)}2Re6S8(CN)6]·bpy·2H2O (1), [{Cu(bpy)}2(μ-CN)][{Cu(bpy)}3Re6S8(CN)6] (2) and [{Cu(bpy)(μ-CN)Cu(bpy)}2{Cu(bpy)}2Re6Q8(CN)6] (3, Q=Se; 4, Q=Te) have been synthesized by the reactions of hexanuclear rhenium(III) chalcocyanide complexes, CuCN and 2,2′-bipyridine (bpy) under hydrothermal conditions. Compound 1 was characterized by single-crystal X-ray diffraction analysis, compounds 2–4 were characterized by a set of methods, including IR spectroscopy, thermal analysis, elemental analysis, EDS, and single-crystal X-ray diffraction analysis. 1 has a molecular structure, in which two cyano groups of cluster anion [Re6S8(CN)6]4− are N-bonded to the dimeric mixed-valent cations {Cu(bpy)2(μ-CN)Cu(bpy)}2+ through axial coordination to the Cu+ cations. In 2, the cluster anions provide all of their six cyano groups for coordination with the copper centers of {Cu(bpy)}+ moieties, giving rise to a 3D framework in which the cavities are occupied by the dimeric cations [{Cu(bpy)}2(μ-CN)]+. 3 and 4 are isostructural and have a 1D chain structure, in which four equatorial cyano groups of cluster anions are N-bonded to the {Cu(bpy)}+ bridging fragments, while two axial cyano groups are terminated by dimeric cations {Cu(bpy)(μ-CN)Cu(bpy)}+. The compounds synthesized appear to be the first examples which combine hexanuclear rhenium(III) chalcocyanide clusters and monovalent Cu(I) ions.

A double complex Cd(II) salt containing the Keggin polyoxoanion and the supramolecular cation of the [amide…H…amide]+ type

The complex [H(DMFA)2]2[Cd(DMFA)6][SiW12O40] (I) was obtained by a reaction between H4[SiW12O40], CdI2, and AgNO3 in N,N-dimethylformamide (DMF) and characterized by X-ray diffraction (CIF file CCDC No. 1041227). Structure I is made up of the complex cations [Cd(DMF)6]2+, the Keggin polyoxoanions [SiW12O40]4–, and the dimeric cations [DMF...H...DMF]+, in which two DMF molecules are linked by a short hydrogen bond (O...O, 2.39(2) A). The cations of this type were compared with similar dimeric cations containing the fragment {amide...H...amide}.