Dimer Cation Research Articles

Dissecting Noncovalent Interactions in Carboxyl-Functionalized Ionic Liquids Exhibiting Double and Single Hydrogens Bonds Between Ions of Like Charge.

We show that the carboxyl‐functionalized ionic liquid 1‐(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC‐CH2‐py][NTf2] exhibits three types of hydrogen bonding: the expected single hydrogen bonds between cation and anion, and, surprisingly, single and double hydrogen bonds between the cations, despite the repulsive Coulomb forces between the ions of like charge. Combining X‐ray crystallography, differential scanning calorimetry, IR spectroscopy, thermodynamic methods and DFT calculations allows the analysis and characterization of all types of hydrogen bonding present in the solid, liquid and gaseous states of the ionic liquid (IL). We find doubly hydrogen bonded cationic dimers (c+=c+) in the crystalline phase. With increasing temperature, this binding motif opens in the liquid and is replaced by (c+−c+−a− species, with a remaining single cationic hydrogen bond and an additional hydrogen bond between cation and anion. We provide clear evidence that the IL evaporates as hydrogen‐bonded ion pairs (c+−a−) into the gas phase. The measured transition enthalpies allow the noncovalent interactions to be dissected and the hydrogen bond strength between ions of like charge to be determined.

Influence of cation shape asymmetry on the interfacial features and capacitance curve of ionic liquids inside the spherical cavity of the porous electrode as an ionic liquid-based supercapacitor

Two imidazolium-based ionic liquids with the same anions but different cations (one was considered as a dimer model while the other as a monomer one) have been compared in some interfacial phenomena such as overscreening, surface charge amplification, and crowding, besides the shape of the capacitance curve inside the spherical cavity of the porous electrode. The coarse-grained model in the framework of classical density functional theory, CDFT, has been used to investigate the different electric double layer structures of these two models through different behaviors for their mean electric potential, MEP, local volume charge density, LVCD, and normalized integrated charge density, NICD, at negative electrode potentials. Although both models illustrate charge reversal, CR, and crowding phenomena, respectively, at moderate and high electrode potentials, at low electrode potentials close to the potential of zero charge, PZC, the monomer model illustrates CR, while the dimer model shows surface charge amplification, SCA. This different behavior may be explained by the fact that anions, due to their small sizes, are positioned in the spatial region that is created by the orientations of dimer cations at the electrode surface. This leads to a negative LVCD and partial charge density in the first layer near the electrode. The normalized integrated charge density as a function of r/σ can be used as a criterion for determining the type of screening phenomenon with curvature. Our results show that regardless of the cavity size, the screening behavior of ionic liquids in different cavities is similar, although its intensity varies with size. Other consequences of cation shape asymmetry for dimer models are related to fewer stored net ion charges inside the cavity at the negative potential region and also an asymmetric shape for the capacitance curve. It has also been identified that the shape of the capacitance curve is a charge concentration effect rather than a geometric effect.

Capture of Hydroxyl Radicals by Hydronium Cations in Water Microdroplets

AbstractDespite the high stability of bulk water, water microdroplets possess strikingly different properties, such as the presence of hydroxyl radicals (OH⋅) at the air–water interface. Previous studies exhibited the recombination of OH⋅ into H2O2 molecules and the capture of OH⋅ by oxidizing other molecules. By spraying pure water microdroplets into a mass spectrometer, we detected OH⋅ in the form of (H4O2)+ that is essentially OH⋅−H3O+, a hydroxyl radical combined with a hydronium cation through hydrogen bonding. We also successfully captured it with two OH⋅ scavengers, caffeine and melatonin, and key oxidation radical intermediates that bear important mechanistic information were seen. It is suggested that some previous reactions involving (H4O2)+ should be attributed to reactions with OH⋅−H3O+ rather than with the water dimer cation (H2O−OH2)+.

Above-room-temperature ferroelectricity and piezoelectric activity of dimethylglycinium-dimethylglycine chloride

Heavy-metal-free ferroelectrics are sought as environmentally compatible alternatives to commonly used inorganic oxides. Here, we demonstrate direct evidence of the ferroelectric properties of a hybrid organic–inorganic material, dimethylglycinium-dimethylglycine chloride. At room temperature, the compound crystallizes in the polar space group P21 and exhibits a switchable spontaneous polarization of 1.9 μC cm−2. Ferroelectric properties are preserved in a wide temperature range up to about 401 K, where the crystal undergoes the transition to the paraelectric phase of the space group P21/c. The temperature-dependent single-crystal X-ray diffraction study and the calorimetric data indicate an order–disorder contribution to the transition mechanism, which is consistent with the critical slowing down of the dielectric relaxation observed near the Curie point. The spontaneous polarization results from ionic displacements that are induced by changes in the disordering of the dimeric cations. In the ferroelectric phase, the crystal exhibits remarkable piezoelectric activity. The electromechanical and elastic properties of the material were thoroughly characterized.

Comparison between polycrystalline Au and single-crystalline Au(1 1 1) electrodes as the substrate of a cationic organic monolayer based on their anion dependent redox activities

The structures of organic monolayers immobilized on a metal electrode and their functions depend on the substrate surface chemistry and morphology. A better defined crystalline flat electrode usually provides a structurally more ordered organic monolayer, although the use of a single-crystalline electrode costs more for mass production. Because the monolayer structures at both molecular and mesoscopic levels are critical to the functions of the modified electrode, in-depth experimental studies to clarify the relationship between the functions and the surface morphology of the electrode are crucial. In this work, we compared polycrystalline Au (poly-Au) and single-crystalline Au(111) electrodes as the substrates for a redox-active, self-assembled monolayer (SAM) of an alkyl-viologen thiol, 1-(10-mercaptodecyl)-1′-methyl-4,4′-bipyridinium dibromide (MeVC10-SH 2Br−). The voltammetric and electroreflectance measurements were performed at a mirror-polished poly-Au electrode and a flame-annealed single-crystalline Au(111) electrode after their surface modification with a MeVC10-SH monolayer. We found significant differences in the anion effects on the voltammetric behavior between the two substrates, especially in the dependence on anion hydrophobicity. The electroreflectance spectra revealed a tendency that a greater abundance of the viologen radical cation dimers at a Au(111) than that at a poly-Au. A considerable Br− adsorption at the SAM-modified Au(111) electrode was observed because of the loose packing of immobilized MeVC10-SH molecules. We discussed the origins of the contrasting behavior resulting from the difference of the surface.

Sulfamates with dimeric cations of glycine, sarcosine and dimethylglycine

Crystalline sulfamate salts of glycine (GlyH⋅⋅⋅Gly)(NH2SO3) (I), sarcosine (SarH⋅⋅⋅Sar)(NH2SO3) (II) and dimethylglycine (DMGH⋅⋅⋅DMG)(NH2SO3)⋅H2O (III) with dimeric cations, obtained from aqueous solutions, were structurally and spectroscopically characterized and further compared with the previously known salt of betaine (Bet⋅⋅⋅H⋅⋅⋅Bet)(NH2SO3). Compounds (I) and (II) crystallize in polar symmetry (space groups Pn and Pna21, respectively), while the salt (III) and that of betaine are centrosymmetric (space group settings P21/n and P21/c, respectively).

Organometallic and Organic Dimers: Moderately Air-Stable, Yet Highly Reducing, n-Dopants.

ConspectusElectrical doping using redox-active molecules can increase the conductivity of organic semiconductors and lower charge-carrier injection and extraction barriers; it has application in devices such as organic and perovskite light-emitting diodes, organic and perovskite photovoltaic cells, field-effect transistors, and thermoelectric devices. Simple one-electron reductants that can act as n-dopants for a wide range of useful semiconductors must necessarily have low ionization energies and are, thus, highly sensitive toward ambient conditions, leading to challenges in their storage and handling. A number of approaches to this challenge have been developed, in which the highly reducing species is generated from a precursor or in which electron transfer is coupled in some way to a chemical reaction. Many of these approaches are relatively limited in applicability because of processing constraints, limited dopant strength, or the formation of side products.This Account discusses our work to develop relatively stable, yet highly reducing, n-dopants based on the dimers formed by some 19-electron organometallic complexes and by some organic radicals. These dimers are sufficiently inert that they can be briefly handled as solids in air but react with acceptors to release two electrons and to form two equivalents of stable monomeric cations, without formation of unwanted side products. We first discuss syntheses of such dimers, both previously reported and our own. We next turn to discuss their thermodynamic redox potentials, which depend on both the oxidation potential of the highly reducing odd-electron monomers and on the free energies of dissociation of the dimers; because trends in both these quantities depend on the monomer stability, they often more-or-less cancel, resulting in effective redox potentials for a number of the organometallic dimers that are approximately -2.0 V vs ferrocenium/ferrocene. However, variations in the dimer oxidation potential and the dissociation energies determine the mechanism through which a dimer reacts with a given acceptor in solution: in all cases dimer-to-acceptor electron transfer is followed by dimer cation cleavage and a subsequent second electron transfer from the neutral monomer to the acceptor, but examples with weak central bonds can also react through endergonic cleavage of the neutral dimer, followed by electron-transfer reactions between the resulting monomers and the acceptor. We, then, discuss the use of these dimers to dope a wide range of semiconductors through both vacuum and solution processing. In particular, we highlight the role of photoactivation in extending the reach of one of these dopants, enabling successful doping of a low-electron-affinity electron-transport material in an organic light-emitting diode. Finally, we suggest future directions for research using dimeric dopants.

Thermodynamically Stable Cationic Dimers in Carboxyl-Functionalized Ionic Liquids: The Paradoxical Case of "Anti-Electrostatic" Hydrogen Bonding.

We show that carboxyl-functionalized ionic liquids (ILs) form doubly hydrogen-bonded cationic dimers (c+=c+) despite the repulsive forces between ions of like charge and competing hydrogen bonds between cation and anion (c+–a−). This structural motif as known for formic acid, the archetype of double hydrogen bridges, is present in the solid state of the IL 1−(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC−CH2−py][NTf2]. By means of quantum chemical calculations, we explored different hydrogen-bonded isomers of neutral (HOOC–(CH2)n–py+)2(NTf2−)2, single-charged (HOOC–(CH2)n–py+)2(NTf2−), and double-charged (HOOC– (CH2)n−py+)2 complexes for demonstrating the paradoxical case of “anti-electrostatic” hydrogen bonding (AEHB) between ions of like charge. For the pure doubly hydrogen-bonded cationic dimers (HOOC– (CH2)n−py+)2, we report robust kinetic stability for n = 1–4. At n = 5, hydrogen bonding and dispersion fully compensate for the repulsive Coulomb forces between the cations, allowing for the quantification of the two equivalent hydrogen bonds and dispersion interaction in the order of 58.5 and 11 kJmol−1, respectively. For n = 6–8, we calculated negative free energies for temperatures below 47, 80, and 114 K, respectively. Quantum cluster equilibrium (QCE) theory predicts the equilibria between cationic monomers and dimers by considering the intermolecular interaction between the species, leading to thermodynamic stability at even higher temperatures. We rationalize the H-bond characteristics of the cationic dimers by the natural bond orbital (NBO) approach, emphasizing the strong correlation between NBO-based and spectroscopic descriptors, such as NMR chemical shifts and vibrational frequencies.

A plethora of isomerization processes and hydrogen scrambling in the fragmentation of the methanol dimer cation: a PEPICO study.

The valence photoionization of light and deuterated methanol dimers was studied by imaging photoelectron photoion coincidence spectroscopy in the 10.00-10.35 eV photon energy range. Methanol clusters were generated in a rich methanol beam in nitrogen after expansion into vacuum. They generally photoionize dissociatively to protonated methanol cluster cations, (CH3OH)nH+. However, the stable dimer parent ion (CH3OH)2+ is readily detected below the dissociation threshold to yield the dominant CH3OH2+ fragment ion. In addition to protonated methanol, we could also detect the water- and methyl-loss fragment ions of the methanol dimer cation for the first time. These newly revealed fragmentation channels are slow and cannot compete with protonated methanol cation formation at higher internal energies. In fact, the water- and methyl-loss fragment ions appear together and disappear at a ca. 150 meV higher energy in the breakdown diagram. Experiments with selectively deuterated methanol samples showed H scrambling involving two hydroxyl and one methyl hydrogens prior to protonated methanol formation. These insights guided the potential energy surface exploration to rationalize the dissociative photoionization mechanism. The potential energy surface was further validated by a statistical model including isotope effects to fit the experiment for the light and the perdeuterated methanol dimers simultaneously. The (CH3OH)2+ parent ion dissociates via five parallel channels at low internal energies. The loss of both CH2OH and CH3O neutral fragments leads to protonated methanol. However, the latter, direct dissociation channel is energetically forbidden at low energies. Instead, an isomerization transition state is followed by proton transfer from a methyl group, which leads to the CH3(H)OH+⋯CH2OH ion, the precursor to the CH2OH-, H2O-, and CH3-loss fragments after further isomerization steps, in part by a roaming mechanism. Water loss yields the ethanol cation, and two paths are proposed to account for m/z 49 fragment ions after CH3 loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH3OH2+, which explains the dominance of the protonated methanol fragment ion in the mass spectrum.

Assembly of Polyiodide Networks with Cu(II) Complexes of Pyridinol-Based Tetraaza Macrocycles.

Polyiodide networks are currently of great practical interest for the preparation of new electronic materials. The participation of metals in the formation of these networks is believed to improve their mechanical performance and thermal stability. Here we report the results on the construction of polyiodide networks obtained using Cu(II) complexes of a series of pyridinol-based tetraazacyclophanes as countercations. The assembly of these crystalline polyiodides takes place from aqueous solutions on the basis of similar structural elements, the [CuL]2+ and [Cu(H–1L)]+ (L = L2, L2-Me, L2-Me3) complex cations, so that the peculiarities induced by the increase of N-methylation of ligands, the structural variable of ligands, can be highlighted. First, solution equilibria involving ligands and complexes were analyzed (potentiometry, NMR, UV–vis, ITC). Then, the appropriate conditions could be selected to prepare polyiodides based on the above complex cations. Single-crystal XRD analysis showed that the coordination of pyridinol units to two metal ions is a prime feature of these ligands, leading to polymeric coordination chains of general formula {[Cu(H–1L)]}nn+ (L = L2-Me, L2-Me3). In the presence of the I–/I2 couple, the polymerization tendency stops with the formation of [(CuL)(CuH–1L)]3+ (L = L2-Me, L2-Me3) dimers which are surrounded by polyiodide networks. Moreover, coordination of the pyridinol group to two metal ions transforms the surface charge of the ring from negative to markedly positive, generating a suitable environment for the assembly of polyiodide anions, while N-methylation shifts the directional control of the assembly from H-bonds to I···I interactions. In fact, an extended concatenation of iodine atoms occurs around the complex dimeric cations, the supramolecular I···I interactions become shorter and shorter, fading into stronger forces dominated by the orbital overlap, which is promising for effective electronic materials.

Correlation between Supramolecular Connectivity and Magnetic Behaviour of [FeIII(5-X-qsal)2]+-Based Salts Prone to Exhibit SCO Transition

We present an extensive study to determine the relationship between structural features of spin crossover (SCO) systems based on N-(8-quinolyl)salicylaldimine (qsal) ligand derivatives and their magnetic properties. Thirteen new compounds with general formula [FeIII(5-X-qsal)2]+ (X = H, F, Cl, Br and I) coupled to Cl−, ClO4−, SCN−, PF6−, BF4− and BPh4− anions were prepared and magnetically characterized. The structure/properties correlations observed in these compounds were compared to those of salts with the same [FeIII(qsal-X)2]+ cations previously reported in the literature. These cations favour the LS configuration in compounds with the weakest connectivity. As connectivity increases most of them present HS states at room temperature and structures may be described as arrangements of parallel layers of interacting cation dimers. All the compounds based on these cations undergoing complete SCO transitions within the 4–300 K temperature range have high intralayer connectivity. If, however, the interlayer connectivity becomes very strong they remain blocked in the HS or in the LS state. The SCO transition may be affected by the slightest change of solvent molecules content, disorder or even crystallinity of the sample and it remain difficult to predict which kind of ligand substituent should be selected to obtain compounds with the desired connectivity.

Salts containing different amino acids: Salts with β-alaninium L-proline dimeric cation

Recently we have discovered a new class of amino acids salts containing different amino acids. In the present paper we report crystal structures of salts with the β-alaninium L-proline dimeric cation: β-alaninium L-proline chloride, (β-AlaH⋅⋅⋅l-Pro)Cl (I), β-alaninium L-proline bromide, (β-AlaH⋅⋅⋅l-Pro)Br (II), β-alaninium L-proline iodide, (β-AlaH⋅⋅⋅l-Pro)I (III), β-alaninium L-proline tetrafluoroborate, (β-AlaH⋅⋅⋅l-Pro)BF4 (IV), β-alaninium L-proline perchlorate, (β-AlaH⋅⋅⋅l-Pro)ClO4 (V) and β-alaninium L-proline nitrate, (β-AlaH⋅⋅⋅l-Pro)NO3 (VI). The salts (I–III) are isostructural and crystallize in the polar space group P21 with one formula unit in the asymmetric unit. The salts (IV-VI) crystallize in space group P212121 with two (isotypic compounds (IV) and (V)) resp. one (VI) formula units in the asymmetric unit. The O⋅⋅⋅O distances in the dimeric cations are 2.490(5) Å (I), 2.5001(19) Å (II), 2.529(3) Å (III) and 2.556(4), 2.581(4) Å (IV), 2.566(5), 2.570(5) Å (V) and 2.561(2) Å (VI). These values are compared with those in salts with dimeric cations (β-AlaH⋅⋅⋅β-Ala) and (l-ProH⋅⋅⋅l-Pro) with respective anions. Infrared spectra of obtained salts were registered and are discussed.

L-Prolinium l-proline halogenides: a comparison of the respective chloride, bromide and iodide salts

• # Chloride, bromide and iodide salts with l -prolinium l -proline dimeric cation were studied • # Tetragonal chloride and bromide are isostructural, while iodide is orthorhombic • # Short O-H…O hydrogen bonds of dimeric cations are reflected in the infrared spectra • # Isostructural chloride and bromide are not SHG-active in agreement with Kleinman condition Structures and vibrational spectra of l -prolinium l -proline chloride (I), l -prolinium l -proline bromide (II) and l -prolinium l -proline iodide (III) were analyzed at 100K, 200K and room temperature (RT). The salts (I) and (II) are isostructural and crystallize in the tetragonal space group P 4 1 2 1 2, Z=8 at RT and in orthorhombic P 2 1 2 1 2 1 , Z=8 at 100K and 200K. The salt (III) is orthorhombic, space group P 2 1 2 1 2 1 , Z=4 at all three temperatures. The O⋅⋅⋅O distances in l -prolinium l -proline dimeric cations are equal to 2.464(3) Å (I), 2.456(3) Å (II) and 2.456(4) Å (III) at RT. The presence of strong hydrogen bonds in the crystal structures is reflected in the infrared spectra. The influence of symmetry on second harmonic activity is discussed.

Tc(NO)Cl(Cp)(PPh3)] – A Technetium(I) Compound with an Unexpected Synthetic Potential

AbstractReactions of the chiral organometallic technetium(I) compound [Tc(NO)Cl(Cp)(PPh3)] (1 a) with phosphines and alkynes give unexpected products such as the dimeric cation [{Tc(NO)(Cp)(PPh3)}2Cl]+ (4) or the Fischer‐type carbene [Tc(NO)(Cp)(PPh3){C(OR)C2H4PPh3}]2+ (5). Both products could be isolated as PF6− salts and studied by spectroscopic methods and X‐ray diffraction. While [Tc(NO)(Cp)(PMe3)(PPh3)](PF6) (2PF6) can be obtained from 1 a, PMe3 and Ag(PF6) under ambient conditions, the synthesis of the corresponding bis‐triphenylphosphine complex [Tc(NO)(Cp)(PPh3)2](PF6) (3PF6) requires high temperatures and long reaction times. The products are stable in air.

Exploring Curious Covalent Bonding: Raman Identification and Thermodynamics of Perpendicular and Parallel Pancake Bonding (Pimers) of Ethyl Viologen Radical Cation Dimers.

Viologen radical cations can dimerize in solutions, and the resulting "pimers" were predicted to assemble into parallel and perpendicular conformers by density functional theory (DFT) calculations. Using resonance Raman, we could identify both perpendicular and parallel forms of ethyl viologen dimers. The distinction between the two forms was accomplished by studying the formation of a host-guest complex with γ-cyclodextrin. The dimer's perpendicular form was excluded due to the host cavity size, and γ-cyclodextrin addition caused a decrease in peak intensities at 1171, 1511, and 1602 cm-1 that could be assigned to the perpendicular form. DFT modeling of the vibrational spectra under preresonance conditions allowed us to assign the remaining vibrational modes for the parallel and perpendicular forms. Using variable-temperature UV-vis, the bond dissociation energy (ΔH) for this pancake-bonded dimer was measured as 13.1 ± 0.2 kcal/mol. This type of covalent pancake bonding is a challenge to properly describe using DFT methods. Previously, B97D was found to best describe the ΔG of this dimerization (Angew. Chem. 2017, 129, 9563-9567), but this method underestimates the ΔH by 6 kcal/mol. Of the 11 functionals tested, we found that B3LYP with Grimme's D3 dispersion effect can best reproduce the ΔH. Energy decomposition analysis of the bonding energy showed that solvation effects were the most important contributor-polar solvents are needed to overcome the Coulomb repulsion between the two positively charged monomers. Dispersion effects are second in importance and appear larger than the favorable orbital interaction obtained by singly occupied molecular orbital (SOMO)-SOMO orbital overlap. This study brings forth important insights into the curious cases of covalent bonding between two π-delocalized radicals.

DEER Spectroscopy of Channelrhodopsin-2 Helix B Movements in Trapped Photocycle Intermediates

The light-gated dimeric cation channel channelrhodopsin-2 (ChR2) has been established as one of the most important optogenetic tools. During its functional cycle, ChR2 undergoes conformational changes, the most prominent ones include a movement of transmembrane helix B. In the present work, we assign this movement to a trapped photocycle intermediate using DEER spectroscopy combined with sample illumination inside the microwave resonator, allowing trapping and relaxation of defined ChR2 intermediates at different temperatures between 180 and 278 K. Intradimer distances measured between spin-labeled positions 79 located in helix B of ChR2 in the dark state and upon light activation and relaxation at 180 K were similar. In contrast, light activation at 180 K and 30 min relaxation at between 230 and 255 K results in significant changes of the distance distribution. We show that the light-induced movement of helix B is correlated with the presence of the P480 state of ChR2. We hypothesize that conformational changes occurring in this area are key elements responsible for desensitizing the channel for cation conduction.

Self-aggregation, Temperature-responsive Agglutination, and pH-induced Disaggregation of Amphiphilic Cyclophane Dimer Having a PEG Linkage

An amphiphilic cyclophane dimer (1), which was constructed with two cyclophane skeletons, each having three hydrophobic side-chains with Boc-protecting amino groups, and a hydrophilic polyethylene glycol (PEG) linkage, was synthesized. Amphiphilic compound 1 was found to form self-aggregate in aqueous media. In addition, 1 exhibited lowest critical solution temperature (LCST) behavior and its aqueous solution became turbid above the LCST upon heating. Moreover, pH-induced disaggregation of the self-aggregates of 1 was successfully performed upon addition of hydrochloric acid, to produce a cationic water-soluble cyclophane dimer (2) by the removal of the Boc-protecting groups of 1.

Pulse Radiolysis Investigation of Radicals Derived from Water-Soluble Cyanine Dyes: Implications for Super-resolution Microscopy.

Light-induced blinking, an inherent feature of many forms of super-resolution microscopy, has been linked to transient reduction of the fluorescent cyanine dye used as an imaging agent. There is, however, only scant literature information related to one-electron reduced cyanine dyes, especially in an aqueous environment. Here, we examine a small series of cyanine dyes, possessing disparate π-conjugation lengths, under selective reducing or oxidizing conditions. The experiment allows recording of both differential absorption spectra and decay kinetics of the resultant one-electron reduced or oxidized transient species in water. Relative to the ground state, absorption transitions for the various radicals are weak and somewhat broadened but do allow correlation with the π-conjugation length. In all cases, absorption maxima lie to the blue of the main ground-state transition. Under anaerobic conditions, the transient species decay on the microsecond to millisecond time scale, with the mean lifetime depending on molecular structure, radiation dose, and dye concentration. The experimental absorption spectra recorded for the one-electron reduced radicals and the presumed dimer cation radical compare well to spectra obtained from time-dependent density functional theory calculations. The results allow conclusions to be drawn regarding the plausibility of the reduced species being responsible for light-induced blinking in direct stochastic optical reconstruction microscopy.

Asymmetric Solvation of the Zinc Dimer Cation Revealed by Infrared Multiple Photon Dissociation Spectroscopy of Zn2+(H2O)n (n = 1-20).

Investigating metal-ion solvation—in particular, the fundamental binding interactions—enhances the understanding of many processes, including hydrogen production via catalysis at metal centers and metal corrosion. Infrared spectra of the hydrated zinc dimer (Zn2+(H2O)n; n = 1–20) were measured in the O–H stretching region, using infrared multiple photon dissociation (IRMPD) spectroscopy. These spectra were then compared with those calculated by using density functional theory. For all cluster sizes, calculated structures adopting asymmetric solvation to one Zn atom in the dimer were found to lie lower in energy than structures adopting symmetric solvation to both Zn atoms. Combining experiment and theory, the spectra show that water molecules preferentially bind to one Zn atom, adopting water binding motifs similar to the Zn+(H2O)n complexes studied previously. A lower coordination number of 2 was observed for Zn2+(H2O)3, evident from the highly red-shifted band in the hydrogen bonding region. Photodissociation leading to loss of a neutral Zn atom was observed only for n = 3, attributed to a particularly low calculated Zn binding energy for this cluster size.

A fast and robust trajectory surface hopping method: Application to the intermolecular photodissociation of a carbon dioxide dimer cation (CO2)2+

Our recently developed trajectory surface hopping method uses numerical time derivatives of adiabatic potential gradients to estimate the nonadiabatic transition probability and the hopping direction. To demonstrate the practicality of the novel method, we applied it to the intermolecular photodissociation of a carbon dioxide dimer cation (CO2)2 +. Our simulations reproduced the measured velocity distribution of CO2 + fragments consisting of two (fast and slow) components and revealed that nonadiabatic transitions occur promptly toward the electronic ground state regardless of the fragment velocity. The structure of (CO2)2 + at optical excitation governs the fate of subsequent nonadiabatic dynamics leading to a fast or slow dissociation. Our method gave similar results to the fewest switches algorithm at lower computational expense. Our fast and robust surface hopping method is promising for the investigation of nonadiabatic dynamics in large and complex systems.