Ion-pair States Research Articles

The RgI2 (ion-pair states) van der Waals complexes

This paper is an overview of our recent experimental investigations of the RgI2 van der Waals complexes, Rg = He, Ar, Kr, performed by means of laser induced fluorescence spectroscopy, optical-optical double resonance and supersonic molecular beam techniques. Spectroscopic parameters of these complexes in the E0 g + ion-pair state, such as binding energies and several spectroscopic constants, have been determined. Most likely, the potential energy surfaces of the all RgI2(E) complexes under study present T-shaped minima. Vibrational and electronic predissociations of the RgI2(E) complexes were analyzed. Relative contributions of the different electronic predissociation channels and vibrational distributions of the decay products were determined. Possible mechanisms of the complexes decay are suggested and discussed.

Understanding Charge-Transfer Characteristics in Crystalline Nanosheets of Fullerene/(Metallo)porphyrin Cocrystals.

Cocrystals in the form of crystalline nanosheets comprised of C70 and (metallo)porphyrins were prepared by using the liquid-liquid interfacial precipitation (LLIP) method where full control over the morphologies in the C70/(metallo)porphyrins nanosheets has been accomplished by changing the solvent and the relative molar ratio of fullerene to (metallo)porphyrin. Importantly, the synergy of integrating C70 and (metallo)porphyrins as electron acceptors and donors, respectively, into nanosheets is substantiated in the form of a near-infrared charge-transfer absorption. The presence of the latter, as reflection of ground-state electron donor-acceptor interactions in the nanosheets, in which a sizable redistribution of charge density from the electron-donating (metallo)porphyrins to the electron-accepting C70 occurs, leads to a quantitative quenching of the localized (metallo)porphyrin fluorescence. Going beyond the ground-state characterization, excited-state electron donor-acceptor interactions are the preclusion to a full charge transfer featuring formation of a radical ion pair state, that is, the one-electron reduced fullerene and the one-electron oxidized (metallo)porphyrin.

Rydberg atom scattering in K(12p)-CH3NO2 collisions: role of transient ion pair formation

SynopsisStudies of K(12p)-CH3NO2 collisions reveal unusually strong Rydberg atom scattering which is attributed to the formation of transient K+..CH3NO2− ion-pair states.

Dipolar dissociation dynamics in electron collisions with carbon monoxide

SynopsisDipolar dissociation (DD) dynamics of carbon monoxide with electron collision has been studied using velocity slice imaging (VSI) technique. Threshold of the process has been obtained from the ion yield curve. Using the time sliced images kinetic energy and angular distribution have been obtained providing the detailed dynamics involved in the DD process. Two ion-pair states have been identified from the angular distribution measurements.

Very strong Rydberg atom scattering in K(12p)–CH3NO2 collisions: Role of transient ion pair formation

Collisions between K(12p) Rydberg atoms and CH3NO2 target molecules are studied. Whereas CH3NO2 can form long-lived valence-bound CH3NO2− ions, the data provide no evidence for production of long-lived K+⋯CH3NO2− ion pair states. Rather, the data show that collisions result in unusually strong Rydberg atom scattering. This behavior is attributed to ion-ion scattering resulting from formation of transient ion pair states through transitions between the covalent K(12p) + CH3NO2 and ionic K+ + (dipole bound) CH3NO2− terms in the quasimolecule formed during collisions. The ion-pair states are destroyed through rapid dissociation of the CH3NO2− ions induced by the field of the K+ core ion, the detached electron remaining bound to the K+ ion in a Rydberg state. Analysis of the experimental data shows that ion pair lifetimes ≳10 ps are sufficient to account for the present observations. The present results are consistent with recent theoretical predictions that Rydberg collisions with CH3NO2 will result in strong collisional quenching. The work highlights a new mechanism for Rydberg atom scattering that could be important for collisions with other polar targets. For purposes of comparison, results obtained following K(12p)–SF6 collisions are also included.

Electrostatic Control of Regioselectivity in Au(I)-Catalyzed Hydroarylation.

Competing pathways in catalytic reactions often involve transition states with very different charge distributions, but this difference is rarely exploited to control selectivity. The proximity of a counterion to a charged catalyst in an ion paired complex gives rise to strong electrostatic interactions that could be used to energetically differentiate transition states. Here we investigate the effects of ion pairing on the regioselectivity of the hydroarylation of 3-substituted phenyl propargyl ethers catalyzed by cationic Au(I) complexes, which forms a mixture of 5- and 7-substituted 2H-chromenes. We show that changing the solvent dielectric to enforce ion pairing to a SbF6- counterion changes the regioselectivity by up to a factor of 12 depending on the substrate structure. Density functional theory (DFT) is used to calculate the energy difference between the putative product-determining isomeric transition states (ΔΔE‡) in both the presence and absence of the counterion. The change in ΔΔE‡ upon switching from the unpaired transition states in high solvent dielectric to ion paired transition states in low solvent dielectric (Δ(ΔΔE‡)) was found to be in good agreement with the experimentally observed selectivity changes across several substrates. Our calculations indicate that the origin of Δ(ΔΔE‡) lies in the preferential electrostatic stabilization of the transition state with greater charge separation by the counterion in the ion paired case. By performing calculations at multiple different values of the solvent dielectric, we show that the role of the solvent in changing selectivity is not solely to enforce ion pairing, but rather that interactions between the ion paired complex and the solvent also contribute to Δ(ΔΔE‡). Our results provide a foundation for exploiting electrostatic control of selectivity in other ion paired systems.

Sulfur rich electron donors - formation of singlet versus triplet radical ion pair states featuring different lifetimes in the same conjugate.

An unprecedented family of novel electron-donor acceptor conjugates based on fullerenes as electron acceptors, on one hand, and triphenyl amines as electron donors, on the other hand, have been synthesized and characterized in a variety of solvents using steady state absorption/emission as well as transient absorption spectroscopy. These are unprecedented in terms of their outcome of radical ion pair formation, that is, the singlet versus triplet excited state. This was corroborated by femto/nanosecond pump probe experiments and by molecular orbital calculations. Not only has the donor strength of the triphenylamines been systematically altered by appending one or two sulfur rich dithiafulvenes, but the presence of the latter changed the nature of the radical ion pair state. Importantly, depending on the excitation wavelength, that is, either where the fullerenes or where the triphenylamines absorb, short-lived or long-lived radical ion pair states, respectively, are formed. The short-lived component with a lifetime as short as 6 ps has singlet character and stems from a fullerene singlet excited state precursor. In contrast, the long-lived component has a lifetime of up to 130 ns in THF, has triplet character, and evolves from a triplet excited state precursor. Key in forming more than three orders of magnitude longer lived radical ion pair states is the presence of sulfur atoms, which enhance spin-orbit coupling and, in turn, intersystem crossing. Independent confirmation for the singlet versus triplet character came from temperature dependent measurements with a focus on the radical ion pair state lifetimes. Here, activation barriers of 2.4 and 10.0 kJ mol-1 for the singlet and triplet radical ion pair state, respectively, were established.

Long-range Rydberg molecules, Rydberg macrodimers and Rydberg aggregates in an ultracold Cs gas

We present an overview of our recent investigations of long-range interactions in an ultracold Cs Rydberg gas. These interactions are studied by high-resolution photoassociation spectroscopy, using excitation close to one-photon transitions into np3/2 Rydberg states with pulsed and continuous-wave ultraviolet laser radiation, and lead to the formation of long-range Cs2 molecules. We observe two types of molecular resonances. The first type originates from the correlated excitation of two atoms into Rydberg-atom-pair states interacting at long range via multipole-multipole interactions. The second type results from the interaction of one atom excited to a Rydberg state with one atom in the electronic ground state. Which type of resonances is observed in the experiments depends on the laser intensity and frequency and on the pulse sequences used to prepare the Rydberg states. We obtain insights into both types of molecular resonances by modelling the interaction potentials, using a multipole expansion of the long-range interaction for the first type of resonances and a Fermi-contact pseudo-potential for the second type of resonances. We analyse the relation of these long-range molecular resonances to molecular Rydberg states and ion-pair states, and discuss their decay channels into atomic and molecular ions. In experiments carried out with a two-colour two-photon excitation scheme, we observe a large enhancement of Rydberg-excitation probability, which we interpret as a saturable autocatalytic antiblockade phenomenon.

Molecular simulation study on K+–Cl− ion pair in geological fluids

This paper reports a classical molecular dynamics study of the potential of mean forces (PMFs), association constants, microstructures K+–Cl− ion pair in supercritical fluids. The constrained MD method is used to derive the PMFs of K+–Cl− ion pair from 673 to 1273 K in low-density water (0.10–0.60 g/cm3). The PMF results show that the contact ion-pair (CIP) state is the one most energetically favored for a K+–Cl− ion pair. The association constants of the K+–Cl− ion pair are calculated from the PMFs, indicating that the K+–Cl− ion pair is thermodynamically stable. It gets more stable as T increases or water density decreases. The microstructures of the K+–Cl− ion pair in the CIP and solvent-shared ion-pair states are characterized in detail. Moreover, we explore the structures and stabilities of the KCl–Au(I)/Cu(I) complexes by using quantum mechanical calculations. The results reveal that these complexes can remain stable for T up to 1273 K, which indicates that KCl may act as a ligand complexing ore-forming metals in hydrothermal fluids.

Solvation structures of sodium halides in dimethyl sulfoxide (DMSO)–methanol (MeOH) mixtures

A constrained molecular dynamics technique has been used to study the structures and dynamics of the solvation shells of three sodium halides, namely sodium chloride (Na+–Cl−), sodium bromide (Na+–Br−) and sodium iodide (Na+–I−) in DMSO–MeOH mixtures. In the case of Na+–Cl− and Na+–Br−, Na+ is preferentially solvated by DMSO and Cl− and Br− are preferentially solvated by methanol in the contact ion pair (CIP) state. In the solvent-assisted ion pair (SAIP) configuration, Na+ ions of Na+–Cl− and Na+–Br− are preferentially solvated by methanol and Cl− and Br− also show preferential solvation by methanol over DMSO. In the case of Na+–I−, the only preferential solvation is in the SAIP state for I− ion by methanol. These observations are supported by the calculated excess coordination numbers and spatial density maps. The heights of the transition states barriers for CIPs and SAIPs/solvent-shared ion pairs (SSHIPs) are significantly affected when the mole fraction of methanol (xMeOH) changes from 0.0 to 0.25 because of a significant increase in the methanol density around halides. From the analysis of angular distribution functions of DMSO and methanol around the cations and anions, it is seen that DMSO and methanol molecules are present in parallel dipolar orientations (with respect to cation–solvent vector) in the first coordination shell of these three ion pairs at the CIP and SAIP states. Methanol molecules are nearly in an antiparallel (with respect to ion–solvent vector) orientation around the three halide ions.

Probing dissociative electron attachment through heavy-Rydberg ion-pair production in Rydberg atom collisions.

Electron transfer in collisions between low-n, n = 12, Rydberg atoms and targets that attach low-energy electrons can lead to the formation of heavy-Rydberg ion-pair states comprising a weakly-bound positive-negative ion pair that orbit each other at large separations. Measurements of the velocity and angular distribution of ion-pair states produced in collisions with 1,1,1-C2Cl3F3, CBrCl3, BrCN, and Fe(CO)5 are used to show that electron transfer reactions furnish a new technique with which to examine the lifetime and decay energetics of the excited intermediates formed during dissociative electron capture. The results are analyzed with the aid of Monte Carlo simulations based on the free electron model of Rydberg atom collisions. The data further highlight the capabilities of Rydberg atoms as a microscale laboratory in which to probe the dynamics of electron attachment reactions.

Epimeric Face-Selective Oxidations and Diastereodivergent Transannular Oxonium Ion Formation Fragmentations: Computational Modeling and Total Syntheses of 12-Epoxyobtusallene IV, 12-Epoxyobtusallene II, Obtusallene X, Marilzabicycloallene C, and Marilzabicycloallene D.

The total syntheses of 12-epoxyobtusallene IV, 12-epoxyobtusallene II, obtusallene X, marilzabicycloallene C, and marilzabicycloallene D as halogenated C15-acetogenin 12-membered bicyclic and tricyclic ether bromoallene-containing marine metabolites from Laurencia species are described. Two enantiomerically pure C4-epimeric dioxabicyclo[8.2.1]tridecenes were synthesized by E-selective ring-closing metathesis where their absolute stereochemistry was previously set via catalytic asymmetric homoallylic epoxidation and elaborated via regioselective epoxide-ring opening and diastereoselective bromoetherification. Epimeric face-selective oxidation of their Δ12,13 olefins followed by bromoallene installation allowed access to the oppositely configured 12,13-epoxides of 12-epoxyobtusallene II and 12-epoxyobtusallene IV. Subsequent exploration of their putative biomimetic oxonium ion formation-fragmentations reactions revealed diastereodivergent pathways giving marilzabicycloallene C and obtusallene X, respectively. The original configurations of the substrates evidently control oxonium ion formation and their subsequent preferred mode of fragmentation by nucleophilic attack at C9 or C12. Quantum modeling of this stereoselectivity at the ωB97X-D/Def2-TZVPPD/SCRF = methanol level revealed that in addition to direction resulting from hydrogen bonding, the dipole moment of the ion-pair transition state is an important factor. Marilzabicycloallene D as a pentahalogenated 12-membered bicyclic ether bromoallene was synthesized by a face-selective chloronium ion initiated oxonium ion formation-fragmentation process followed by subsequent bromoallene installation.

The diffusion and conduction of lithium in poly(ethylene oxide)-based sulfonate ionomers

Pulsed field gradient nuclear magnetic resonance spectroscopy and dielectric relaxation spectroscopy have been utilized to investigate lithium dynamics within poly(ethylene oxide) (PEO)-based lithium sulfonate ionomers of varying ion content. The ion content is set by the fraction of sulfonated phthalates and the molecular weight of the PEO spacer, both of which can be varied independently. The molecular level dynamics of the ionomers are dominated by either Vogel-Fulcher-Tammann or Arrhenius behavior depending on ion content, spacer length, temperature, and degree of ionic aggregation. In these ionomers the main determinants of the self-diffusion of lithium and the observed conductivities are the ion content and ionic states of the lithium ion, which are profoundly affected by the interactions of the lithium ions with the ether oxygens of the polymer. Since many lithium ions move by segmental polymer motion in the ion pair state, their diffusion is significantly larger than that estimated from conductivity using the Nernst-Einstein equation.

Temperature-Induced Coil-Globule Transition of Alkali Metal Polyacrylates in Aqueous Organic Solvent Mixtures

ABSTRACTA counterion-specific coil-globule transition (CGT) has been observed for alkali metal poly(acrylate)s (PAAM, M; Li, Na, K, Cs) in aqueous organic solvent (ethanol, dimethyl sulfoxide, acetone, acetonitrile) mixtures. In the present study we found that the reduced viscosity of PAAM, except for PAALi, in the relevant mixed solvents, except for those containing acetonitrile, significantly increased with increasing temperature. The observed temperature-induced CGT was interpreted as being caused by disintegration of the ion-clusters, i.e., the associated state of contact ion-pairs formed between the polymer charge and the counterion. In fact, the NMR line width of 23Na and 133Cs, which were used as a measure of the counterion mobility, indicated that the counterion binding was significantly weakened with increasing temperature, while the temperature effects for the PAALi and acetonitrile systems were only marginal. All these results suggested that the temperature-induced CGT behavior occurs only for systems where the ion-cluster formation promotes the CGT in the relevant solvent systems, which is also consistent with the known similar behavior of some betaine polymers.

Toward More Complete Descriptors of Reactivity in Catalysis by Solid Acids

Density functional theory and classical electrostatics are used to develop reactivity descriptors for catalysis by solid acids. Acid strength, as deprotonation energies (DPE), reflects the charge reorganization required to disrupt covalent OH bonds in inorganic acids and the electrostatic forces that resist the separation of protons from conjugate anions. Both charge reorganization (covalent) and electrostatic (ionic) components vary monotonically with DPE on solid acids with different heteroatoms within a given type of oxide framework, but their relative contributions differ among different acid types. Ion-pair transition states recover predominantly the ionic part of the DPE, and the extent to which they recover each component is a unique property of a transition state and thus of an acid-catalyzed reaction, independent of the acid strength or type. These fractional recoveries, together with the ionic and covalent DPE components, a unique property of a solid acid, provide a general and complete descript...

A quantum defect analysis of heavy Rydberg behaviour in the [formula omitted] and [formula omitted] states of H2 and the [formula omitted] state of D2

Heavy Rydberg behaviour in the BΣu+1 and B″B‾Σu+1 ion-pair states of H2 and the BΣu+1 state of D2, is analysed in terms of the absolute quantum defects of the vibronic levels. The influence of the inner repulsive wall of ion-pair potentials on heavy Rydberg behaviour is considered and shown to determine the size of both absolute quantum defects and their energy dependence.

Molecular parameters for weakly bound 2g(aa, ab) and states of molecular iodine and dipole moment functions of transitions to these states

Weakly bound valence states of 2g symmetry, correlating with the I(2P3/2) + I(2P3/2) (aa) and I(2P3/2) + I(2P1/2) (ab) dissociation limits, as well as (ab) state, were studied using vibrationally resolved luminescence spectra corresponding to transitions from δ2u(3P2) and g(3P1) ion-pair states, in molecular iodine, respectively, populated using a three-step three-color laser excitation scheme. Spectroscopic constants and potential energy curves of the valence states are determined for the first time. Dipole moment functions of δ2u → 2g(aa, ab) and g → (ab) transitions are found to exponentially decrease.

A model for charge transfer in ultracold Rydberg ground-state atomic collisions

In excited molecules, the interaction between the covalent Rydberg and ion-pair channels forms a unique class of excited states, in which the infinite manifold of vibrational levels are the equivalent of atomic Rydberg states with a heavy electron mass. Production of the ion-pair states usually requires excitation through one or several interacting Rydberg states; these interacting channels lead to loss of flux, diminishing the rate of ion-pair production. Here, we develop an analytical, asymptotic charge-transfer model for the interaction between ultracold Rydberg molecular states, and employ this method to demonstrate the utility of off-resonant field control over the ion-pair formation, with near unity efficiency.

Ab initio MRCI + Q calculations on the low-lying excited states of the MgBr radical including spin–orbit coupling

Accurate theoretical calculations on the MgBr radical have been carried out by using the high-level relativistic multireference configuration interaction method with Davidson correction (MRCI+Q) using correlation-consistent Quintuple-ζ quality basis set. The potential energy curves (PECs) of the 14 Λ-S states of MgBr have been computed. In order to improve the PECs, the core–valence correlation, scalar relativistic effect, and spin–orbit coupling effect are taken into account in the computations. The spectroscopic constants of the bound states have been determined from the computed PECs. The results of the ground state X2Σ+ and the first excited state A2Π are in good agreement with those from the available experiments, while spectroscopic constants of the other electronic states are firstly reported. The low-lying ion-pair state B2Σ+ correlated to ion-pair dissociation limit Mg+ (2Sg)+Br− (1Sg) is characterized. The permanent dipole moments (PDMs) of Λ-S states and the R-dependent spin–orbit (SO) matrix elements are computed. The results indicate that the abrupt changes of PDMs and the SO matrix elements are attributed to the changes of the electronic configurations near the avoided crossing point. After taking the SOC effect into account, the 14 Λ-S states split into 30Ω states, and the SOC splitting for the A2Π is calculated to be 102.58cm−1. The SOC effect, leading to the double-well potential of the Ω=(3)1/2 state, is found to be substantial for MgBr. In order to further illustrate the SOC effect and the avoided crossing phenomenon of the PECs, the Λ-S compositions in the Ω state wavefunctions are analyzed in detail. Finally, the transition dipole moments (TDMs) of several transitions from upper Ω states to the ground X2Σ+1/2 state and the corresponding radiative lifetimes have been studied. It is shown that the (1)3/2–X2Σ+1/2 and (2)3/2–X2Σ+1/2 are particularly important to the observed transitions A2Π–X2Σ+ and C2Π–X2Σ+. The present study should shed more light on the electronic structures and transition properties of electronic states of the MgBr radical.

An ab initio study of ion-pair states of the Br2 molecule

This contribution reports an ab initio study of 18 ion-pair states of the Br2 molecule correlating with the Br+(3PJ=2,1,0,1D2) +Br–(1S0) dissociation asymptotes. Calculations were performed on the CASSCF/CASPT2 level of theory taking into account electron correlations and with inclusion of the spinorbit coupling. Ab initio results are compared with the experiment. For gerade and ungerade ion-pair potentials, the difference between calculated and experimental values of the equilibrium internuclear distance does not exceed ΔRe = ±0.01 and ±0.05 A, respectively. For the states correlating to the lowest dissociation asymptote Br+(3P2) + Br–(1S0), the accuracy of relative energies is postilions ΔTe ± 0.02 eV. The dipole moment functions for some transitions between the ion-pair and valence states were calculated as well.