Contact Ion Pair State Research Articles

Thermodynamic stability of finely dispersed Na+Cl−(H2O) n aerosol particles in water vapor

Four qualitatively different thermodynamic states have been identified for hydrated sodium chloride ion pairs in water vapor. Two of the states are stable, and the other two are unstable. The stable states of a contact ion pair (CIP) and a solvent-separated ion pair (SSIP) represent sites of heterogeneous nucleation in supersaturated vapors. The unstable states form free energy barriers that separate the stable states. As the temperature is elevated, the SSIP states become more stable than the CIP states already in unsaturated water vapors. Hydrated ion pairs that contain hydroxonium ions instead of alkali metal cations are absolutely stable in the SSIP state already in the range of unsaturated vapors and low temperatures. Computer-assisted calculations of the free energy and work of hydration have been carried out by the Monte Carlo bicanonical statistical ensemble method in terms of a detailed model of interactions comprising polarization, many-particle covalent interactions, and charge transfer phenomena.

Ion Pair Structures in Aqueous KSCN Solution: Classical and Quantum Mechanical/Molecular Mechanical Molecular Dynamics Simulation Study#

Ion pair formation and dissociation are fundamental processes in electrolyte solutions so that understanding thermodynamic stabilities and dynamic aspects of ion pairs is of great importance. The structures of various ion pair states are here studied by carrying out classical and quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations of aqueous KSCN solutions. A few different solvent potential models are considered and the resulting ion pair structures are directly compared. In particular, when KSCN is treated quantum mechanically and effective fragment potential (EFP) model for water is used, we found two stable forms of ion pairs that can be considered as contact ion pair (CIP) and solvent separated ion pair (SSIP), where the interionic distances between K+ and SCN − are found to be 4.0 and 5.5 Å, respectively. QM/EFP‐MD further indicates that the nitrogen side of SCN − is preferentially interacting with K+ in CIP. However, the corresponding CIP appears at 3.0 Å interionic distance in fully classical MD simulation at a higher KSCN concentration. Nonetheless, both QM/EFP and classical MD simulation results show that the CIP state appears to be more stable than SSIP. From the site‐site radial distribution functions (RDF) calculated from the QM/MM and classical MD trajectories, detailed ion pair structures and surrounding solvent configurations are further elucidated.

Ion Pairing Kinetics Does not Necessarily Follow the Eigen-Tamm Mechanism

The most recognized and employed model of the solvation equilibration in the ionic solutions was proposed by Eigen and Tamm, in which there are four major states for an ion pair in the solution: the completely solvated state, 2SIP (double solvent separate ion pair), SIP (single solvent separate ion pair), and CIP (contact ion pair). Eigen and Tamm suggested that the transition from SIP to CIP is always the slowest step during the whole pairing process, due to a high free energy barrier between these two states. We carried out a series of potential of mean force calculations to study the pairing free energy profiles of two sets of model monoatomic 1:1 ion pairs 2.0:x and xx:2.0. For 2.0:x pairs the free energy barrier between the SIP and CIP states is largely reduced due to the salvation shell water structure. For these pairs the SIP to CIP transition is thus not the slowest step in the ion pair formation course. This is a deviation from the Eigen-Tamm model.

Experimental and Computational Exploration of Ground and Excited State Properties of Highly Strained Ruthenium Terpyridine Complexes

Dissociative electron transfer reactions are prevalent in one-electron reduced aryl halides; however, calculations applied to charge-transfer excited states of metal complexes suggest that this reaction would be strongly endergonic unless attention is paid to specific structural details. In this current study, we explore the effect of introducing intramolecular strain into a series of halogenated ruthenium(II) polypyridyls. Parent [Ru(tpy)2](2+) (1) (tpy = 2,2':6',2″-terpyridine) is compared with two complexes, [Ru(6,6″-Br2-tpy)(tpy)](2+) (2) and [Ru(6,6″-Br2-tpy)2](2+) (3) (6,6″-Br2-tpy = 6,6″-dibromo-tpy) that incorporate interligand van der Waals strain derived from the large halogen substituents. DFT calculations and the crystal structure of 3 show how this strain distorts the geometry of 3 as compared to 1. Time-dependent DFT calculations are used to explain the effect of this strain on electronic absorption spectra where, in particular, a transition observed in 3 is attenuated in 2 and absent in 1 and heralds interligand charge transfer mediated by the halogen substituent. Ultrafast transient absorption spectroscopy reveals coherent vibrational dynamics particularly in 3 but also in 2 that is interpreted as reflecting heavy-atom motion. Surprisingly, in spite of the additional strain, the excited-state lifetime of 3 is observed to be approximately a factor of 6 longer than 2. Constrained-DFT calculations show that while the excited behavior of 2 is similar to 1, the strain-induced geometric distortions in 3 cause a nesting of excited state triplet surfaces resulting in a longer excited state lifetime. This may afford the additional time needed to engage in photochemistry, and kinetic evidence is observed for the breaking of a C-Br bond in 3 and formation of a contact ion pair state.

Understanding ion–ion interactions in bulk and aqueous interfaces using molecular simulations

In addition to its scientific significance, the distribution of ions in the bulk and at aqueous interfaces is also very important for practical reasons. Providing a quantitative description of the ionic distribution, and describing interactions between ions in different environments, remains a challenge, and is the subject of current debate. In this study, we found that interionic potentials of mean force (PMFs) and interfacial properties are very sensitive to the ion-ion interaction potential models. Our study predicted a Sr(2+)--CI- PMF with no contact ion-pair state and a shallow solvent-separated ion-pair state. In addition, we were able to quantitatively capture the experimental X-ray reflectivity results of the aqueous salt interface of the Sr(2+)--Cl- ion-pair, and provided a detailed physical description of the interfacial structure for this system. We also predicted the Xray reflectivity results for SrBr2 and SrI2 systems.

Enthalpy-Entropy of Cation Association with the Acetate Anion in Water.

Negatively charged carboxylate and phosphate groups on biomolecules have different affinity for Na(+) and K(+) ions. We performed molecular simulations and studied the pair potential of mean force between monovalent cations and the carboxylate group of the acetate anion in aqueous solution at 298 K. The simulations indicate that a larger affinity of Na(+) over K(+) in the contact ion pair (CIP) state is of entropic origin with the CIP state becoming increasingly populated at higher temperature. Differences between the osmotic activities of these two ions are however governed by interactions with acetate in the solvent-shared ion pair (SIP) state as was previously shown (Hess, B.; van der Vegt, N. F. A. Proc. Natl. Acad. Sci. U.S.A.2009, 106, 13296). SIP states with Na(+) are slightly more stable than SIP states with K(+), resulting in a smaller osmotic activity of sodium. We discuss the different affinities of Na(+) and K(+) in the SIP state in terms of an enthalpy-entropy reinforcement mechanism which involves a water-mediated hydrogen-bonding interaction between the oppositely charged ions. SIP states are enthalpically favorable and become decreasingly populated at higher temperature.

Note: Interionic potentials of mean force for Ca2+-Cl− in polarizable water

We describe our molecular dynamics simulation studies of the inter-ionic potentials of mean force (PMF) for the Ca2+-Cl− ion pair in water. Ion-water and ion-ion polarizable potential models were constructed to reproduce experimental ion-hydration enthalpies as well as the electronic-structure calculations of the minimum energy for the Ca2+-Cl− interactions. Our study predicted a PMF with no contact ion pair state and a shallow solvent-separated ion-pair state. In addition, our computed radial distribution function, gCa-Cl, for a 1-M CaCl2 electrolyte solution indicated that there is no contact ion pair but, instead, showed a maximum near 5 Å. This observation is consistent with our PMF calculation. These new results are discussed and compared to the recent classical and ab initio PMF calculations on the same system reported by Timko et al. [J. Chem. Phys. 134, 204510 (2011)]10.1063/1.3595261.

Ion-pairing dynamics of Li+ and SCN− in dimethylformamide solution: Chemical exchange two-dimensional infrared spectroscopy

Ultrafast two-dimensional infrared (2DIR) spectroscopy has been proven to be an exceptionally useful method to study chemical exchange processes between different vibrational chromophores under thermal equilibria. Here, we present experimental results on the thermal equilibrium ion pairing dynamics of Li(+) and SCN(-) ions in N,N-dimethylformamide. Li(+) and SCN(-) ions can form a contact ion pair (CIP). Varying the relative concentration of Li(+) in solution, we could control the equilibrium CIP and free SCN(-) concentrations. Since the CN stretch frequency of Li-SCN CIP is blue-shifted by about 16 cm(-1) from that of free SCN(-) ion, the CN stretch IR spectrum is a doublet. The temperature-dependent IR absorption spectra reveal that the CIP formation is an endothermic (0.57 kJ∕mol) process and the CIP state has larger entropy by 3.12 J∕(K mol) than the free ion states. Since the two ionic configurations are spectrally distinguishable, this salt solution is ideally suited for nonlinear IR spectroscopic investigations to study ion pair association and dissociation dynamics. Using polarization-controlled IR pump-probe methods, we first measured the lifetimes and orientational relaxation times of these two forms of ionic configurations. The vibrational population relaxation times of both the free ion and CIP are about 32 ps. However, the orientational relaxation time of the CIP, which is ∼47 ps, is significantly longer than that of the free SCN(-), which is ∼7.7 ps. This clearly indicates that the effective moment of inertia of the CIP is much larger than that of the free SCN(-). Then, using chemical exchange 2DIR spectroscopy and analyzing the diagonal peak and cross-peak amplitude changes with increasing the waiting time, we determined the contact ion pair association and dissociation time constants that are found to be 165 and 190 ps, respectively. The results presented and discussed in this paper are believed to be important, not only because the ion-pairing dynamics is one of the most fundamental physical chemistry problems but also because such molecular ion-ion interactions are of critical importance in understanding Hofmeister effects on protein stability.

Generation of singlet oxygen by indotricarbocyanine dyes in low-polarity media

We present the results of a study of the spectral luminescence properties of three groups of indotricarbocyanine dyes, each of which is formed from compounds with the same cation and different anions. In high-polarity solvents, in the absorption and emission spectra of the dyes we see one type of center; in low-polarity solvents, due to the presence of different ionic forms of the dyes (free ions, contact ion pairs), we observe either one type or two types of centers. By analysis of the luminescence of molecular oxygen in the 1.27 µm spectral region, we determined the efficiency of photosensitization of 1O2 formation by dyes in deuterated solvents. We have shown that in low-polarity solvents, the yield for singlet oxygen generation is higher for indotricarbocyanine dyes which are found in the contact ion pair state and which also contain a heavy atom (I) in the anion. We have observed that an increase in the fraction of contact ion pairs in solution as the dye concentration increases or when an additional salt is introduced leads to an increase in the quantum yield for generation of singlet oxygen. In polar deuterated acetonitrile, the counterion has no effect on the efficiency of photosensitization of oxygen by the dyes.

Microscopic Hydration of the Sodium Chloride Ion Pair

Hydration of ion pairs is an essential process in various physicochemical phenomena occurring in solutions. Isolated clusters of an ion pair solvated with finite number of waters have been considered as a model system for the critical evaluation of microscopic interactions involved in the process, and theoretical studies have contributed exclusively to the subject up to now. Here we report the first experimental characterization of structure and internal dynamics of hydrated ion pairs, NaCl-(H2O)n (n = 1-3). The measurements of their rotational spectra have proven that the clusters have cyclic forms, in which Na+ and Cl- ions are strongly interacted with the O and H atoms of the solvent molecules, respectively. The Na-Cl distance shows a pronounced increase with the successive addition of water molecules. The separation for n = 3 approaches the value predicted for the contact ion-pair state in aqueous solution by recent molecular dynamics simulations.

Co-existence of hydrated electron and metal di-cation in [Mg(H2O)n]+Dedicated to Professor F. Dörr on the occasion of his 80th birthday.

DFT/BLYP based investigations of hydrated magnesium mono-cations [Mg(H2O)n]+, 1 ≤ n ≤ 19, reveal an electronic structure of [Mg(H2O)n]+ that evolves from a valence state in Mg+(H2O)n, n ≤ 5, via a contact ion pair state Mg2+(H2O)n−, 6 ≤ n < 17, to a solvent separated ion pair state Mg2+(H2O)ne⊖ for n ≥ 17. In [Mg(H2O)n]+, n ≥ 9, O–H bonds of adjacent H2O ligands form “molecular tweezers” HO–H⋯e⊖⋯H–OH, which induce a localization of the odd electron within the hydration sphere. The shape of the singly occupied molecular orbitals (SOMOs) and their position within the clusters indicate the co-existence of a hydrated electron and of a magnesium di-cation in [Mg(H2O)n]+, n ≥ 8. The now calculated energetics of previously observed competing decay channels — reactive hydroxide formation versus H2O monomer evaporation — predict the evaporation process to be favored for [Mg(H2O)n]+, 6 ≥ n ≥ 17, in very nice agreement with experiment. The switchings in the decay propensities are analysed on the basis of the computed electronic and geometrical structures of product and reactant clusters.

Vertical and Relaxed Structures of a Reactive Organosilane Radical Cation from CW and Transient Resonance Raman Spectra

Resonance Raman spectra of two p-methoxybenzyltrialkylsilanes (alkyl = methyl and ethyl) have been obtained both as their neutral charge-transfer complexes with tetracyanoethylene in steady-state cw experiments and as their radical cations via two-color pump−probe transient measurements. The ground-state charge-transfer resonant spectra exhibit intensity predominantly in phenyl-localized modes, suggesting that vertical excitation to the contact ion-pair state involves little participation of the bond that is known to undergo subsequent nucleophile-assisted cleavage in the separated radical cation. Quantitative modeling of the absolute cross sections for the methyl compound is used to determine the mode-specific reorganization energies accompanying vertical electron transfer. Transient spectra of the relaxed radical cations show more than 20 resonance-enhanced modes, several of which have significant contributions from the C−Si stretching coordinate based on frequency shifts between the natural abundance a...

Interionic potentials of mean force for SrCl 2 in polarizable water : A computer simulation study

Potentials of mean force (PMFs) and solvation properties of SrCl 2 in polarizable water have been studied using molecular dynamics simulations. The PMF between isolated Sr 2+ and Cl − ions involves a stable contact ion-pair state with a binding free energy of −1.8 kcal mol and a dissociation barrier of 3.5 kcal/mol. A solvent-separated state is observed to have comparable free energy and a minimal barrier for dissociation to free ions. The PMF between a second Cl − ion and the Sr 2+⋯Cl − contact ion pair has also been calculated. The Cl −⋯Sr 2+⋯Cl − complex is nonlinear and involves a bridging water molecule strongly hydrogen-bound to both chlorine ions.

Methanol-Incorporated Photoaddition of N-Methyl-9,10-phenanthrenedicarboximide to Alkenes. Addition at Carbonyl and Phenanthrene-Ring Carbon Atoms

Abstract Irradiation of acetonitrile–methanol (2:1 v/v) solutions of N-methyl-9,10-phenanthrenedicarboximide (3) with alkenes 5a–g gave methanol-incorporated adducts (9c–f) at a carbonyl carbon atom in 3, those (10c,f) at a phenanthrene-ring carbon atom, and a methanol-adduct 11g to alkene 5g, together with products 4, 6b,e–g, 7b,e–g, and 8e,f obtained by the photoreactions in benzene. The concentration effect of alkenes 5e,f on yields of products 6e,f, 7e,f, 8e,f, 9e,f, and 10f showed that both 9e,f and 10f arose from a relatively short-lived singlet excited state of 3. The values of the free-energy change associated with an electron transfer (ΔGet) from alkenes 5a–d,g to the singlet excited state of 3 and fluorescence quenching rate constants support a photochemical electron transfer mechanism for the methanol-incorporated addition. Both LiClO4 and BU4NClO4 caused only slight decreases in the relative ratios 10c/9c indicating that both the methanol-incorporated addition at the phenanthrene-ring carbon atom and that at the carbonyl carbon atom might arise from a contact ion pair state. The fact that the methanol-incorporated addition at the arene-ring carbon atom was observed only in the reaction of 3 in a series of N-methylarenedicarboximides, possessing a five-membered imide ring, is interpreted on the basis of the spin density of the radical anions of the imides derived from the photochemical electron transfer.