Ion Pair Structure Research Articles

Direct photoisomerization of CH2I2vs. CHBr3 in the gas phase: a joint 50 fs experimental and multireference resonance-theoretical study.

Femtosecond transient absorption measurements powered by 40 fs laser pulses reveal that ultrafast isomerization takes place upon S1 excitation of both CH2I2 and CHBr3 in the gas phase. The photochemical conversion process is direct and intramolecular, i.e., it proceeds without caging media that have long been implicated in the photo-induced isomerization of polyhalogenated alkanes in condensed phases. Using multistate complete active space second order perturbation theory (MS-CASPT2) calculations, we investigate the structure of the photochemical reaction paths connecting the photoexcited species to their corresponding isomeric forms. Unconstrained minimum energy paths computed starting from the S1 Franck-Condon points lead to S1/S0 conical intersections, which directly connect the parent CHBr3 and CH2I2 molecules to their isomeric forms. Changes in the chemical bonding picture along the S1/S0 isomerization reaction path are described using multireference average coupled pair functional (MRACPF) calculations in conjunction with natural resonance theory (NRT) analysis. These calculations reveal a complex interplay between covalent, radical, ylidic, and ion-pair dominant resonance structures throughout the nonadiabatic photochemical isomerization processes described in this work.

Synthesis, crystal structure, and magnetic property of a cyano involved ion-pair complex based on macrocyclic metallic tecton and [Ni(CN)4]2–

A cyano-bridged nickel(II) complex [NiL]2{[NiL][Ni(CN)4]2}2 · H2O (III) (L = 1,3,10,12,15,18-hexaazatetracyclo[16,2,1,112.15,04.9]docosane), based on macrocyclic metallic tecton and diamagnetic [Ni(CN)4]2– has been synthesized and characterized by physico-chemical methods, as well as single crystal X-ray determination (CIF file CCDC no. 1038635). The crystal belongs to orthorhombic, space group: P212121, with a = 15.410(3), b = 15.752(3), c = 19.134(4) A, Mr = 2137.74 g mol–1, Z = 2, V = 4644.6(16) A3 (CIF file CCDC no. 1038635). Single X-ray diffraction analysis reveals that complex I shows a ion-pair type structure which consists of two fragments [NiL]2+ and {[NiL][Ni(CN)4]2}2–. In this complex, each {[NiL][Ni(CN)4]2}2– binds two [NiL]2+, resulting a hydrogen-bonded 1D chain. In additional, the magnetic properties were studied.

Structural studies on choline-carboxylate bio-ionic liquids by x-ray scattering and molecular dynamics.

We report a X-ray diffraction and molecular dynamics study on three choline-based bio-ionic liquids, choline formate, [Ch] [For], choline propanoate, [Ch][Pro], and choline butanoate, [Ch][But]. For the first time, this class of ionic liquids has been investigated by X-ray diffraction. Experimental and theoretical structure factors have been compared for each term of the series. Local structural organization has been obtained from ab initio calculations through static models of isolated ion pairs and dynamic simulations of small portions of liquids through twelve, ten, and nine ion pairs for [Ch][For], [Ch][Pro], and [Ch][But], respectively. All the theoretical models indicate that cations and anions are connected by strong hydrogen bonding and form stable ion pairs in the liquid that are reminiscent of the static ab initio ion pairs. Different structural aspects may affect the radial distribution function, like the local structure of ion pairs and the conformation of choline. When small portions of liquids have been simulated by dynamic quantum chemical methods, some key structural features of the X-ray radial distribution function were well reproduced whereas the classical force fields here applied did not entirely reproduce all the observed structural features.

Accessing sodium ferrate complexes containing neutral and anionic N-heterocyclic carbene ligands: structural, synthetic, and magnetic insights.

This study reports the synthesis and single-crystal X-ray crystallographic, NMR spectroscopic, and magnetic characterization of a series of sodium ferrates using bis(amide) Fe(HMDS)2 as a precursor (HMDS = 1,1,1,3,3,3-hexamethyldisilazide). Reaction with sodium reagents NaHMDS and NaCH2SiMe3 in hexane afforded donor-solvent-free sodium ferrates [{NaFe(HMDS)3}∞] (1) and [{NaFe(HMDS)2(CH2SiMe3)}∞] (2), respectively, which exhibit contacted ion pair structures, giving rise to new polymeric chain arrangements made up of a combination of inter- and intramolecular Na···Me(HMDS) electrostatic interactions. Addition of the unsaturated NHC IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to 1 and 2 caused deaggregation of their polymeric structures to form discrete NHC-stabilized solvent-separated ion pairs [Na(IPr)2](+)[Fe(HMDS)3](-) (3) and [(THF)3·NaIPr](+)[Fe(HMDS)2CH2SiMe3](-) (4), where in both cases, the NHC ligand coordinates preferentially to Na. In contrast, when IPr is sequentially reacted with the single-metal reagents NaCH2SiMe3 and Fe(HMDS)2, the novel heteroleptic ferrate (THF)3Na[:C{[N(2,6-(i)Pr2C6H3)]2CHCFe(HMDS)2}] (5) is obtained. This contains an anionic NHC ligand acting as an unsymmetrical bridge between the two metals, coordinating through its abnormal C4 position to Fe and its normal C2 position to Na. The formation of 5 can be described as an indirect ferration process where IPr is first metalated at the C4 position by the polar sodium alkyl reagent, which in turn undergoes transmetalation to the more electronegative Fe(HMDS)2 fragment. Treatment of 5 with 1 molar equiv of methyl triflate (MeOTf) led to the isolation and structural elucidation of the neutral abnormal NHC (aNHC) tricoordinate iron complex [CH3C{[N(2,6-iPr2C6H3)]2CHCFe(HMDS)2}] (6) with the subsequent elimination of NaOTf, disclosing the selectivity of complex 5 to react with this electrophile via its C2 position, leaving its Fe-C4 and Fe-N bonds intact. The magnetic susceptibility properties of compounds 1-6 have been examined. This study revealed a drastic change of magnetic susceptibility in replacing a pure σ donor from an idealized trigonal coordination environment by an NHC π donating character.

En route formation of ion pairs at the ionic liquid–vacuum interface

An increasing lack of single ion cation–anion associations (ion pairing) in ionic liquids suggests a structural motif that stands in contradiction to the single ion pair structure of their vapor phase, which was evidenced by different experimental and theoretical studies. Therefore, a structural rearrangement has to occur en route from the liquid to the vapor. In this study, we propose a detailed four-step evaporation mechanism for ionic liquids, providing a refined perspective on the theory of this process based on the connection between ion pairing and volatility. The process involves diffusion of ions from the bulk to the surface, where they float around until a well-defined ion pair is formed with a counterion, leading to the departure from the surface into the vacuum. To assess the validity of this scheme, we performed a series of classical and ab initio molecular dynamics simulations based on the most sophisticated methods and force fields available for ionic liquids.

Computational insights into the molecular interaction and ion-pair structures of a novel zinc-functionalized ionic liquid, [Emim][Zn(TFSI)3

The ion pair structures of a novel CO2 capture material in the form of a metal chelate anion-containing room temperature ionic liquid (IL), 1-ethyl-3-methylimidazolium tri[bis(trifluoromethylsulfonyl)imide]zincate(II), [Emim][Zn(TFSI)3], were elucidated by correlating the infrared spectra generated using density functional theory (DFT) calculations with the experimental spectrum derived from a room temperature infrared spectroscopic measurement. A free volume energy minimization algorithm revealed stable structures where the zinc ion forms an octahedral, homoleptic complex with the ligand bis(trifluoromethylsulfonyl)imide through coordination with the oxygen of the sulfone group, with 1-ethyl-3-methylimidazolium acting as the counterion. The method of analysis was built around 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [Emim][TFSI], involving direct comparison with published data, and extended to the more complex [Emim][Zn(TFSI)3] system. The DFT calculations reproduced the vibrational spectra of [Emim][Zn(TFSI)3] and [Emim][TFSI] using their optimized geometries, with correlation slopes of 0.9996 and 1.0022, respectively. Comparison of the vibrational modes of [Emim][TFSI] and [Emim][Zn(TFSI)3] provided insights into the ion pair structure of, and molecular interactions in the ILs analyzed.

Microsolvation of sodium acetate in water: Anion photoelectron spectroscopy and ab initio calculations.

To understand the microsolvation of sodium acetate (CH3COONa, NaOAc) in water, we studied NaOAc(H2O)n(-) (n = 0-3) clusters by photoelectron spectroscopy. We also investigated the structures of NaOAc(H2O)n(-) (n = 0-5) anions and NaOAc(H2O)n (n = 0-7) neutrals by quantum chemistry calculations. By comparing the theoretical results with the photoelectron experiment, the most probable structures of NaOAc(H2O)n(-/0) (n = 0-3) were determined. The study also shows that, with increasing n, the solvent-separated ion pair (SSIP) structures of NaOAc(H2O)n(-) anions become nearly energetically degenerate with the contact ion pair (CIP) structures at n = 5, while the SSIP structures of the neutral NaOAc(H2O)n clusters appear at n = 6 and become dominant at n = 7.

Hydrogen Bonding, (1)H NMR, and Molecular Electron Density Topographical Characteristics of Ionic Liquids Based on Amino Acid Cations and Their Ester Derivatives.

Amino acid ionic liquids (AAILs) have attracted significant attention in the recent literature owing to their ubiquitous applications in diversifying areas of modern chemistry, materials science, and biosciences. The present work focuses on unraveling the molecular interactions underlying AAILs. Electronic structures of ion pairs consisting of amino acid cations ([AA(+)], AA = Gly, Ala, Val, Leu, Ile, Pro, Ser, Thr) and their ester substituted derivatives [AAE(+)] interacting with nitrate anion [NO3(-)] have been obtained from the dispersion corrected M06-2x density functional theory. The formation of ion pair is accompanied by the transfer of proton from quaternary nitrogen to anion facilitated via hydrogen bonding. The [Ile], [Pro], [Ser], and [Thr] and their esters reveal relatively strong inter- as well as intramolecular hydrogen-bonding interactions. Consequently, the hierarchy in binding energies of [AA][NO3] ion pairs and their ester analogues turns out to be [Gly] > [Ala] > [Ser] ∼ [Val] ∼ [Ile] > [Leu] ∼ [Thr] > [Pro]. The work underlines how the interplay of intra- as well as intermolecular hydrogen-bonding interactions in [AA]- and [AAE]-based ILs manifest in their infrared and (1)H NMR spectra. Substitution of -OCH3 functional group in [AA][NO3] ILs lowers the melting point attributed to weaker hydrogen-bonding interactions, making them suitable for room temperature applications. As opposed to gas phase structures, the presence of solvent (DMSO) does not bring about any proton transfer in the ion pairs or their ester analogues. Calculated (1)H NMR chemical shifts of the solvated structures agree well with those from experiment. Correlations of decomposition temperatures in [AA]- and [AAE]-based ILs with binding energies and electron densities at the bond critical point(s) in molecular electron density topography, have been established.

Ion association characteristics in MgCl2 and CaCl2 aqueous solutions: a density functional theory and molecular dynamics investigation

In this work, ion association characteristics in MgCl2 and CaCl2 aqueous solutions have been investigated using density functional theory (DFT) and molecular dynamics (MD) simulation at various temperatures and pressures. Results all indicate that the ion association is more obvious in MgCl2 solution than in CaCl2 solution, due to the hydration characteristics of Mg2+ and Ca2+. Although ion association can occur in concentrated MgCl2 solution, contact ion pair (CIP) structures and solvent-shared ion pair (SSIP) with single Cl− dissociated (SSIP/s) ([MgCl(H2O)6Cl]0) are quite unstable. CIP and SSIP/s conformers are almost iso-energetic even in CaCl2 solution, and the lifetime of CIP conformer is much longer than that of SSIP/s conformer in concentrated CaCl2 solution. However, the possibility of CIP conformer found in concentrated CaCl2 solution is obviously lower than its SSIP/s isomers. MD simulations exhibit that ion association characteristics do not change as pressure ranges from 0.5 to 20.0 bar in MgCl2 solution while ion association in CaCl2 solution does change as pressure rises. Moreover, ion association decreases with temperature increasing in MgCl2 solution, but the situation is reversed in CaCl2 solution.

Structure and dynamics of room temperature ionic liquids with bromide anion: results from 81Br NMR spectroscopy.

We report the results of a comprehensive (81)Br NMR spectroscopic study of the structure and dynamics of two room temperature ionic liquids (RTILs), 1-butyl-3-methylimidazolium bromide ([C(4)mim]Br) and 1-butyl-2,3-dimethylimidazolium bromide ([C(4)C(1)mim]Br), in both liquid and crystalline states. NMR parameters in the gas phase are also simulated for stable ion pairs using quantum chemical calculations. The combination of (81)Br spin-lattice and spin-spin relaxation measurements in the motionally narrowed region of the stable liquid state provides information on the correlation time of the translational motion of the cation. (81) Br quadrupolar coupling constants (C(Q)) of the two RTILs were estimated to be 6.22 and 6.52 MHz in the crystalline state which were reduced by nearly 50% in the liquid state, although in the gas phase, the values are higher and span the range of 7-53 MHz depending on ion pair structure. The C(Q) can be correlated with the distance between the cation-anion pairs in all the three states. The (81)Br C(Q) values of the bromide anion in the liquid state indicate the presence of some structural order in these RTILs, the degree of which decreases with increasing temperature. On the other hand, the ionicity of these RTILs is estimated from the combined knowledge of the isotropic chemical shift and the appropriate mean energy of the excited state. [C(4)C(1)mim]Br has higher ionicity than [C(4)mim]Br in the gas phase, while the situation is reverse for the liquid and the crystalline states.

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.

Synthesis, crystal structure, theoretical calculations and antimicrobial properties of [Pt(tetramethylthiourea)4] [Pt(CN)4]·4H2O

A new platinum(II) complex, [Pt(Tmtu)4][Pt(CN)4]·4H2O (1) was synthesized by reaction of K2[PtCl4], KCN and tetramethylthiourea (Tmtu). Its structure was determined by X-ray crystallography. The [Pt(CN)4]2− anion shows regular square planar geometry at platinum, while in the [Pt(Tmtu)4]2+ cation the geometry at platinum is somewhat distorted. Hydrogen bonding between water molecules and the cyanide nitrogen of [Pt(CN)4]2− ions stabilizes the structure and leads to a supramolecular 2D network. DFT calculations support the experimentally found dinuclear (homocoordinated) ion-pair structure 1 as the most stable in comparison to noncovalent dimer [Pt(CN)2(Tmtu)2]222 that could, in turn, be involved in the formation sequence of 1. Antimicrobial activities of the complex were evaluated by minimum inhibitory concentration and the results showed that the complex exhibited moderate activities against gram-negative bacteria (Escherichiacoli, Pseudomonas aeruginosa) and molds (Aspergillus niger,Penicilliumcitrinum).

Relationship between the anion/cation relative orientation and the catalytic activity of nitrogen acyclic carbene–gold catalysts

The role of the ligand in determining the ion pair structure of the [(NAC)Au(η2-3-hexyne)]+ BF4− catalysts and how the position of the anion (positions A, B and C) influences their catalytic performance in the intermolecular alkoxylation of alkynes have been studied.

Multiscale modeling of the trihexyltetradecylphosphonium chloride ionic liquid.

A multiscale modeling protocol was sketched for the trihexyltetradecylphosphonium chloride ([P6,6,6,14]Cl) ionic liquid (IL). The optimized molecular geometries of an isolated [P6,6,6,14] cation and a tightly bound [P6,6,6,14]Cl ion pair structure were obtained from quantum chemistry ab initio calculations. A cost-effective united-atom model was proposed for the [P6,6,6,14] cation based on the corresponding atomistic model. Atomistic and coarse-grained molecular dynamics simulations were performed over a wide temperature range to validate the proposed united-atom [P6,6,6,14] model against the available experimental data. Through a systemic analysis of volumetric quantities, microscopic structures, and transport properties of the bulk [P6,6,6,14]Cl IL under varied thermodynamic conditions, it was identified that the proposed united-atom [P6,6,6,14] cationic model could essentially capture the local intermolecular structures and the nonlocal experimental thermodynamics, including liquid density, volume expansivity and isothermal compressibility, and transport properties, such as zero-shear viscosity, of the bulk [P6,6,6,14]Cl IL within a wide temperature range.

The molecular complexes of boron trifluoride with nitrosyl fluoride and nitrosyl chloride. Ion-pair formation

The electron donor–acceptor complexes formed between boron trifluoride and the nitrosyl halides ONF and ONCl have been studied by means of ab initio molecular orbital theory at the second order level of Møller–Plesset perturbation theory and with Dunning’s augmented correlation-consistent polarized valence triple-zeta basis set. The focus of the calculations was on the structures, interaction energies and vibrational spectra of the complexes. A variety of trial structures were examined, with electron donation occurring from the oxygen, nitrogen and halogen atoms, in an attempt to establish the most favorable site for interaction. In addition, a number of rotational isomers for each adduct were investigated. It was found that, in both cases, the halogen atom was the preferred donor atom. Several of the optimized structures suggested that the formation of ion pairs would lead to stable complexes, and four separate ion-pair structures were included among the possible associated species for each combination of interacting molecules. In those cases the computed spectra were more consistent with those of the NO+ and BF4− or BClF3− ions than of the corresponding neutral components. The relative stabilities of the two families of complexes have been rationalized and some differences between the properties of the ONF and ONCl adducts have been observed and explained.

A DFT investigation into the structure and energetics for nonadiabatic proton transfer in the benzophenone/N,N-dimethylaniline contact radical ion pair

For the past 60 years, the standard model for the interpretation of the mechanism for proton transfer has been based upon transition-state theory, which posits that the transition state is found in the proton transfer coordinate involving the breaking and making of bonds. However, the observed dynamics of proton transfer within the triplet contact radical ion pair, derived from a variety of substituted benzophenones complexed with N,N-dimethylaniline, cannot be accounted for within the standard model for proton transfer. Instead, the kinetic behavior is in accord with nonadiabatic proton transfer theory that has the transition state in the solvent coordinate. Evidence for the importance of the solvent coordinate comes from the existence of an inverted region; as the driving force for reaction increases, the rate of proton transfer decreases. This kinetic behavior is not found in the standard model. The present paper employs density function theory to examine the question as to whether the inverted region can be attributed to the transition state being in the solvent coordinate or whether the inverted region is an artifact produced by changes in the structure of the triplet contact radical ion pair with the placement of substituents upon the p,p′ positions of benzophenone. It is concluded that the inverted region is not an artifact of substituent effects upon structure. These results support the conclusion that the transition state for proton transfer resides in the solvent coordinate and challenges the validity of the standard model for interpreting the mechanism of proton transfer. Copyright © 2014 John Wiley & Sons, Ltd.

Persistent ion pairing in aqueous hydrochloric acid.

For strong acids, like hydrochloric acid, the complete dissociation into an excess proton and conjugated base as well as the formation of independent solvated charged fragments is assumed. The existence of chloride-hydronium (Cl(-)···H3O(+)) contact ion pairs even in moderate concentration hydrochloric acid (2.5 m) demonstrates that the counterions do not behave merely as spectators. Through comparison of recent extended X-ray absorption fine structure (EXAFS) measurements to state-of-the-art density functional theory (DFT) simulations, we are able to obtain a unique view into the molecular structure of medium-to-high concentrated electrolytes. Here we report that the Cl(-)···H3O(+) contact ion pair structure persists throughout the entire concentration range studied and that these structures differ significantly from moieties studied in microsolvated hydrochloric acid gas phase clusters. Characterizing distinct populations of these ion pairs gives rise to a novel molecular level description of how to view the reaction network for acid dissociation and how it relates to our picture of acid-base equilibria.

Preparation and Microbiological Evaluation of Amphiphilic Kanamycin-Lipoamino Acid Ion-Pairs.

Amphiphilic ion-pairs of kanamycin (KAN) were prepared by evaporation of a water-ethanol co-solution of KAN base and a lipoamino acid bearing a 12-carbon atoms alkyl side chain (LAA12), at different molar ratios. Infrared spectroscopy confirmed the structure of ion-pairs, while differential scanning calorimetry (DSC) and powder X-ray diffractometry (PXRD) studies supported the formation of new saline species with a different crystalline structure than the starting components. The solubility pattern shown in a range of both aqueous and organic solvents confirmed that the ion-pairs possess an amphiphilic character. The LAA12 counter-ion showed not to improve the antibacterial activity of KAN, suggesting that such chemical strategy is not able to favor the penetration of this drug inside the bacteria cells. Nevertheless, a slight improving, i.e., a one-fold dilution, was observed in E. coli. The present study can also serve as the basis for a further evaluation of LAA ion-pairing of antibiotics, as a means to improve the loading of hydrophilic drugs into lipid-based nanocarriers.

Probing ion-specific effects on aqueous acetate solutions: Ion pairing versus water structure modifications.

The effect of monovalent cations (Li+, K+, NH4+, Na+) on the water structure in aqueous chloride and acetate solutions was characterized by oxygen K-edge X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, and resonant inelastic X-ray scattering (RIXS) of a liquid microjet. We show ion- and counterion dependent effects on the emission spectra of the oxygen K-edge, which we attribute to modifications of the hydrogen bond network of water. For acetates, ion pairing with carboxylates was also probed selectively by XAS and RIXS. We correlate our experimental results to speciation data and to the salting-out properties of the cations.

Regional Electrophilic and Nucleophilic Fukui Functions Efficiently Highlight the Lewis Acidic/Basic Regions in Ionic Liquids

The origin of catalysis and selectivity induced by room temperature ionic liquids in several organic reactions has putatively been associated with the concept of cation effect (hydrogen bond donor ability of the ionic liquids) or anion effect (hydrogen bond accepting ability of the ionic liquids). We show that there may be cases where this a priori classification may not be correctly assigned. Cations may concentrate both Lewis acidity and basicity functions in one fragment of the ionic liquid: an effect we tentatively call bifunctional distribution of the molecular Lewis acidity/basicity. Bifunctionality on the cation is however anion dependent through electronic polarization effects. The molecular distribution of the Lewis acidity/basicity may simply be assessed by evaluating the regional Fukui function within a reference ion pair structure. The model is tested for a set of nine ionic liquids based on the 1-butyl-3-methylimidazolium cation commonly used as solvent to run organic reactions.