Spectroscopic studies of ionic solvation. XX. Cesium-133 NMR study of cesium salts in different solvents

Cesium-133 chemical shifts were measured in a number of solvents as a function of salt concentration and of the counterion. Infinite-dilution chemical shifts (vs. aqueous Cs⁺ ion at infinite dilution) ranged from +59.8 ppm for nitromethane solutions to –29.4 ppm for pyridine. In general, the magnitude of the downfield chemical shift reflected the donor ability of the solvents. Ion-pair formation constants were calculated from the concentration dependences of¹³³Cs chemical shifts in several nonaqueous solvents.     

Spectroscopic studies of ionic solvation. XXI. A raman,infrared, and NMR study of sodium perchlorate solutions in nonaqueous solvents

Sodium perchlorate solutions in several nonaqueous solvents were examined by²³Na,³⁵Cl-NMR, infrared, and Raman spectroscopic techniques. The formation of contact ion pairs lowers the symmetry of ClO ₄ ^– ion from T_d to C_3v or C_2v provided that the interaction is fairly strong. This was manifested for NaClO₄ solutions in acetonitrile, tetrahydrofuran, and pyridine. Combination of the vibrational measurements with NMR shows that in the above three cases the anion-cation interactions are quite strong. Sodium-23 NMR studies confirm the above results and, being a more sensitive technique, also indicates weak cation-anion interaction in propylene carbonate, formic acid, acetone, methanol, ethanol, dimethyl sulfoxide, and water. In all solvents the²³Na resonance shifts upfield with increasing concentration of NaClO₄, indicating that the replacement of solvent by ClO ₄ ^– ion decreases electron density around the cation.     

Complex formation of hydronium ion with several crown ethers in 1,2-dichloroethane,acetonitrile, and nitrobenzene solutions

A conductance study concerning the interaction between hydronium ion and several crown ethers in acetonitrile, nitrobenzene and 1,2-dichloroethane solutions has been carried out at 25°C. The stability constants of the resulting 1:1 complexes in acetonitrile and nitrobenzene solutions were determined from the molar conductance-mole ratio data and found to vary in the order 18C6>DB30C10>DC18C6>DB18C6>DB21C7>DB24C8>B15C5. In 1,2-dichloroethane solution, the complexation process results in the dissociation of ion pairs. There is an inverse relationship between the stabilities of the complexes and the Gutmann donicity of the solvents. In nitrobenzene solution, some evidence for the formation of a 2:1 sandwich adduct between the smaller crowns (i.e., B15C5 and 18-crowns) are observed from the molar conductance-mole ratio data which is supported by the¹H NMR data.     

Zinc-67 NMR study of zinc ions in water and in some nonaqueous and mixed solvents

Solutions of zinc salts in water, methanol (MeOH), dimethylformamide (DMF) and binary mixtures of water with the two nonaqueous solvents were studied by zinc-67 NMR measurements. Anhydrous zinc nitrate solutions in DMF and MeOH show upfield, concentration independent, chemical shifts at –27 and –19 ppm, respectively, vs. the aqueous solution standard. Addition of DMF or MeOH to an aqueous solution of a zinc salt results in a diamagnetic shift but for the addition of acetonitrile a paramagnetic shift results. In all cases the signal was broadened very considerably, e.g., in ZnCl₂ solution the linewidth increased from 40 to 600 Hz in going from water to 35% aqueous MeOH. Both⁶⁷Zn and¹³C NMR failed to show any complexation of Zn²⁺ ion by crown ethers in aqueous solution. A gradual addition of EDTA, of diaza-18-crown-6 or of tetraazacyclotetradecane resulted in an immediate broadening of the⁶⁷Zn signal which became undetectable when one equivalent of a ligand was added.     

Study of association of alcohols in aprotic solvents by multinuclear NMR

Binary mixtures of alcohols(ethyl,n-butyl and n-amyl)with several aprotic sol-vents,such as acetone,dioxane,THF,DMSO and DMF have been studied systematically by~1H,~(13)C,~(15)N and ~(17)O NMR measurements.The concentration dependence of chemical shift ofthe solvent was used to evaluate equilibrium constants of the complexation of alcohols withthe solvents.A relationship between the proton shift of alcohol uncombined with the solventand its concentration was found,and the fraction of unassociated hydroxyl groups was thusquantitatively described.The effect of solvent on self association and complexation of thealcohol is discussed on the basis of the electron donicity of the solvents.     

Measurement of pH by NMR Spectroscopy in Concentrated Aqueous Fluoride Buffers

An NMR spectroscopic technique has been developed to give rapid, accurate pH measurements on tenth-milliliter samples of concentrated acidic aqueous solutions buffered by fluoride ion in the pH 1.5-4.5 range. The fluoride ¹⁹F chemical shift has been calibrated as a function of pH at 0.1 and 1.0 M concentration by reference to an internal 3-fluoropyridine standard. Subsequent measurements of fluoride buffer pH required no additives and only two NMR spectra in the presence of an external reference standard.     

205Tl NMR studies: Ion pairing of Tl+ and (CH3)2Tl+ with NO 3 − and ClO 4 − in various solvents

The concentration dependence of the²⁰⁵Tl chemical shifts of Tl⁺ and of (CH₃)₂Tl⁺ ions was determined in several solvents with NO ₃ ^– and ClO ₄ ^– counterions. In general, increased ion pairing caused a low-frequency shift of the²⁰⁵Tl resonance, with the exceptions of (CH₃)₂TlNO₃ inn-butylamine and TlNO₃ in N,N-dimethylformamide (DMF) and in hexamethylphosphorotriamide (HMPA). In HMPA,²⁰⁵Tl linewidths of both Tl⁺ and (CH₃)₂Tl⁺ increased dramatically with dilution below 0.1M. Analysis of the data allowed ion-pair formation constants and²⁰⁵Tl chemical shifts for the ion-paired cation and for the free (solvated) cation to be estimated for some of the solvents.     

NMR studies of interionic interactions in N-methylformamide

The NMR chemical shifts of alkali and thallium(I) salts with various monovalent anions have been measured in N-methylformamide solution. Lithium-7 chemical shifts are virtually concentration and counter-ion independent, presumably due to an absence of direct cation-anion interactions. The sodium-23, potassium-39 and cesium-133 chemical shifts of the salts studied depend on the anion and vary linearly with the concentration. The observed behavior can be accounted for by the formation of collisional ion pairs. On the other hand, the thallium-205 chemical shifts of thallium(I) nitrate and perchlorate were anion-dependent and varied non-linearly with the salt concentration. These results are indicative of contact ion pair formation; formation constants were calculated to be 2.6±0.4 M ^–1 for TlNO ₃ and 1.7±0.5 M ^–1 for TlClO ₄ . The cesium-133 NMR spectra of several mixed electrolyte systems also have been measured in N-methylformamide solution. The¹³³Cs chemical shifts also change linearly with the concentrations of the salts added to 0.10 M CsI/NMF solutions. The influence of the anions on the chemical shifts is the same as that observed for cesium salts alone.     

14N NMR studies of thallium nitrate solutions in liquid ammonia

¹⁴N chemical shifts and linewidths were determined for NO ₃ ^– and NH₃ in liquid ammonia solutions of thallium nitrate at concentrations between 0.07 and 10 M. The concentration dependences of the¹⁴NO ₃ ^– shift and linewidth are consistent with the presence of C_2v ion pairs at a 2:1 mole ratio of NH₃ to TINO₃ and C_3v ion pairs at mole ratios of 3:1 or higher. Previous studies had indicated the formation of ion pairs at low concentrations. The small value of the¹⁴NO ₃ ^– linewidth below 1 M suggests that these are contact ion pairs. Studies of the¹⁴NH₃ linewidth as a function of thallium salt concentration indicate slow solvent exchange at very high concentrations.¹⁴NH₃ exhibits a downfield shift upon incorporation into the solvation sphere of the Tl⁺NO ₃ ^– ion pair, in constrast to upfield shifts reported for complexation with transition metal cations.     

Influence of solvent properties on cation-macrocycle complexation: Cesium cryptates

The chemical shift of the¹³³Cs resonance has been measured as a function of the mole ratio of cryptands C222, C222B, C221, and C211 to the Cs⁺ ion inN-methylformamide and dimethylacetamide solutions. The overall formation constants of the cesium cryptates were calculated from the NMR titration curves. From these and previously reported data it is clear that solvent properties such as dipole moment, donicity, dielectric constant and hydrogen bonding ability affect macrocyclic ligand-cation complexation equilibria through specific solvation of the cation, the ligand and the complex. A model of M⁺ · L bonding in solution, involving a rather intimate electrostatic attraction between the cation and the local electron distribution of the macrocycle accompanied by little perturbation of the charge distributions of the separated partners, is presented. In addition to the specific interactions previously noted, the presence of the solvent mitigates the ion-multipole forces which would otherwise govern the long-range attraction.This paper is dedicated to the memory of the late Dr C. J. Pedersen.Deceased.     

23Na NMR in urethane cross-linked polyether solid polymer electrolytes

Sodium triflate/polyether urethane polymer electrolytes ranging in concentration from 0.05 molal to 1.75 molal have been investigated via ²³Na static solid-state NMR. Room temperature spectra and spin lattice relaxation times were consistent with a single narrow resonance indicating the presence of only mobile ionic species. The concentration and temperature dependence of relaxation times, chemical shifts, and linewidth have been investigated. The results suggest either a single species or rapid exchange between a number of species (even at temperatures below the glass transition temperature, T_g). The linewidth decreases with increasing concentration of ions and remains temperature independent below T_g. Below T_g a maximum quadrupolar interaction constant of 2 MHz is calculated. The addition of plasticizer to the polymer electrolyte causes significant chemical shift changes that depend on the solvent donicity of the plasticizer. The linewidth and T₁ relaxation times also depend on the T_g of the plasticized systems. Previous ²³Na NMR literature results are reviewed and qualitative models developed to account for the variation in results. © 1994 John Wiley & Sons, Inc.     

Magnesium-25 NMR studies of magnesium salts and complexes in nonaqueous solvents

Magnesium-25 NMR measurements were carried out on aqueous and non-aqueous solutions of magnesium salts. In the former case the²⁵Mg resonance frequency was independent of the concentration or of the counterion. In nonaqueous solvents, however, the resonance frequency was dependent on the solvent, the concentration, and on the nature of the counterion, indicating some cation-anion interactions. Measurements on Mg²⁺—phosphonoacetic acid mixtures in aqueous solutions gave strong indications of complexation. Only inconclusive evidence was obtained on the complexation of Mg²⁺ by macro-bicyclic cryptand C211 in methanol solutions, and no evidence of complexation was obtained with macrocycle 12-crown-4 in dimethylformamide solutions.     

The first automated 470.59 MHz 19F NMR‐controlled titration: dissociation constants and ion‐specific chemical shifts of 2‐amino‐4‐fluoro 2‐methylpent‐4‐enoic acid

2‐Amino‐4‐fluoro‐2‐methylpent‐4‐enoic acid, obtained as a 1 : 1 salt with trifluoro‐acetic acid, was characterized by ¹H and ¹⁹F high‐resolution NMR spectroscopy. High‐precision potentiometry led to the dissociation constants pK = 1.879 and pK = 9.054. The first automated 470.59 MHz ¹⁹F NMR‐controlled titration yielded the dynamic chemical shift 〈δ_F〉 as a function of pcH or τ and the ion‐specific chemical shifts: δ_F(H₂L⁺) = ?94.81 ppm, δ_F(HL) = ?94.21 ppm, δ_F(L^?) = ?92.45 ppm. The deprotonation gradients were found to be Δ₁ = ?0.60 ppm and Δ₂ = ?1.76 ppm. Copyright © 2002 John Wiley & Sons, Ltd.     

Counterion binding in Na poly(acrylate) gel. An 23Na‐NMR study

Counterion binding in Na poly(acrylate) gel immersed in water/organic solvent [ethanol (EtOH), acetonitrile (AcN), or tetrahydrofuran (THF)] mixtures was investigated by ²³Na‐NMR spectroscopy. With an increase in the content of an organic solvent (～40–50 vol %), the ²³Na chemical shift significantly moved downfield on a gel collapse. This downfield shift strongly suggests that the gel collapse was induced by contact ion‐pair formation between the counterion and the carboxyl anion on the polymer. With a further increase in the solvent content (～90 vol %), the chemical shift for an EtOH system showed a slight upfield shift, while THF and AcN systems maintained downfield shifts. The contrasting behaviors for EtOH and the latter two solvent systems were interpreted as being caused by desolvation and resolvation of bound Na⁺ counterions because of deswelling and reswelling of the respective gels in the pertinent solvent concentration regions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4412–4420, 2004     

Synthesis of methyl orange using ionic liquids

Two ionic liquids were synthesized, each system consisting of the 1-hexyl-3-methylimidazolium ion ([hmim]⁺) as the cation and either hexafluorophosphate ([PF₆]⁻), or perchlorate ([ClO₄]⁻) as the anions. This study involves the synthesis of methyl orange (4-[[(4-dimethylamino)phenyl]-azo] benzene sulfonic acid sodium salt) using the ionic liquids as replacement solvents for the reaction. The advantage of using ionic liquids as substitutes for organic solvents includes: recyclable/reclaimable solvents, stabilization of intermediates and higher product yields. The 1-hexyl-3-methylimidazolium derivatives can be used for syntheses conducted at low temperatures and are less toxic than typical organic solvents.     

Formation of ionic liquid submicron particles. 1H and 19F nuclear magnetic resonance spectroscopic studies

We prepared novel ionic liquid submicron particles (ILSPs) in water by emulsifying the ionic liquid (IL) N-(2-methoxyethyl)-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr_12O1TFSI), which is immiscible with water, with the nonionic surfactants Tween 20 and Span 80. The mean particle size and zeta potential of the ILSPs were about 580?nm and ?30?mV, respectively. The ILSPs were characterized using ¹H and ¹⁹F nuclear magnetic resonance (NMR) spectroscopic methods. The chemical shifts of the Pyr_12O1⁺ cation and TFSI^? anion in the ¹H and ¹⁹F NMR spectra of the ILSP suspension were consistent with those corresponding to pure Pyr_12O1TFSI. This indicated that most of the Pyr_12O1TFSI was still in the IL state in the ILSP suspension. In addition, ¹H–¹H nuclear Overhauser effect correlated spectroscopic measurements showed that the ILSP droplets contained Pyr_12O1TFSI in the neat IL steric state even in the ILSP suspension, and Pyr_12O1TFSI species localized in the droplet surfaces interacted with the hydrophobic acyl side chains of the surfactants. These NMR spectroscopic results show that the ILSPs formed an IL-in-water microemulsion in which droplets of neat Pyr_12O1TFSI were surrounded by the two surfactants, that is, Tween 20 and Span 80.     

Nuclear magnetic resonance of donor atoms as a tool for determination of the structure of platinum metal complexes in solutions

Data on the NMR spectroscopy of C, N, O, Si, P, and Sn donor atoms of platinum metal complexes in solutions are surveyed. The chemical shift of a donor atom mainly depends on the ligand in the trans-position (due to the trans-effect). The chemical shift of a donor atom on a particular coordinate of the complex (coordinate shift, CSh) is an attribute of this coordinate and can be used to identify such a coordinate in platinum metal complexes and to determine the structures of complexes. Based on the known data, CSh diagrams were composed for ¹H, ¹³C, ¹⁴N, ¹⁷O, ¹⁹F, ³¹P, and ¹¹⁹Sn. Examples of using the CShs for determining the structures of platinum metal complexes in solutions are presented.     

Dielectric relaxation and conductivity studies on (PEO:LiClO4) polymer electrolyte with added ionic liquid [BMIM][PF6]: Evidence of ion–ion interaction

Polymer electrolytes, (PEO:LiClO₄)+x IL (1‐Buty‐3‐methylimidazolium hexafluorophosphate) with varying concentration of IL; x = 0,5,10,15,20 wt % have been prepared by solution cast technique and characterized by X‐Ray diffraction, differential scanning calorimetery, FTIR, conductivity and dielectric relaxation measurements in the frequency range of 100 Hz–5 MHz. Temperature dependence of relaxation frequency and conductivity were found to be typical of thermally activated process both at T > T_m and T < T_m. Composition dependence of conductivity, dielectric relaxation, and degree of crystallinity has also been studied. On addition of IL, the degree of crystallinity after a decrease at 5 wt % IL increases slightly at 10 wt % and then finally decreasing. Variation of conductivity and relaxation frequency with composition could only be partly explained on the basis of variation of degree of crystallinity. An additional feature of ion–ion interaction (contact ion pair formation between IL or salt cations and their associated anions) has been invoked which was supported by FTIR studies. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Thallium-205 NMR spectroscopy. Solvation of the thallous ion in dilute aqueous-amide and amide-amide binary solvent systems of formamide,N-methylformamide,andN-ethylformamide

Solvation of the thallous ion in dilute solutions of six binary solvent systems (formamide/water,N-methylformamide/water,N-ethylformamide/water, formamide/N-methylformamide, formamide/N-ethylformamide, andN-methylformamide/N-ethylformamide) was studied with²⁰⁵Tl NMR spectroscopy. An attempt was made to separate solvation effects related to the electrondonating ability (Lewis basicity) of the solvents from effects resulting from structural changes in the solvation sphere. Structural effects were found to be greatest in theN-methylformamide/water system and least in theN-methylformamide/formamide system.     

Design Principles and Theory of Paramagnetic Fluorine‐Labelled Lanthanide Complexes as Probes for 19F Magnetic Resonance: A Proof‐of‐Concept Study

The synthesis and spectroscopic properties of a series of CF₃‐labelled lanthanide(III) complexes (Ln=Gd, Tb, Dy, Ho, Er, Tm) with amide‐substituted ligands based on 1,4,7,10‐tetraazacyclododecane are described. The theoretical contributions of the ¹⁹F magnetic relaxation processes in these systems are critically assessed and selected volumetric plots are presented. These plots allow an accurate estimation of the increase in the rates of longitudinal and transverse relaxation as a function of the distance between the Ln^III ion and the fluorine nucleus, the applied magnetic field, and the re‐rotational correlation time of the complex, for a given Ln^III ion. Selected complexes exhibit pH‐dependent chemical shift behaviour, and a pK_a of 7.0 was determined in one example based on the holmium complex of an ortho‐cyano DO3A‐monoamide ligand, which allowed the pH to be assessed by measuring the difference in chemical shift (varying by over 14 ppm) between two ¹⁹F resonances. Relaxation analyses of variable‐temperature and variable‐field ¹⁹F, ¹⁷O and ¹H NMR spectroscopy experiments are reported, aided by identification of salient low‐energy conformers by using density functional theory. The study of fluorine relaxation rates, over a field range of 4.7 to 16.5 T allowed precise computation of the distance between the Ln^III ion and the CF₃ reporter group by using global fitting methods. The sensitivity benefits of using such paramagnetic fluorinated probes in ¹⁹F NMR spectroscopic studies are quantified in preliminary spectroscopic and imaging experiments with respect to a diamagnetic yttrium(III) analogue.