Symmetric Cations Research Articles

Temperature dependence of viscosity for room temperature ionic liquids

We have measured the absolute viscosity of 23 room temperature ionic liquids (RTILs) (13 air/moisture-tolerant single salts and 10 air/moisture-sensitive binary salts) using a rolling-ball viscometer, which was constructed to enable the viscosity measurements to be performed outside a glove-box. The viscometer was non-destructive to the RTIL and the viscosity measurements were made under typical levels of O 2 and H 2O of less than 1 ppm. The viscometer was calibrated with Milli-Q water and sucrose solutions (20, 40 and 60% w/w) at 14 temperatures (10 °C ⩽ T⩽ 70 °C). Three balls of diameters 3.2, 4 and 5 mm were used for the calibration. We found that the ball with the smallest diameter (3.2 mm) was more accurate for measuring the viscosities. The activation energies for viscous flow ( E η ) for each RTIL were calculated, from the slopes of the Arrhenius plots. E η was correlated with changes in molecular structure of the RTIL; those containing high molar mass and less symmetric cations had higher E η values. The Arrhenius plot was not linear for the majority of the RTILs and some of these RTILs were better fitted with the Vogel–Tammann–Fulcher (VTF) equation. Those RTILs which obeyed the Arrhenius law typically contained asymmetric cations and the majority did not contain functional groups. Those RTILs which obeyed the VTF law contained small and symmetrical cations with low molar mass. Those RTILs which obeyed neither the Arrhenius nor the VTF law consisted of cations which were less symmetric, contained functional groups (e.g., OH and CO) and had higher molar mass. The glass transition temperatures T o (calculated from the VTF fittings) generally decreased with increasing size and molar mass of the cation and anion. The chloroaluminate RTILs containing oxygenated cations were more volatile than the corresponding de-oxygenated RTILs making them unsuitable for applications as solvents and/or electrolytes above ambient temperature.

Room-Temperature Ionic Liquids with High Conductivities and Wide Electrochemical Windows

Room-temperature ionic liquids (RTILs), N-alkyl-N-methylpylrolidinium (RMPyr + ) and N-alkyl-N-methylpiperidinium (RMPip + ) fluorohydrogenates formulated by RMPyr(HF) 2 . 3 F and RMPip(HF) 2 . 3 F (R = ethyl, propyl, and butyl), have been synthesized by the reactions of the corresponding chlorides and anhydrous hydrogen fluoride. These RTILs exhibit relatively low viscosities (11.5-34.4 cP), high conductivities (12.3-74.6 mS cm - 1 ), and wide electrochemical widows around 5 V using glassy carbon electrodes. The fluorohydrogenate salts of symmetric cations, N,N-dimethylpyrrolidinium (DMPyr + ) and N,N-dimethylpiperidinium (DMPip + ) obtained by evacuation at room temperature are solids, exhibiting the composition of DMPyrF-2HF and DMPipF-2HF.

Enthalpy Barriers for Asymmetric SN2 Alkyl Cation Transfer Reactions between Neutral and Protonated Alcohols

Enthalpy barriers for gas phase asymmetric alkyl cation transfer reactions between neutral and protonated alcohols in mixed alcohol systems have been calculated by B3LYP, MP2, and G3(MP2) computational methods. As expected, on the basis of the proton affinities of the alcohols, the enthalpy barrier for transfer of the larger alkyl cation is calculated to be lower than that for transfer of the smaller alkyl cation. A linear relationship between the difference in gas basicity (GB) between the alcohols and the enthalpy barrier is observed for the less favorable alkyl cation transfer reactions. The slope of the regression line is 0.55, and the intercept (at ΔGB = 0) is found to be −22.5 kJ mol-1, which is extremely close to the enthalpy barriers for symmetric alkyl cation transfer. This relationship is discussed in the context of the Hammett relationship. Experiments conducted on the reaction of protonated ethanol with 18O-labeled methanol established that about 7.8% of the protonated ethyl methyl ether formed was via the less favorable methyl cation transfer. This experimental value agrees well with the 8.5% predicted based upon the calculated difference in energy barriers between ethyl and methyl cation transfer (6.1 kJ mol-1). This work shows that in the gas phase, the protonated ether products in mixed alcohol systems are formed via two competing SN2 reactions and that the efficiency of the less favorable alkyl cation transfer reaction increases as the difference in proton affinity of the two alcohols decreases.

Local hybrid functionals

We present a novel approach for constructing hybrid functionals by using a local mix of regular density functional theory (DFT) exchange and exact Hartree–Fock (HF) exchange. This local hybrid approach is computationally feasible for a wide range of molecules. In this work, the local mix of HF and DFT exchange is driven by the ratio of τW=|∇ρ|2/8ρ, the Weizsäcker kinetic energy density, with τ, the exact kinetic energy density. This particular choice of local mix yields 100% of exact exchange in one-electron regions. Dissociation energy curves, binding energies, and equilibrium geometries for two-center, three-electron symmetric radical cations can be modeled accurately using this scheme. We also report encouraging results for reaction energy barriers, and somewhat disappointing atomization energies for the small G2 set.

D 2–D 0 Fluorescence of pseudo-D 3h symmetric radical cations in solution and their `invisible' D 1 state as a route to non-radiative decay

Photophysical properties of three radical cations with a pseudo-D 3h electronic structure in acetonitrile were studied by laser flash photolysis and their tandem excitation to their D 2 state. The theoretical calculations of their electronic levels by the INDO/1 method have indicated that their D 1 and D 0 states deviate from the degenerate state and D 1←D 0 transition has excitation energies of 0.5–1.1 eV with oscillator strengths of nearly zero. The rate constants for their non-radiative decays from the D 2 state increased exponentially with the decrease of the calculated D 2–D 1 energy gap obeying the energy gap law.

CASPT2 calculation of the excited states of symmetric polyenyl cations

Ab initio quantum mechanical calculations have been performed to study the excited state properties of the symmetric all-E-configurated polyenyl cations, [R2C–(CH–CH)n–CH–CR2]+, with R=H and CH3 and n from 1 to 5. All geometries were fully optimised within the C2v space group using the DFT method and the Becke LYP3 exchange correlation functional in conjunction with the 6-31G** basis set. Excited state energies were calculated using multi-configurational second-order perturbation theory (CASPT2) within the framework of CASSCF and a 4s3p1d/2s ANO basis set. The vinylene shift of 76 nm which is experimentally observed for these compounds is reproduced with remarkable accuracy; the CASPT2 corrected vertical energies of the lowest excited state (1 1B2) are within 0.07 eV of the experimental values. There is a steady increase of the bathochromic shift induced by the four terminal methyl groups, from 75 nm for the shortest cation 1 (which has five conjugated π-orbitals) to 115 nm in 5 (with 13 π-orbitals). The symmetry forbidden 2 1A1 state is slowly decreasing with respect to the 1 1B2 state as the length of the chromophore increases, with the energy gap going from 1.53 eV for 1 to 0.54 eV in 5. However, it appears that the 2 1A1 state stays above the 1 1B2 state even for very long chromophore lengths. There is a close correlation between the electronic states of symmetric polyenyl cations and streptocyanine dyes.

Metallocene-terminated allylium salts: the effect of end group on localization in polymethines.

A range of 1,3-di(metallocenyl)allylium salts [Mc(CH)(3)Mc'](+)[X](-) [Mc, Mc' = ferrocenyl (Fc), 2,3,4,5,1',2',3',4'-octamethylferrocen-1-yl (Fc' '), ruthenocenyl (Rc); X = BF(4), PF(6)] was synthesized by reaction of (2-lithiovinyl)metallocenes with formylmetallocenes, followed by treatment of the resulting alcohols with HX. Two salts with X = BAr'(4) [Ar' = 3,5-(CF(3))(2)C(6)H(3)] were synthesized by anion metathesis from the corresponding PF(6) salts. The crystal structure of [Fc' '(CH)(3)Fc' '](+)[PF(6)](-) contains symmetrical termethine cations, while the same appears to be true in the disordered structure of [Fc(CH)(3)Fc](+)[PF(6)](-). The formally unsymmetrical cation in [Fc(CH)(3)Fc' '](+)[BF(4)](-) is only slightly unsymmetrical with little bond-length alternation in the allylium bridge. In contrast, the crystal structures of [Rc(CH)(3)Rc](+)[PF(6)](-) and [Rc(CH)(3)Rc](+)[BAr'(4)](-) both contain a bond-alternated "Peierls-distorted" cation, which can be considered as a ruthenocene bridged to a [(eta(6)-fulvene)(eta(5)-cyclopentadienyl)ruthenium] cation by a vinylene moiety. The strong similarity between solid-state and solution infrared and Raman spectra of [BF(4)](-), [PF(6)](-), and [BAr'(4)](-) salts of [Rc(CH)(3)Rc](+) indicates that the C-C stretching constant in the allylium chain and, therefore, the structure, of this ion are largely independent of the local environment, suggesting that the unsymmetrical structures observed in the crystal structures are not simply an artifact of packing. Differences in the solvatochromism of [Rc(CH)(3)Rc](+) and [Fc(CH)(3)Fc](+) also suggest a localized structure for the former cation in solution. Electrochemistry, UV-visible-NIR spectroscopy, and DF calculations give insight into the electronic structure of the metallocene-terminated allylium cations. Using an analogy between polymethines and mixed-valence compounds, the difference between the behaviors of [Fc(CH)(3)Fc](+) and [Rc(CH)(3)Rc](+) is attributed to larger reorganization energy associated with the geometry differences between metallocene and [(eta(6)-fulvene)(eta(5)-cyclopentadienyl)metal] structures in the ruthenium case.

Novel highly symmetrical cube-shaped cation with 16-nitrogen donors

A novel highly symmetrical cube-shaped trication [C36H75N16]3+ with 16-nitrogen donors is achieved via self-assembly in which two water molecules were clathrated within the cavity of the framework.

Symmetry, Radical Ions, and Butadienes: Exploring the Limits of Density Functional Theory

Pure and hybrid density functional theory (DFT) methods are evaluated regarding their ability to correctly describe asymmetric transition structures for symmetric butadiene radical cations. The study covers unsubstituted butadiene radical cation as well as (2,3-X,X)-disubstituted butadiene radical cations, where X is -CH3, -OH, -F, or -SiH3. The DFT methods either converge toward an erroneous symmetric solution or describe the asymmetric transition structure qualitatively correctly. Whether or not the asymmetric transition structure is found depends on the amount of Hartree−Fock density exchange included in the method, as well as on the electronic characteristics of the substituents, as described by their σ+ values. The disparate behaviors encountered are rationalized by a two-state model. It is also shown that molecular symmetry does not automatically induce errors, as reported by earlier studies. Instead, electronic symmetry is introduced as a factor to observe when organic radical cations are studied c...

Entropic origin of the sulfonate groups’ electrostatic assistance in the complexation of quaternary ammonium cations by water soluble calix[4]arenes

The inclusion of symmetrical tetramethylammonium cation (TEMA) by the water soluble calixarene hosts 1–8 was studied at neutral pH by 1H NMR spectroscopy and compared with that of the ditopic trimethylanilinium cation (TMA). The hosts blocked in the cone conformation and bearing sulfonate groups at the upper rim (2, 3, 5, 7 and 8) bind selectively the aromatic portion of TMA, whereas compound 4 which lacks sulfonate groups interacts only with the charged head group of TMA. The conformationally mobile compound 1 and the partial cone calixarene 6 include TMA cation in an unselective fashion. TEMA is complexed by hosts 1–7, but not by the tetraether-tetrasulfonate receptor 8. The binding constants for all the systems, as determined by 1H NMR spectroscopy, show that inclusion is favoured by the presence of the sulfonate groups and that the complexes of the conformationally mobile receptor 1 with both guests are more stable. The thermodynamic parameters of inclusion determined by direct calorimetry for 2–TEMA and 4–TEMA systems show that in both cases the inclusion process is enthalpically driven and that the greater stability constant observed for 2–TEMA with respect to that of 4–TEMA mainly results from a less unfavourable entropic contribution, suggesting that in the 2–TEMA complex the charged sulfonate groups cause a better desolvation of the host–guest system upon inclusion.

Some Di- and Tri-nuclear Compounds of Copper(II) with 4-Amino-4-methylpentan-2-one Oxime; Structures and Magnetism

Green [{Cu(amox)(amoxH)}2](NO3)2 (amoxH = 4-amino-4-methylpentan-2-one oxime) has twofold symmetrical, oximato-bridged, dinuclear cations. Each copper(II) ion is bonded to the nitrogen atoms of one bidentate amox – and one bidentate amoxH (marked *) ligand {Cu–Namine 2.039(4), 2.006(4)* Å and Cu–Noxime 1.975(5), 2.192(5)* Å}. The dinuclear cations have bonds between each copper(II) ion and the oximato oxygen atom of the other (marked #) unit {Cu–O# 2.062(4) Å, Cu–Cu# 3.801(5) Å}. The copper(II) ion is in asymmetrical trigonal-bipyramidal coordination, with Namine*–Cu–Noximate axial {176.5(2)˚}, and Namine–Cu–O# {136.6(2)˚}, Namine–Cu–Noxime* {126.2(2)˚} and Namine–Cu–O# {96.9(2)˚} equatorial [orthorhombic, Pbcn, a 12.218(1), b 23.361(3), c 12.275(1) Å, Z 4, R1 0.057 for 2239 reflections]. The magnetic susceptibility values follow the Curie Weiss relation, with q = –10(2) K. Dark-green [{{Cu(amox)}3O}2H] (ClO4)3·3H2O has threefold symmetrical trinuclear cations, with each copper(II) ion chelated by the amine {Cu–N 1.977(3) Å} and oxime {Cu–N 1.994(2) Å} nitrogen atoms of one amox – ligand, and coordinated by the bridging oximato oxygen atom of another amox – ligand {Cu–O# 1.938(2) Å}. A central oxygen atom on the threefold axis completes square-planar tetracoordination about each copper(II) ion {Cu–O 1.910(1) Å}. Cu–Cu# distances are 3.283(2) Å, with the central oxygen atom 0.233(2) Å out of the Cu3 plane. Two of these tri-nuclear cations are linked confacially by a proton on the common threefold axis {O–O# 2.474(1) Å}, to form novel hexanuclear clusters, with dihedral angles about the central Cu–O–O#–Cu# axis of 37.4(1)˚. Distances Cu–Cu# between the triangles are 3.180(5), 3.860(5) and 4.738(5) Å [trigonal, R3 – c, a 23.932(5), c 18.581(4) Å, Z 12, R1 0.034 for 1660 reflections]. The magnetic susceptibility values indicate extensive antiferromagnetic spin-coupling within the triangles. A blue compound of stoichiometry Cu(amox)(H2O)(NO3) has susceptibility values supporting a di-nuclear spin-coupled structure.

Structure and IR spectroscopic behaviour of 2,7‐dichloro‐1,8‐bis(dimethylamino)naphthalene and its protonated form

X-ray diffraction and IR spectroscopic studies on the proton sponge 2,7-dichloro-1,8-bis(dimethylamino)naphthalene and its adduct with HBr were performed. It was shown that the presence of the chlorine atoms in positions 2 and 7 leads to some change of conformation with respect to the free molecule. The main factor determining the conformation remains the repulsion of the nitrogen lone electron pairs. However, the repulsion between the methyl groups and chlorine atoms causes the dimethylamino groups to be slightly less twisted compared with unsubstituted dimethylaminonaphthalene. The protonation leads to the formation of a symmetrical cation (symmetry plane passing through the C9—C10 axis) with the N…N distance equal to 2.561(3) Å. This corresponds to the situation where the barrier to the proton transfer should be very low. As in other short [NHN]+ bridges, one observes the IR absorption band at about 500 cm−1, assigned to the transition between the split O+ → O− vibrational levels. A very high frequency isotopic ratio (νH/νD = 1.8) is observed for this transition. Copyright © 1999 John Wiley & Sons, Ltd.

Unusual M (super 3+) cations in synthetic amphiboles with nominal fluoro-eckermannite composition; deviations from stoichiometry and structural effects of the cummingtonite component

Single-crystal structure-refinement and electron-microprobe analysis of synthetic amphiboles with nominal fluoro-eckermannite composition and different trivalent cations (Al, Sc, Ti (super 3+) , V (super 3+) , Cr (super 3+) , Ga,) show significant deviations from nominal stoichiometry. Synthetic Sc- and Cr-bearing fluoro-eckermannite are close to nominal stoichiometry, whereas the corresponding nominal Al-, V (super 3+) -, Ga-, and Ti-bearing species contain very few trivalent cations and approximate Na(NaMg)Mg 5 Si 8 O 22 F 2 , a composition that has not been found in natural systems. The presence of a significant cummingtonite component strongly affects unit-cell parameters, coordination geometry around the B-, C-, and T-group sites, and cation ordering at the A-group sites. The high-charge cations are completely ordered at the M2 site, and there is a well-developed linear relationship between M2-O and the constituent-cation radius at the M2 site. The synthetic fluoro-eckermannite structure is stabilized by large spherically symmetric trivalent cations at the M2 site; for small spherically symmetric trivalent cations, it is not stable (at least at the synthesis conditions used here). Synthetic chromium-fluoro-eckermannite is stabilized by the non-spherically symmetric 3d 3 electronic arrangement, whereas Ga and V (super 3+) (3d 2 ) do not stabilize the fluoroeckermannite structure.

Liquid–liquid distribution of ion associates of hexabromotellurate (IV) with quaternary ammonium counter ions

The distribution behavior of ion associates (Q +) 2. TeBr 6 2− of TeBr 6 2− with quaternary ammonium cations (Q +) between aqueous phase and two organic phases (1,2-dichloroethane (DCE) and chloroform (CF)) was examined, and the extraction constants (log K ex) were determined. The extractability with long-chain alkyltrimethylammonium cations (group I) is greater than that with symmetrical tetraalkylammonium cations (group II) in the 1,2-dichloroethane extraction system and the difference in log K ex for two cations (one of each type) with the same number of carbon atoms was about 3 on the average. The contributions of a methylene group in the cations of groups I and II to log K ex were found to be 0.43∼1.10 and about 0.74 on the average. Among the ion associates examined, the extractability of the extracting solvent was in the DCE>CF; the difference in log K ex was about 5 on the average.

Liquid-liquid distribution of ion associates of tetrabromoindate(III) with quaternary ammonium counter ions

The solvent extraction of an ion associate of tetrabromoindate(III) ion, InBr − 4, with quaternary ammonium cations (Q +) has been studied. The extraction constant ( K ex) were determined for the ion associates of InBr − 4 with Q + between an aqueous phase and several organic phases (chloroform, chlorobenzene, benzene and toluene). A linear relationship was found between log K ex and the total number of carbon atoms in Q +; from the slope of the lines, the contribution of a methylene group to log K ex was calculated to be 0.91 for the chloroform extraction system and 0.52 for the other extraction systems. The extractability with alkyltrimethylammonium cations was larger than that with symmetrical tetraalkylammonium cations and the mean difference in log K ex for two cations (one of each type) with the same number of carbon atoms was about 1.3. From the extraction constant obtained, the extractability of InBr − 4 among metal-halogeno complex anions was in the order TlBr − 4 > BiI − 4 > AuBr − 4 > AuCl − 4 > TlCl − 4 > InBr − 4 > CuCl − 2.

Perturbed-angular-correlation study of the electric-field gradient indoped181and implanted indium sesquioxide

We studied the hyperfine interactions of $^{181}\mathrm{Ta}$ in ${\mathrm{In}}_{2}$${\mathrm{O}}_{3}$ by means of perturbed-angular-correlation (PAC) measurements. We prepared thin films of indium sesquioxide with different degrees of initial amorphism and implanted them with $^{181}\mathrm{Hf}$. Chemically prepared indium-sesquioxide powder samples were also made starting from neutron-irradiated ${\mathrm{HfCl}}_{4}$, which provides the $^{181}\mathrm{Hf}$ PAC probes. PAC experiments were performed on each sample at room temperature, after each step of annealing programs at increasing temperatures up to the full crystallization of the samples. The results indicate that the PAC probe occupies preferentially the axially symmetric cation site. Point-charge-model calculations were performed. The calculated asymmetry parameters \ensuremath{\eta} were compared with those obtained in $^{181}\mathrm{Hf}$ PAC experiments performed also on other binary oxides, showing that the symmetry of the electric-field-gradient (EFG) tensor at $^{181}\mathrm{Ta}$ cation sites in binary oxides is mainly determined by the nearest-neighbor oxygen-ion distribution around the probe. Comparisons of the experimental results in bixbyites obtained for both PAC probes, $^{111}\mathrm{Cd}$ and $^{181}\mathrm{Ta}$, show that the local EFG in bixbyites, are strongly dependent on the geometry of the sites and the electronic configuration of the probes.

MCSCF vibrational spectra of the symmetric and asymmetric dihydronium cations

We report the first full geometry optimizations of the dihydronium cation performed at the MCSCF level. Large basis set effects are encountered. Depending on the basis set, we found, either the C2 symmetric or the Cs asymmetric species as global minima of the purely electronic PES. However, after harmonic ZPE correction, the C2 symmetric structure appears to be favoured. Supported by a display of MCSCF natural orbitals, we propose an analysis of the H5O2+ electronic structure in terms of a chemical-type binding. A “soft mattress” model of proton transfer, linked to this “chemical” picture, is set forward. MCSCF harmonic OH stretch frequencies, corrected for anharmonicity are compared with experimental data, and support the evidence of a C2 structure. The nature of proton motions is examined through a pictorial analysis of intermolecular vibrational motions. A tentative explanation is proposed for the very wide spectra starting near 3500 cm−1 down to 800 cm−1 in concentrated acidic solutions. Prospects related to on-going large scale MCSCF computations are sketched.

MCSCF vibrational spectra of the symmetric and asymmetric dihydronium cations

We report the first full geometry optimizations of the dihydronium cation performed at the MCSCF level. Large basis set effects are encountered. Depending on the basis set, we found, either the C 2 symmetric or the C s asymmetric species as global minima of the purely electronic PES. However, after harmonic ZPE correction, the C 2 symmetric structure appears to be favoured. Supported by a display of MCSCF natural orbitals, we propose an analysis of the H 5O 2 + electronic structure in terms of a chemical-type binding. A “soft mattress” model of proton transfer, linked to this “chemical” picture, is set forward. MCSCF harmonic OH stretch frequencies, corrected for anharmonicity are compared with experimental data, and support the evidence of a C 2 structure. The nature of proton motions is examined through a pictorial analysis of intermolecular vibrational motions. A tentative explanation is proposed for the very wide spectra starting near 3500 cm −1 down to 800 cm −1 in concentrated acidic solutions. Prospects related to on-going large scale MCSCF computations are sketched.

The chemistry of cobalt acetate: The isolation and crystal structure of the symmetric trimer,hexakis( μ-acetato)- μ3-oxo-tris(pyridine)tricobalt(III) perchlorate water solvate, [Co 3O(CH 3CO 2) 6(C 5H 5N) 3][CiO 4]·H 2O

The isolation and characterization by single-crystal X-ray diffraction (refined to R = 0.066, R w = 0.058) of the perchlorate salt of the symmetric trimeric cation, [Co 3O(CH 3 CO 2) 6(C 5H 5N) 3] +, are reported. The compound crystallizes in space group P2 1/n (No. 14) as its water solvate, [Co 3O(CH 3CO 2) 6(C 5H 5N) 3][CiO 4]·H 2O. The oxo-centred trimeric cations have a planar Co 3ON 3 core, with an equilateral triangle of cobalt atoms around the central oxygen and average distances 3.194(1) (Co3.Co) and 1.845(8)A˚(Co μ 3-O).

Liquid-liquid distribution of ion associates of hexachloroplatinate(IV) with quaternary ammonium counter ions

The distribution behavior of ion associates of PtCl 6 2− with quaternary ammonium cations (Q +) between aqueous phase and two organic phases (chloroform and carbon tetrachloride) was examined, and the extraction constants (log K ex) were determined. A linear relationship was found between log K ex and the total number of carbon atoms in Q +; from the slope of the line, the contribution of a methylene group to log K ex was calculated to be 0.6. The extractability with alkyltrimethylammonium cations was larger than that with symmetrical tetraalkylammonium cations. From the extraction constants obtained, an extraction method based on the ion association of PtCl 6 2− with Q + can be used for the separation and determination of platinum at the 10 −6 M level.