A covalent attraction between two molecular cation TTF·+

The optimized structure of the tetrathiafulvalence radical-cation dimer (TTF·⁺-TTF·⁺) with all-real frequencies is obtained at MP2/6-311G level, which exhibits the attraction between two molecular cation TTF·⁺. The new attraction interaction is a 20-center-2-electron intermolecular covalent π/π bonding with a telescope shape. The covalent π/π bonding has the bonding energy of about ?21 kcal·mol^?1 and is concealed by the Coulombic repulsion between two TTF·⁺ cations. This intermolecular covalent attraction also influences the structure of the TTF·⁺ subunit, i.e., its molecular plane is bent by an angle θ = 5.6°. This work provides new knowledge on intermolecular interaction.

     

NMR STUDY OF EXCHANGE KINETICS OF THE LITHIUM ION WITH CRYPTAND C222 IN BINARY ACETONITRILE-NITROMETHANE MIXTURES

Abstract

The exchange kinetics of the lithium ion with cryptand C222 were studied in acetonitrile-nitromethane mixtures by lithium-7 NMR line-shape analysis. In all solvent mixtures used, and over the entire temperature range studied, the chemical exchange of the Li⁺ ion between the solvated and complexed sites was found to occur via a bimolecular mechanism. The activation parameters E_a, δH?, δS? and δG? for the exchange have been determined. The free energy barrier for the exchange process appears to be nearly independent of the binary mixture composition. The results confirm the preferential solvation of the lithium ion with acetonitrile in the binary mixed solvent systems used.     

A covalent attraction between two molecular cation TTF·+

The optimized structure of the tetrathiafulvalence radical-cation dimer (TTF·⁺-TTF·⁺) with all-real frequencies is obtained at MP2/6-311G level, which exhibits the attraction between two molecular cation TTF·⁺. The new attraction interaction is a 20-center-2-electron intermolecular covalent π/π bonding with a telescope shape. The covalent π/π bonding has the bonding energy of about −21 kcal·mol⁻¹ and is concealed by the Coulombic repulsion between two TTF·⁺ cations. This intermolecular covalent attraction also influences the structure of the TTF·⁺ subunit, i.e., its molecular plane is bent by an angle θ = 5.6°. This work provides new knowledge on intermolecular interaction.     

Ion-solvent molecule interaction in nonaqueous solutions: Spin-lattice relaxation time of7Li

The spin-lattice relaxation time (T ₁) of⁷Li⁺ was measured in solutions of LiCl and LiClO₄ in protic (MeOH, EtOH,n-PrOH,i-PrOH,n-BuOH, sec-BuOH, formamide, N-methylformamide) and aprotic (MeCN, acetone, methyl ethyl ketone, propylene carbonate, dimethyl sulfoxide, dimethylformamide, hexamethylphosphotriamide) solvents and in mixtures of H₂O-formamide, H₂O–N-methylformamide, H₂O–N,N-dimethylformamide, H₂O-DMSO, H₂O-hexamethylphosphotriamide, and formamide-N,N-dimethylformamide at 25°C. The values of (1/T ₁)₀ obtained by extrapolation are discussed in terms of current theories of the magnetic relaxation of ionic nuclei. Linear correlations were found between (1/T ₁)₀ and Gutmann's donor numbers and Kosower's Z-values. These correlations indicate that relaxation of⁷Li⁺ is dominated by donor-acceptor interaction of the cation with solvent molecules. Concentration dependences of 1/T ₁ for LiCl and LiClO₄ differ from one another in a given solvent, a fact which is accounted for by a specific cation-anion short-range potential. The quantity 1/T ₁ of⁷Li⁺ atC=1 mole per 55.5 moles of mixed solvent as a function of solvent composition show characteristic features, which are discussed in terms of the relaxation mechanism proposed.     

Rotational molecular motion and solvation of Na+ and Cs+ in water-dimethylacetamide. A nuclear magnetic relaxation study

Nuclear magnetic relaxation by intra- and intermolecular quadrupoleelectric field gradient interaction has been used for the study of the systems DMA-water-NaI and DMA-water-CsI at 25°C.¹⁴N relaxation of DMA and²H relaxation of D₂O measured over the complete mixture range reveal the behavior of the rotational molecular motion of the two solvent components. For both solvent components a marked maximum of the reorientational correlation time has been found, reflecting hydrophobic effects and strong DMA-water interaction. The quadrupolar relaxation rates of²³Na⁺ and¹³³Cs⁺ in pure DMA were evaluated giving an indication that the electric solvent dipoles in the solvation shell are not located on positions of cubic symmetry. A quantitative study of preferential solvation of the cations in the mixed solvent has been performed by using the H₂O-D₂O isotope effect on²³Na⁺ and¹³³Cs⁺ relaxation. For both cations an obviously typical change in the selectivity occurs. In the range l>x _{H2 O}>0.7 we find weak preferential hydration, but in the range 0.7>xH ₂ O>0 strong preferential solvation by DMA is reflected.     

The Synthesis and the Cationic Fluorescence Role of Glycols with Aromatic End Groups, Part III

Some novel bis-(substituted-phenoxy) ended glycols were synthesised usinghydroxy aromatics of vanillin, o-vanillin, iso-vanillin and 4-hydroxy coumarin which reacted with bis-dihalides of polyglycols in the presence ofDMSO/alkali carbonate. The novel podands, Ar-(CH₂CH₂O)_m-Ar,(m = 1–4), were identified with IR, ¹H-NMR, ¹³C-NMR and mass spectrometry. The various (formyl-methoxy)phenyl and 4-oxycoumarin derivatives of glycols were studied to estimate the cation binding selectivity of SCN^- salts ofLi⁺, Na⁺, K⁺ and Zn²⁺ cations in acetonitrile using steady statefluorescence spectroscopy. The relevant structures of podands have shown goodselectivity depending on the cation and the glycollength, although the chromophoreend groups have no specific contribution on binding.     

Réduction du cryptate de thallium(I) par polarographie impulsionnelle normale: Constante de réduction électrochimique (hétérogène) et constante de vitesse (homogène) de décomplexation

In propylene carbonate as solvent (+(n-hex)₄NClO₄ 0.1 M) the cryptate [222 Tl]⁺ and the cation Tl⁺ are electroducible. The reduction step is monoelectronic and slowed down for the cryptate compared to the uncomplexed cation. For the cryptate [222 Tl]⁺, by correlation of n.m.r. results on homogeneous ion exchange and combination with the electrochemical heterogeneous data, the standard redox potential of the couple [222 Tl]⁺/[222 Tl]⁰ is estimated to be ?0.73±0.02 V/SCE. This value is in agreement with the electrochemical results. The stability constant of [222 Tl]⁺ is calculated by two independent procedures, from electrochemical results and from n.m.r. data. The results are concordant: log K_s=9.0±0.3 at 25°C, in propylene carbonate (+(n-hex)₄NClO₄ 0.1 M).     

Molecular dynamics simulations on the protonated 222. H+ and 222.2H+ cryptands in water:Endo versusexo conformations

MD simulations on the 222 cryptand, monoprotonated 222.H⁺ and diprotonated 222.2H⁺ in theendo-endo andexo-exo conformations have been performedin vacuo and in a bath of water molecules. It is found that intrinsicallyendo protonation is favoured overexo protonation due to internal N-H⁺ ... O hydrogen bonding which makes the cage more rigid. On the other hand,endo protonated forms display hydrophobic hydration compared toexo forms. For the monoprotonated 222. H⁺ endo conformer, one water molecule is hydrogen bonded inside the cage thereby forming a water cryptate. From the hydration pattern found previously for the neutral 222 cryptand and for its cation complexes, we suggest mechanisms (not involvingexo toendo conversions) for the protonation of 222 and for acid catalysed decomplexation of cryptates in theendo-endo form.     

FORMATION AND DISSOCIATION KINETICS OF THE STRONTIUM(II) COMPLEX OF CRYPT AND C222 IN BINARY DIMETHYLSULFOXIDE-ACETONITRILE MIXTURES

Abstract

The dissociation and the formation rates of the Sr ²⁺ - C222 cryptate in different dimethylsulf-oxide-acetonitrile mixtures have been studied spectrophotometrically using murexide as a metallochromic indicator. In all solvent mixtures used, the ratio of the rate constants for dissociation and formation of the cryptate was found to be in agreement with the formation constants obtained by an independent equilibrium spectrophotometric technique. The activation parameters E _a, δH ^? and δS ^? for the formation and dissociation of the cryptate were determined from the temperature dependence of the corresponding rate constants, and found to be strongly solvent dependent. There is a linear relationship between the logarithm of the formation constant as well as the activation parameters and the mol fraction of acetonitrile in the mixed solvent.     

Free energies and entropies of transfer from water to methanol of cations complexed by 18-crown-6

Free energies and entropies of transfer from water to methanol have been obtained for [M⁺18C6] complexes, where M⁺ = Na⁺, K⁺, Rb⁺, Cs⁺, and Ag⁺. The variation of ΔG_t° and ΔS_t° with the central metal cation is smaller than with the [M⁺222] complexes and it is concluded that 18-crown-6 shields the metal cation from the solvent more effectively than crystal structure determinations would suggest.     

Solvation of Na+ in N,N-dimethylformamide + methanol solutions. A nuclear magnetic relaxation study using dynamic solvent isotope effects

Inter- and intramolecular nuclear magnetic quadrupole relaxation measurements have been used to study the system methanol (CH₃OH)+ N,N-dimethylformamide (DMF)+NaI at 25°C. The dynamic behavior of the solvent molecules was investigated, throughout the composition range of the binary mixtures, by means of ¹⁴ N relaxation of DMF and ² H of methanol-d ₁ (CH ₃ OD). The intermolecular relaxation of ²³ Na⁺ in pure DMF was used to obtain information about the symmetry of the solvent electric dipole arrangement in the solvation sphere of the ion. The investigation of preferential solvation around Na⁺ in the binary mixtures was carried out by means of ²³ Na⁺ relaxation measurements using, for the first time, both the CH ₃ OH/CD ₃ OD and the DMF/DMF-d ₇ dynamic isotope effect. The results show that, throughout the composition range, there is preferential solvation by DMF. Furthermore, the use of the isotope effects of both components allowed for the first time a basic check of the reliability of the method since we obtained two independent sets of data for the composition of the Na⁺ solvation shell in the mixtures. The consistency of the two separate data sets demonstrates that the application of the dynamic isotope effect represents a powerful tool in preferential solvation studies.     

Thermodynamics of cesium cation complexation with dibenzo-24-crown-8 in mixed nonaqueous solvents

Thermodynamics of complexation of cesium cation by dibenzo-24-crown-8 was studied in three binary solvent mixtures: acetonitrile-nitromethane (AN/NM),N,N-dimethylformamide-nitromethane (DMF/NM) and acetonitrile-propylene carbonate (AN/PC) using the¹³³Cs-NMR technique. In all cases the variation of the formation constant,K _f, with the solvent composition was monotonic:K _f increased as the mole-% of the solvent of low donicity was increased. The temperature dependence ofK _f indicated that the complexes are generally enthalpy stabilized, but entropy destabilized. The enthalpy and entropy of complexation reactions are quite sensitive to the solvent composition. However, their variation with solvent composition was not monotonic but showed maxima or minima at the isosolvation points of the cation or the complexed cation. In all cases an enthalpy-entropy compensating effect was observed.     

Conductance Studies on Complex Formation Between Aza-18-Crown-6 with Ag+, Hg2+ and Pb2+ Cations in DMSO–H2O Binary Solutions

The complexation reactions between Ag⁺, Hg²⁺ and Pb²⁺ metal cations with aza-18-crown-6 (A18C6) were studied in dimethylsulfoxide (DMSO)–water (H₂O) binary mixtures at different temperatures using the conductometric method. The conductance data show that the stoichiometry of the complexes in most cases is 1:1(ML), but in some cases 1:2 (ML₂) complexes are formed in solutions. A non-linear behaviour was observed for the variation of log K _f of the complexes vs. the composition of the binary mixed solvents. Selectivity of A18C6 for Ag⁺, Hg²⁺ and Pb²⁺ cations is sensitive to the solvent composition and in some cases and in certain compositions of the mixed solvent systems, the selectivity order is changed. The values of thermodynamic parameters (ΔH _c^o, ΔS _c^o) for formation of A18C6–Ag⁺, A18C6–Hg²⁺ and A18C6–Pb²⁺ complexes in DMSO–H₂O binary systems were obtained from temperature dependence of stability constants and the results show that the thermodynamics of complexation reactions is affected by the nature and composition of the mixed solvents.     

Diatomic interactions in momentum space. The 1sσg and 2pσ u states of H 2 + system

The method of momentum electron density for interatomic interactions has been applied to the two lowest σ states of the H ₂ ⁺ system. For attractive (1sσ_g) and repulsive (2pσ _u ) interactions, the behaviour of momentum density and its effect on the stabilization energy of the system are examined quantitatively. The concept of contraction and expansion of the momentum density is shown to form an important guiding principle in this approach. The origin of covalent bonding is discussed based on the energy partitioning proposed previously.     

A Proton NMR Study of the Stoichiometry and Stability of 18-Crown-6 Complexes with K+, Rb+ and Tl+ Ions in Binary Dimethyl Sulfoxide-Nitrobenzene Mixtures

Proton NMR spectroscopy was used to study the complexation reaction of 18-crown-6 (18C6) with K⁺, Rb⁺ and Tl⁺ ions in a number of binary dimethyl sulfoxide-nitrobenzene mixtures. In all cases, the exchange between free and complexed crowns was fast on the NMR time scale and only a single population average ¹H signal was observed. Formation constants of the resulting 1:1 complexes in different solvent mixtures were determined by computer fitting of the chemical shift-mole ratio data. There is an inverse relationship between the complex stability and the amount of dimethyl sulfoxide in the mixed solvent. It was found that, in all solvent mixtures used, Rb⁺ ion forms the most stable complex with 18-crown-6 in the series.     

Raman spectra and structural features of ClOF2 + cation in a solution of anhydrous HF

The Raman spectra of ClOF₂ ⁺ cation in solutions of anhydrous HF were studied. In the ClOF₂ ⁺HF₂ ⁻ and ClOF₂ ⁺BF₄ ⁻−HF systems, this cation exists as a pyramidal structure (C _s symmetry), while in the ClOF₂ ⁺AuF₆ ⁻−HF system, it exists as a planar structure (C _2v symmetry). Based on nonempirical calculations by the Hartree-Fock-Roothaan method, an explanation for the dependence of the structure of the ClOF₂ ⁺ cation on the nature of the anion was proposed. For the Cl−O bond vibrations, the correlation functions of vibrational and rotational relaxations were calculated, and the characteristic times of these processes were determined. The main contribution to the formation of the band contours corresponding to the above-mentioned modes is made by the vibrational dephasing. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 432–437, March, 1998.     

Conductance Study of the Thermodynamics of Complexation of K+, Rb+, Cs+ and Tl+ Ions with Dibenzo-24-crown-8 in Binary Acetonitrile–Nitromethane Mixtures

The complexation reactions between dibenzo-24-crown-8 (DB24C8) and K⁺, Rb⁺, Cs⁺ and Tl⁺ ions were studied conductometrically in different acetonitrile–nitromethane mixtures at various temperatures. The formation constants of the resulting 1:1 complexes were calculated from the computer fitting of the molar conductance–mole ratio data at different temperatures. At 25 °C and in all solvent mixtures used, the stability of the resulting complexes varied in the order Tl⁺ > K⁺ > Rb⁺ > Cs⁺. The enthalpy and entropy changes of the complexation reactions were evaluated from the temperature dependence of formation constants. It was found that the stability of the resulting complexes increased with increasing nitromethane in the solvent mixture. The TΔS° vs. ΔH° plot of all thermodynamic data obtained shows a fairly good linear correlation indicating the existence of enthalpy–entropy compensation in the complexation reactions.     

Complex Formation of 1,10-Dibenzyl-1,10-diaza-18-crown-6 with Ni2+, Cu2+, Ag+ and Cd2+ Metal Cations in Acetonitrile–dimethylformamide Binary Solutions

Conductance measurements are reported for nickel(II), cupper(II), silver(I) and cadmium(II), salts in acetonitrile (AN)–dimethylformamide (DMF) binary solvents containing macrocyclic ligand, 1,10-dibenzyl-1,10-diaza-18-crown-6 (DBDA18C6) at different temperatures. The changes in molar conductance caused by addition of DBDA18C6 to solutions were analyzed by non-linear least squares to give stability constants of 1:1 metal cation–DBDA18C6 complexes. The results show that the stabilities of the complexes are sensitive to solvent composition and in some cases the sequence of stabilities is changed with changing the composition of the mixed solvents. The values of thermodynamic quantities (ΔH°c and ΔS°c) for formation of DBDA18C6-Ni²⁺, DBDA18C6-Cu²⁺, DBDA18C6-Ag⁺ and DBDA18C6-Cd²⁺ complexes were obtained from temperature dependence of the stability constants and the results show that the values of ΔH°c and ΔS°c for these complexes are sensitive to the nature and composition of AN–DMF binary solutions, but they do not vary monotonically with the solvent composition.     

Structures of 18-Crown-6 Complexes with Alkali Cations in Methanolic Solution as Studied by X-ray Absorption Fine Structure

The structures of 18-crown-6 (18C6) complexes with K⁺ and Rb⁺ in methanolic solutions have been studied by X-ray absorption fine structure (XAFS) at the Br K-edge as well as at the K and Rb K-edges. The XAFS spectrum at the K or Rb K-edge has indicated that either Br⁻ or solvent (methanol) molecules are present in the first coordination shell of K⁺ or Rb⁺ complexed by 18C6. However, the spectra obtained at the Br-K edge have strongly suggested that the alkali cations do not exist in the vicinity of Br⁻, indicating that no direct ion-pairing occurs between the 18C6 complex and Br⁻. The 18C6-K⁺ complex maintains D _3d symmetry even in methanol, and two methanol molecules coordinate the cation possibly from above and below the crown plane. In contrast, the corresponding Rb⁺ complex possibly forms an umbrella-shaped complex, in which Rb⁺ is situated slightly off the crown plane and three solvent molecules bind With the cation.     

Cationic Exciplexes: Role of Hydrogen Bonding in Deactivation and Electronic Coupling

Emissive properties for the cationic exciplex (A⁺*/D→A^.D^.+) of an isoquinolinium cation tethered to a substituted arene ( 1⁺ ) are strongly affected by hydrogen bonding solvents. At equal dielectric constant (ϵ), the ground-to-excited state energy gaps (ΔG) and solvent reorganization energies (λ_s) decrease from nitriles to aliphatic alcohols. The corresponding decrease from aliphatic alcohols to high hydrogen bond acidity solvents is ∼3 times larger. The exciplex decay (k_Ex), largely determined by unfolding of the exciplex to a stretched conformer, changes in a complex way depending on the strength of the hydrogen bond ability of these solvents. In contrast, the electronic couplings between the exciplex ground, excited, and charge transfer states do not show a solvent functionality dependence.