Dissociation Quotients for Citric Acid in Aqueous Sodium Chloride Media to 150°C

The three molal dissociation quotients for citric acid were measured potentiometrically with a hydrogen-electrode concentration cell from 5 to 150°C in NaCl solutions at ionic strengths of 0.1, 0.3, 0.6, and 1 molal. The molal dissociation quotients and available literature data at infinite dilution were fitted by empirical equations in the all-anionic form involving an extended Debye-Hückel term and up to five adjustable parameters involving functions of temperature and ionic strength. This treatment yielded the following thermodynamic quantitites for the first dissociation equilibrium at 25°C: logK _1a=−3.127±0.002, ΔH _1a ^o =4.1±0.2 kJ-mol⁻¹, ΔS _1a ^o =−46.3±0.7 J-K⁻¹-mol⁻¹, and ΔCp _1a ^o =−162±7 J-K⁻¹-mol⁻¹; for the second acid dissociation equilibrium at 25°C: logK _2a =−4.759±0.001, ΔH _2a ^o =2.2±0.1, ΔS _2a ^o =−83.8±0.4, and ΔCp _2a ^o =−192±15, and for the third dissociation equilibrium at 25°C: logK _3a=−6.397±0.002, ΔH _3a ^o =−3.6±0.2, ΔS _3a ^o =−134.5±0.7, and ΔCp _3a ^o =−231±7.     

Spin exchange interaction through phenylene-ethynylene bridge in diradicals based on iminonitroxide and nitronylnitroxide radical derivatives. 1. Experimental investigation of the through-bond spin exchange coupling

A series of bis-iminonitroxide diradical derivatives of different lengths and geometry have been prepared that incorporate a conjugated phenylene-ethynylene bridge as a rigid spacer. This paper describes the synthesis of these new components and their main characterizations. An unexpected singlet ground state and substituent effects on the singlet-triplet gap have been found for substituted "m-phenylene"-based diradicals. The effects of the pi-conjugation on the intramolecular through-bond spin coupling have been investigated by changing the length of the spacer within linear derivatives. The EPR studies demonstrate the intramolecular magnetic coupling between the radical spins within all compounds. This result is very attractive and unusual, given the large distance between the radicals from 15 A in the dimer to 36 A in the pentamer.     

Synthèse d'analogues furannique et nitro-2 furannique du benzo[e]pyrène

Pyreno[4,5-b]furan, a heterocyclic analogue of benzo[e]pyrene, and its 10-nitro derivative were prepared in six steps staring from 5-formyl-4-hydroxypyrene. These isomers of the previously described pyreno[1,2-b]-furan, pyreno[2,1-b]furan, 10-nitopyreno[1,2-b]furan and 10-nitropyreno-[2,1-b]furan are of particular interest due to their mutagenic activities.     

Action des halogenures d'alkylmagnesium sur les pyrones-2—VI : Etude de la regioselectivite de l'ion enolate intermediaire de la reaction

Grignard reagents normally react with 2H-pyrones to yield unsaturated ketols, dihydropyranols, dienones and 2H-pyrans. In some cases, products are obtained by attack on the most sterically crowded position of the enolate ion. Since steric hindrance is an unsatisfactory basis for predicting the reactivity, we have applied a theoretical study to this problem. The model used is based on an enlarged electronic delocalization in the transition state, and allows the interpretation of the regioselectivity of the reaction.     

Structural isomerism in crystals of redox-active secondary ortho-diamides: the role of competing intra- and intermolecular hydrogen bonds in directing crystalline topologies

The solid-state chemistry of a series of seven ortho-bis(alkylamido)ethylenedithiotetrathiafulvalene derivatives EDT-TTF-(CONHR)2 (R=Me, 1; Et, 2; Pr, 3; Bu, 4; Pent, 5; Hex, 6; and Bz, 7), in their neutral and one-electron-oxidized, radical cation states, was investigated with an eye on the topology of intra- and intermolecular hydrogen-bond motifs. In the case of neutral, monomolecular solids, an intramolecular N--H.O hydrogen bond seals a constrained seven-membered ring for 1, 2, 3, and 7, which is disrupted in butyl derivative 4 in favor of an antiparallel ladder at the expense of any intramolecular hydrogen bond. In the solid-state, the competition between intra- and intermolecular hydrogen bonding observed in solution depends on the packing of the molecules. Electrocrystallization of methyl derivative 1 with ReO4- or ClO4-, two anions of different volumes but otherwise identical charge and symmetry, revealed a fine sensitivity of the constrained seven-membered ring to the internal chemical pressure exerted by the anion. [1]2 *+ReO4 (sigmaRT=8.5 S cm(-1), activation energy Delta=600 K at high temperature) and beta"-[1]2 *+ClO4 (sigmaRT=0.03 S cm(-1), Delta=1600 K; under pressure at room temperature, the conductivity increases by three orders of magnitude up to 17 kbar with a linear variation of the activation energy with pressure, Delta=aP with a=0.202 10(6) K kbar(-1)) have vastly different architectures, dimensionalities, electronic structures, and collective properties, a consequence of the presence of the closed or open structural isomers in one or the other. This exemplifies the flexibility of functionalized TTF derivatives, even when the functional group is directly attached to the electro-active core. This allows an analogy to be drawn with tetrafunctionalized metallocenes, in which such flexibility has already been observed. The experimental data are supported by theoretical calculations of the energy profile of a model molecule on rotation of the amido groups.     

Etude de la reactivite des coumarines—II : Regioselectivite des reactifs de grignard engages avec la methyl-4, la methyl-3 et la phenyl-3 coumarine

Using the nucleophilicity parameters of Grignard reagents calculation of the electronic delocalization energies permits prediction of the variation of regioselectivity of substituted coumarins towards Grignard reagents.     

Singular crystalline beta'-layered topologies directed by ribbons of self-complementary amide...amide ring motifs in [EDT-TTF-(CONH(2))(2)](2)X (X = HSO(4)(-), ClO(4)(-), ReO(4)(-), AsF(6)(-)): coupled activation of ribbon curvature, electron interactions, and magnetic susceptibility

The deliberate design of a series of single crystals of conducting two-dimensional radical cation salts of o-bis(amide)-appended ethylenedithiotetrathiafulvalene, beta'-[EDT-TTF-(CONH(2))(2)](2)X (X = HSO(4)(-), ClO(4)(-), ReO(4)(-), or AsF(6)(-)) and of their parent monocomponent solid EDT-TTF-(CONH(2))(2) is demonstrated and allows us to reach a level of prediction of the structure of molecular conductors. Their conductivity is activated with a gap of 1650 K and a sizable room-temperature conductivity of 0.15 S.cm(-)(1) (for X = ClO(4)(-)) and a singular spin susceptibility for a beta'-type salt that, in addition, changes very remarkably with the anion. The key design element is that of a recurrent, puckered ribbon constructed out of self-complementary, hydrogen-bonded amide...amide ring motifs whose minute modulations of curvature and shape throughout the series have been shown to correlate to very remarkable differences in the intrastack beta(HOMO)(-)(HOMO) interaction energies and changes in the density of states at the Fermi level and on to important differences of spin susceptibility behavior in a system where electron correlations are significant. The coupled activation of structure, electron interactions, and magnetic susceptibility discovered and discussed throughout the paper is unprecedented and is seen as a genuine expression of interfacial hydrogen-bond interactions onto the collective electronic properties.     

An attempt at interpreting competitive Friedel-Crafts reactions : Role of solvation energy

Results of Olah et al. on benzylation of benzene and toluene are studied theoretically. The evolution of intermolecular k_T/k_B and intramolecular o/2p selectivities is interpreted with the aid of a model involving electron affinities of electrophilic entities. Our electrophilic affinities are calculated taking solvation into account, which reverses the order of affinities computed on the naked cations.