On the Role of Symmetry in Vibrational Strong Coupling: The Case of Charge‐Transfer Complexation

It is well known that symmetry plays a key role in chemical reactivity. Here we explore its role in vibrational strong coupling (VSC) for a charge‐transfer (CT) complexation reaction. By studying the trimethylated‐benzene–I₂ CT complex, we find that VSC induces large changes in the equilibrium constant K_DA of the CT complex, reflecting modifications in the ΔG° value of the reaction. Furthermore, by tuning the microfluidic cavity modes to the different IR vibrations of the trimethylated benzene, ΔG° either increases or decreases depending only on the symmetry of the normal mode that is coupled. This result reveals the critical role of symmetry in VSC and, in turn, provides an explanation for why the magnitude of chemical changes induced by VSC are much greater than the Rabi splitting, that is, the energy perturbation caused by VSC. These findings further confirm that VSC is powerful and versatile tool for the molecular sciences.     

Kinetics and mechanism of diels-alder additions of tetracyanoethylene to anthracene derivatives—I : Substituent effects

The rates of addition of tetracyanoethylene to a number of 9- and 9,10-substituted anthracenes have been measured spectrophotometrically in solvent CCl₄. Substituent effects correlated well using the extended form of the Hammett equation. The importance of steric effects on the reaction was assessed by a systematic variation of the components of the data used in the above correlation.Activation parameters (ΔG_exp^≠, etc.) and the corresponding overall thermodynamic parameters for adduct formation (ΔG_ad°, etc.) were evaluated. ΔG_exp^≠ was found to be linearly related to ΔG_c°, the free energy of formation of the intermediate complex which confirms the role of the latter as a true reaction intermediate. From correlations between ΔG_exp^≠ and ΔG_ad°, an “early” transition state is suggested. The above thermodynamic and activation data enable detailed reaction profiles to be drawn.     

Activation of Strong Boron–Fluorine and Silicon–Fluorine σ‐Bonds: Theoretical Understanding and Prediction

The oxidative addition of BF₃ to a platinum(0) bis(phosphine) complex [Pt(PMe₃)₂] ( 1 ) was investigated by density functional calculations. Both the cis and trans pathways for the oxidative addition of BF₃ to 1 are endergonic (ΔG°=26.8 and 35.7 kcal mol^?1, respectively) and require large Gibbs activation energies (ΔG°^≠=56.3 and 38.9 kcal mol^?1, respectively). A second borane plays crucial roles in accelerating the activation; the trans oxidative addition of BF₃ to 1 in the presence of a second BF₃ molecule occurs with ΔG°^≠ and ΔG° values of 10.1 and ?4.7 kcal mol^?1, respectively. ΔG°^≠ becomes very small and ΔG° becomes negative. A charge transfer (CT), F→BF₃, occurs from the dissociating fluoride to the second non‐coordinated BF₃. This CT interaction stabilizes both the transition state and the product. The B?F σ‐bond cleavage of BF₂Ar^F (Ar^F=3,5‐bis(trifluoromethyl)phenyl) and the B?Cl σ‐bond cleavage of BCl₃ by 1 are accelerated by the participation of the second borane. The calculations predict that trans oxidative addition of SiF₄ to 1 easily occurs in the presence of a second SiF₄ molecule via the formation of a hypervalent Si species.     

Solvation and temperature effect on the charge-transfer complex between 2-amino-4-picoline with 2,5-dihydroxy-p-benzoquinone

The charge-transfer (CT) complex between the donor 2-amino-4-picoline (2A4P) and the acceptor 2,5-dihydroxy-p-benzoquinone (DHBQ) was studied spectrophotometrically in different polar and non-polar solvents. The molecular composition of the complex, in all solvents, was determined by Job's method of continuous variation and photometric titrations to be 1:1. Benesi–Hildebrand equation has been applied to estimate the formation constant (K _CT) and molecular extinction coefficient (ε) of the formed complex. The variation in K _CT was rationalised based on Taft–Kamlet and electric permittivity parameters of the used solvents. Thermodynamic parameters ΔH°, ΔG° and ΔS° were estimated, they were all negative so the studied complex is reasonably stable and exothermic in nature. In addition, the thermodynamic properties were observed to be sensitive to the nature of the solvent. Moreover, the solid 1:1 CT complex between 2A4P and DHBQ was isolated and characterised using elemental analysis, FTIR and ¹H NMR measurements.     

Kinetic and thermodynamic properties of the aerial oxidation of hydroquinone in developer solutions

The aerial oxidation kinetics of hydroquinone in a freshly prepared developer solution at different temperatures and pHs has been studied. The activation parameters, Ea, ΔG# , ΔS# , ΔH# and enthalpy of formation of activated complex, ΔHfo(X# ), are determined. The large negative value of free energy of activation ΔG# proves that hydroquinone extremely tends to be oxidized by air at optimum temperature (20℃) and optimum pH (10.5) and converts to the activated complex semiquinone. It was also found that if the pH of the developer solution is increased from 9.3 to 10.5 the reaction rate will increase by a factor of 2.     

Comparison between ab Initio and Modified INDO-MO Calculations of N20

Both STO-3G ab initio and s-p separation-type-modified INDO semiempirical methods were applied to molecular-orbital calculation of the N₂₀ molecule. From these two methods, the optimized bond distances between the nearest N atoms (d_n-n) and the most calculated thermodynamic data are close to each other. The positive values of ΔH_a° and ΔG_a° for the atomization reaction in this work prove that N₂₀ is stable. In contrast to conventional INDO and MINDO/3, but similar to former AMI and MNDO calculations, both ΔH_r° and ΔG_r° are positive in the formation reaction, which indicates that N₂₀ belongs to the category of high-energy molecules.     

Standard states and standard adsorption gibbs energy of substances at electrode interfaces

The physical meaning of ΔG°_ad is discussed in terms of choice of standard states, form of the isotherm equation, true adsorption equilibrium reaction and contributions from interactions in the solution bulk. It is shown that the quantitative value of ΔG°_ad may be misleading if the above factors are not taken into account properly. The particular case of thiourea adsorption on Hg is discussed in detail. It is recommended that surface and bulk concentrations are expressed as mole fractions whenever possible so as to minimize problems of ΔG°_ad interconversion among different isotherms.     

Thermodynamic studies of hydrogen iodide in dioxane-water mixtures from electromotive force measurements

The standard potentials of the silver-silver iodide electrode were measured in 10,20,30 and 40% (w/w) dioxane-water mixtures at 15,25,35 and 45°C. These values have been used to determine the thermodynamic quantities ΔG_t°, ΔS_t°, ΔH_t° for the transfer of H⁺I^? from water to various dioxane-water mixtures. The ionic ΔG_t° values for H⁺, Cl^?, Br^? and I^? are determined using Feakins method. The chemical and electrical contributions of ΔG_t° are also calculated using the method proposed by Roy and co-workers. The significance of these thermodynamic functions is discussed in relation to the acid—base character of the solvents.     

A Thermodynamic and Kinetic Insight into the Pathways Leading to a Highly Functionalized Ketenimine: A Computational Study

All steps of two mechanistic pathways for the synthesized ketenimine through a multicomponent reaction between cyclohexyl isocyanide 1 and acetylen ester 2 in the presence of CH‐acid 3 have been thermodynamically and kinetically evaluated. Intrinsic reaction coordinate calculations were performed for the optimized structures to verify the connectivity of all transition states with reactants and products. The kinetic data showed that the step 1 of the two proposed mechanisms was a rate‐determining step. Also, activation (ΔG^?, ΔH^?, ΔS^?) and thermodynamic (ΔG°, ΔH°, ΔS°) parameters confirmed that the second mechanism for generation product 4b was favored energetically. In addition, the single‐point ¹H and ¹³C NMR (GIAO) chemical shift calculations showed that the product obtained from the approval pathway was according to the experimental data.     

A Thermodynamic Study of Bromo-Lanthanum (III) Complexes from EMF Measurements

The stability constants (for the formation) of LaBr²⁺ and LaBr₂⁺ ions were obtained potentiometrically at various ionic strengths at 20°, 25°, 30°, and 35°. The molal free energy was given by ΔG°=-2,444.04+2.67T log T+1.39T. The thermodynamic quantities for the formation of LaBr²⁺ were evaluated as ΔH_f° = ?207.9, ΔG_f° = ?199.1 kcal mole^?1 and S° = ?29.5 cal. deg^?1 mole^?1 at 25°.     

Calorimetric investigation of the complex formation reaction of 18-crown-6 ether with d,l-alanine in water–ethanol mixtures

The standard thermodynamic parameters (Δ_r G°, Δ_r H°, and TΔ_r S°) of the reaction of molecular complex formation of 18-crown-6 ether (18C6) with d,l-alanine (Ala), [Ala18C6], have been obtained from calorimetric titration experiments carried out using the microcalorimetric system TAM III (TA Instruments, USA) at T = 298.15 K in water–ethanol (H₂O–EtOH) solvents at X _EtOH = 0 ÷ 0.6 mol fractions. Results show that the increase of the EtOH concentration in solvent brings about an increase of the [Ala18C6] complex stability and of the exothermicity of the reaction of complex formation. The solvation contributions of 18C6, Ala, [Ala18C6] to Δ_r G° and Δ_r H° at various X _EtOH values are also analyzed.     

Kinetics and mechanism of decomposition of intermediate complex during oxidation of pectate polysaccharide by potassium permanganate in alkaline solutions

The kinetics of decomposition of an [Pect·Mn^VIO₄^2?] intermediate complex have been investigated spectrophotometrically at various temperatures of 15–30°C and a constant ionic strength of 0.1 mol dm^?3. The decomposition reaction was found to be first‐order in the intermediate concentration. The results showed that the rate of reaction was base‐catalyzed. The kinetic parameters have been evaluated and found to be ΔS^≠ = ? 190.06 ± 9.84 J mol^?1 K^?1, ΔH^≠ = 19.75 ± 0.57 kJ mol^?1, and ΔG^≠ = 76.39 ± 3.50 kJ mol^?1, respectively. A reaction mechanism consistent with the results is discussed. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 67–72, 2003     

Thermodynamic characteristics of alanine-18-crown-6 ether complexes in binary water-acetone solvents

The formation of 18-crown-6 ether (18C6) complexes with D,L-alanine (Ala) in mixed wateracetone solvents with 0.0, 0.08, 0.17, 0.22, and 0.30 mole fractions of acetone (T = 298.15 K) was investigated by means of calorimetry. Thermodynamic characteristics of the reaction of the molecular [Ala18C6] complex formation (Δ_r G°, Δ_r H°, and TΔ_r S°) were calculated on the basis of calorimetric data. Analysis of solvation contributions of reagents into the enthalpy of the [Ala18C6] formation reaction showed that the changes in the reaction energy when the solvent composition is varied are determined by the changes in the solvate state of 18C6.     

Thermodynamic studies of inclusion complex between cetyltrimethylammonium bromide (CTAB) and β -cyclodextrin ( β -CD) in water/n-butanol mixture,using potentiometric technique

The behaviour of the inclusion complex consisting of cetyltrimethylammonium bromide (CTAB) and β-cyclodextrin (β-CD) in water/n-butanol mixture was studied using ion selective electrodes sensitive to surfactant ions. The experiments were carried out at different temperatures and different composition of water/alcohol. The data obtained indicate that the inclusion complexes S(CD) and S(CD)₂ had formed between CTAB and β-CD in water/alcohol mixture environment. In addition to the 1 : 1 complex, CTAB formed 1 : 2 complexes with β-CD. Further investigation showed that K ₁ for S(CD) was greater than K ₂ for S(CD)₂, and the values of K_i were reduced with increasing butanol concentration. Finally, thermodynamic parameters of the complexation, i.e. ΔH°, ΔG° and ΔS° were also calculated. The obtained thermodynamic data showed that the hydrophobic interaction is the main factor for inclusion complex formation and tendency of complex formation has been reduced with increasing of medium hydrophobicity.     

Conformational Analysis. XIX properties and reactions of 1,3-oxathianes VIII A 1H NMR conformational study of methyl-substituted derivatives

The addition of thioacetic acid to unsaturated alcohols or acids was utilized to obtain mercaptoalkanols which were condensed with suitable carybonyl compounds to prepare 24 methyl-substituted 1,3-oxathianes. The ¹H NMR spectra of the 1,3-oxathiane products were recorded at 60, 100 and/or 300 MHz and fully analysed. The results are best explained by a chair form which is completely staggered in the C-4? C-5? C-6 moiety ψ₄₅ or (ψ₅₆=60±1°). 1,3-Oxathianes having syn-axial 2,4- (and/or 2,6-) methyl-methyl interactions exist appreciably, if not exclusively, in twist forms. The vicinal coupling constants lead to the conformational free energies of axial methyl groups at C-4, ΔG°=7.4±0.4 kJ mol^?1, and at C-5, ΔG°=3.7±0.3 kJ mol^?1, in good agreement with previous estimates. They also show that both r-4,cis-5,trans-6- and r-4,trans-5,trans-6- trimethyl-1,3-oxathianes greatly favour the chiar form where the methyl group at C-4 is axial. The chair-twist energy parameters are reestimated at ΔH°_CT 27.0 kJ mol^?1, ΔS°_CT 11.6J mol^?1K^?1, and ΔG°_CT(298) 23.5 kJ mol^?1 for a 2,5-twist form.     

Solvent extraction of copper(II) from sulfate medium with N -(2-hydroxybenzylidene)aniline

Solvent extraction of copper(II) from sulfate medium with N-(2-hydroxybenzylidene)aniline is studied with the following parameters: pH, concentration of the extractant, nature of diluent, and temperature. The extraction of copper(II) proceeds by a cation exchange mechanism and the extracted species are CuL₂ in cyclohexane and toluene and CuL₂ with some CuL₂HL in chloroform. The equilibrium constants have been calculated as well as thermodynamic parameters ΔH°, ΔS°, and ΔG°. The temperature effect on the solvent extraction of copper(II) with N-(2-hydroxybenzylidene)aniline in cyclohexane is discussed.     

Thermochemistry of the Acid-Base Interactions in Aqueous Solutions of Isonicotinic and Picolinic Acids

The calorimetric method was employed to measure the heat effects of the interaction of isonicotinic and picolinic acids with HNO₃in aqueous solutions over different pH ranges at 298.15 K and ionic strengths of 0.5, 1.0, and 1.5 against potassium nitrate. The heat effects of stepwise dissociation of the acids were determined. The standard thermodynamic characteristics (Δ_rH°, Δ_rG°, Δ_rS°, ΔC_p°) of the acid-base reactions in aqueous solutions of isonicotinic and picolinic acids were calculated.

     

Spectrophotometric studies of the formation of charge transfer complex of iron(III) with N,N′-bis(2-pyridylmethylidene)-1,2-diiminoethane di-Schiff base ligand in methanol

The complex formation reaction between N,N′-bis(2-pyridylmethylidene)-1,2-diiminoethane (BPIE) di-Schiff base ligand as an electron donor and iron(III) chloride as an electron acceptor have been studied spectrophometrically in methanol at 28°C. The values of equilibrium constants, K and molar absorptivities, ε were obtained from the Benesi–Hildebrand, Scott and Foster–Hammick–Wardley equations. The results indicate the formation of 1?:?1 charge transfer complex. The absorption band energy of the complex, E _CT, the ionization potential of the BPIE Schiff base ligand, I ^D, and the Gibbs energy changes of the above reaction, ΔG ⁰, were calculated. Finally, the kinetics of the complex formation reaction were studied and was found to be second-order in each reactant. The values of the rate constants of the forward and reverse reactions k ₁ and k _?1 were determined.     

Homolytic substitution in trialkyltin iodides by photochemically generated iodine atoms. I. Kinetics

The kinetics of the homolytic substitution of several trialkyltin iodides by iodine atoms are presented. Rate constants have been determined at three different temperatures and the following activation parameters calculated: A, E_a, and ΔS°^≠. The observation that the activation energy, ΔG^≠, is related to the driving force of the ion-pair formation, leads to the conclusion that the charge-transfer model is a valid approach for substitution in the reaction between R₃SnI compounds and iodine atoms.     

Spectroscopic studies and determination of the formation constants and thermodynamic parameters for a new water-soluble Co(II) Schiff-base complex and some imidazole derivatives

A new water-soluble Co(II) Schiff-base complex, sodium[{N,N′-bis(5-sulfosalicylidene)-1,8-diamino-3,6-dioxaoctan}cobalt] dihydrate, abbreviated as Na₂[Co(II)L], was synthesized and characterized. The formation constants and thermodynamic parameters for the interaction of this complex with imidazole (Im) and 1-methylimidazole (MeIm) were determined spectroscopically in aqueous solution, ethanol/water (10/90), and methanol/water (10/90) under physiological conditions (pH?=?7), constant ionic strength (I?=?0.1?mol?dm^?3 KNO₃), and various temperatures ranging from 294 to 310?K. Our spectroscopic and thermodynamic results show that this adduct formation is endothermic and the positive values of ΔS _f° make ΔG _f° negative. The trend in variation of ΔH _f° and ΔS _f° for Im is in the order water?>?methanol?>?ethanol, but for MeIm it is in the opposite order which is related to the hydrogen bonding between solvents and these donors. Formation constants between MeIm and Na₂[Co(II)L] in these three solvents are larger than for Im which depends on the electron donation of methyl on MeIm.