Thermochemistry of organosilicon compounds VII. Permethylcyclosilazanes and 1,1,3,3-tetramethyldisilazane

The enthalpy of formation (ΔH_f⁰), enthalpy of evaporation (ΔH_v⁰) and enthalpy of atomization (ΔH_a) of permethylcyclosilazanes (Me₂SiNH)_n (n = 3, 4) and 1,1,3,3-tetramethyldisilazane (Me₂SiH)₂NH have been determined. The enthalpies of formation of these compounds were compared with those calculated by the Benson-Buss-Franklin and Tatevskii additive schemes. In higher permethylcyclosilazanes the energy of the endocyclic Si---N bond is 306 ± 2 kJ mol⁻¹ (73 kcal mol⁻¹), that is 12 ± 2 kJ mol⁻¹ (3 kcal mol⁻¹) lower than the energy of the acyclic Si---N bond. The strain energy of the cyclotrisilazane ring is estimated to be 10.5 kJ mol⁻¹ (2.5 kcal mol⁻¹), whereas the energy of the ring Si---N bond is 295 kJ mol⁻¹ (70.5 kcal mol⁻¹).

The thermochemical data for permethylcyclosilazanes were compared with the corresponding values for permethylcyclosiloxanes calculated from the results of previously reported studies.     

The enthalpies of formation of two dibenzocyclooctadienones

The standard molar enthalpies of formation (Δ_fH_m⁰(s)/kJmol⁻¹) for 2,3:6,7-dibenzocycloocta-2,6-dien-1-one and 2,3:7,8-dibenzocycloocta-2,7-dien-1-one [6H-11,12-dihydro-dibenzo[a,e]cycloocten-5-one (ketone 1) and 10H-11,12-dihydrodibenzo[a,d]-cycloocten-5-one (ketone 2), respectively] were derived from enthalpies of combustion, measured by means of a microbomb calorimeter. The fusion and vaporization enthalpies of these compounds were obtained from DSC and correlation gas chromatography measurements. The standard molar enthalpies of formation in the gas phase were calculated by combining the condensed phase standard molar enthalpies of formation with the fusion and vaporization enthalpies adjusted to 298.15 K. Values for Δ_fH_m⁰(g) of (−39.9±5.5) and (−14.8±5.3) kJ mol⁻¹ were obtained for 2,3:6,7-dibenzocycloocta-2,6-dien-1-one and 2,3:7,8-dibenzocycloocta-2,7-dien-1-one, respectively. Quantum chemical calculations are reported for the compounds investigated experimentally and an additional four isomers. Isomerization enthalpies are derived from computed energies. The enthalpies of formation are also calculated by group additivity, compared with the experimental values and then correlated with the structure of the molecules investigated. The X-ray analysis of ketone 1 is also reported.     

Structures and energetics of C3H6S isomers: a Gaussian-3 ab initio study

Twenty-two isomers/conformers of C₃H₆S^+√ radical cations have been identified and their heats of formation (ΔH_f) at 0 and 298 K have been calculated using the Gaussian-3 (G3) method. Seven of these isomers are known and their ΔH_f data are available in the literature for comparison. The least energy isomer is found to be the thioacetone radical cation (4⁺) with C_2v symmetry. In contrast, the least energy C₃H₆O^+√ isomer is the 1-propen-2-ol radical cation. The G3 ΔH_f298 of 4⁺ is calculated to be 859.4 kJ mol⁻¹, ca. 38 kJ mol⁻¹ higher than the literature value, ≤821 kJ mol⁻¹. For allyl mercaptan radical cation (7⁺), the G3 ΔH_f298 is calculated to be 927.8 kJ mol⁻¹, also not in good agreement with the experimental estimate, 956 kJ mol⁻¹. Upon examining the experimental data and carrying out further calculations, it is shown that the G3 ΔH_f298 values for 4⁺ and 7⁺ should be more reliable than the compiled values. For the five remaining cations with available experimental thermal data, the agreement between the experimental and G3 results ranges from fair to excellent.

Cation CH₃CHSCH₂^+√ (10⁺) has the least energy among the eleven distonic radical cations identified. Their ΔH_f298 values range from 918 to 1151 kJ mol⁻¹. Nevertheless, only one of them, CH₂=SCH₂CH₂^+√ (12⁺), has been observed. Its G3 ΔH_f298 value is 980.9 kJ mol⁻¹, in fair agreement with the experimental result, 990 kJ mol⁻¹.

A couple of reactions involving C₃H₆S^+√ isomers CH₂=SCH₂CH₂^+√ (12⁺) and trimethylene sulfide radical cation (13⁺) have also been studied with the G3 method and the results are consistent with experimental findings.     

The gaseous azide ion, N3: Enthalpy of formation, ΔHf(N3(g)) and charge distribution lattice energies of sodium, potassium and rubidium azides

The calculations reported here assign a charge q_N = −0.52 electron units to the terminal nitrogen atoms in the azide ion and a value of 141.9 kJ mole⁻¹ to the enthalpy of formation of the gaseous azide ion, ΔH_f⁰(N₃⁻(g)). The total lattice potential energies are found to be: E_pot(NaN₃) = 725.1 kJ mole⁻¹; E_pot(KN₃) = 650.7 kJ mole⁻¹ and E_pot(RbN₃) = 632.1 kJ mole⁻¹.     

Formation of cobalt, nickel and copper complexes with murexide in ethanol-water mixtures

The complexation reactions between murexide and Co²⁺, Ni²⁺ and Cu²⁺ in C₂H₅OH-H₂O mixtures have been investigated spectrophotometrically. The formation constants of the 1:1 complexes formed increase in the order Co²⁺ < Ni²⁺ < Cu²⁺ for all solvent mixtures studied, and log K_f is a linear function of the mole fraction of ethanol. The heat of complexation was determined calorimetrically for the nickel and copper complexes. The values of ΔH° and ΔS° are solvent-dependent, and all three complexes have negative ΔH° and positive ΔS° values.     

Thermochemistry of aromatic ketones. Experimental enthalpies of formation and structural effects

The standard enthalpies of formation Δ_fH^o (liq. or cr.) at the temperature T = 298.15 K were measured using combustion calorimetry for benzophenone (A), 1-indanone (B), -tetralone (C), 9-fluorenone (D), anthrone (E) and dibenzosuberone (F). The standard enthalpies of vaporization Δ_vH^o or sublimation Δ_sH^o of A-F and 5,7-dihydro-6H-dibenzo[a,c]cyclohepten-6-one (G) were obtained from the temperature function of the vapor pressure measured in a flow system. Enthalpies of fusion Δ_mH of solid compounds were measured by DSC. From the enthalpies of formation of the gaseous compounds of A-G the values of their strain enthalpies were derived and structural effects discussed.

     

Enthalpies of transfer of several ions from water to water-n-propanol mixtures at 298.15 K

Measurements were made of the dissolution heats of NaBPh₄ and Ph₄PCl in water-n-propanol mixtures over the whole range of compositions. Assuming the equality of ΔH_tr+(Ph₄P⁺) and ΔH_tr+(BPh⁻₄), the transfer enthalpies of several ions from water to water-n-propanol mixtures at 298.15 K were calculated.     

Influence of non-covalent interactions on the thermodynamic stereoselectivity of the protonation of some dipeptides

ΔG⁰, ΔH⁰ and ΔS⁰ protonation values of some pairs of diastereoisomeric dipeptides have been determined by potentiometry and calorimetry in aqueous solution at 25°C and I = 0.1 mol dm⁻³ (KNO₃). On the basis of the results obtained it has been possible to assess the role played by two different non-covalent interactions, namely the electrostatic interaction and the solvophobic interaction, on the thermodynamic stereoselectivity in the proton complex formation, shown by the systems investigated.     

A calorimetrical study of the polynuclear Zn(II) and Pb(II) and the polymeric Ni(II) and Cd(II) complexes of 2-mercaptoethanol and 3-mercapto-1,2-propanediol

The ΔG, ΔH and ΔS values for the dissociation of 2-mercaptoethanol (MEL) and 3-mercapto-1,2-propanediol and for the formation of complexes between these ligands and the metal ions Ni²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ have been determined calorimetrically in 0.5 M KNO₃ and at 25°C.     

Kinetische und mechanistische untersuchungen von Übergangsmetall-komplex-reaktionen : III. Abhängigkeit des gleichgewichts der addition von tributylphosphan an pentacarbonyl(arylmethoxycarben)wolfram vom substituenten am ring

The reaction of pentacarbonyl(arylmethoxycarbene)tungsten, (CO)₅W[C(OCH₃(p-C₆H₄R)] [R = OCH₃ (a), CH₃ (b), H (c), Br (d), CF₃ (e)], with tributylphosphane at low temperatures results in a reversible addition of the phosphane to the carbene carbon atom. The addition—dissoziation equilibrium is not only dependent on the temperature but also to a strong degree on the nature of the substituent R. ΔG, ΔH and the equilibrium constant K increase in the series from R = OCH₃ to R = CF₃. With the exception of R = OCH₃ the substituents b to e form an isentropic class. For all substituents (a to e) a linear dependency from Jaffés σ-constants was observed for ΔH. Good linear correlation for the substituents b to e was also found for log K and σ as well as for log K and the CO-force constants k_cis and k_trans.     

Simulated T ← S spectra of benzaldehydes as a function of the energy gap between the ππ and nπ levels

The T_1,2 ← S₀ spectra of benzaldehydes have been studied as a function of the energy separation between the vibrationless levels. It is shown that the spectra are very complicated in the region of ΔE[T₂⁰(nπ^*)-T₁⁰(ππ^*)] = 250–400 cm⁻¹, reflecting effective vibronic interferences between T₂⁰(0-0) and each of the ν₃₆-ν₃₃ out-of-plane vibrational levels of T₁⁰(ππ^*). The simulated spectra correspond to the observed spectra. In the case of T₁⁰ = ³nπ^* and T₂⁰ = ³ππ^* the spectral change is not so drastic as in the reverse case loc. cit. because the optical intensity generally concentrates in the longest wavelength band, i.e., the origin band of the T₁(nπ^*) ← S₀ transition. The simulation spectra are useful for interpretation of the absorption spectra in similar electronic structure systems of substituted benzaldehydes.     

Ab initio calculations on the geometries and thermodynamic stabilities of chlorine trioxides

By applying G1 theory to a potential energy surface of the stoichiometry ClO₃ three equilibrium geometries were found and their zero-point heats of formation calculated. They correspond to the structures ClO·O₂ (ΔH_{f,0 K} = 41 kcal/mol), OCl·O₂ (ΔH_{f,0 K} = 58 kcal/mol), and sym-ClO₃ (ΔH_{f,0 K} = 48 kcal/mol). It is shown that the formation of an adduct by association of O₂ to ClO is endothermic. In the G1 electronic energy calculations the basis set containing additional polarization functions was extended to 6-311G(3df) on chlorine.     

Thermodynamic studies of the solvation of Ph4AsPh4B in mixed solvents (MeOH—DMF)

The thermodynamic data (ΔG⁰, ΔH⁰ and TΔS⁰) of the solvation of tetraphenylarsonium-tetraphenylborate (Ph₄AsPh₄B) and its neutral parts, tetraphenylgermanium (Ph₄Ge) and tetraphenylmethane (Ph₄C) in methanol—N,N-dimethylformamide mixed solvents are discussed.

The values of the free energy of transfer, Δ^s_MG⁰, are calculated from measurements of the solubilities of Ph₄AsPh₄B, Ph₄Ge and Ph₄C in the successive fractions of MeOH in DMF at three different temperatures (15, 25, 35°C). The values of Δ^s_MH⁰ and TΔ^s_MS⁰ for the derivatives are calculated from Δ^s_MG⁰ values.

The values of Δ^s_MG⁰, Δ^s_MH⁰ and TΔ^s_MS⁰ of tetraphenylarsonium and tetraphenylborate ions have also been carefully calculated. The ratios of Δ^s_MG⁰ values (Δ^s_MG⁰ = ΔG⁰(+)/ΔG⁰(−)) were found to be greater than unity. Similarly, the ratios of Δ^s_MH⁰ and TΔ^s_MS⁰ for the positive and negative ions were found to be greater than unity.     

The enthalpy of solution of ferrous chloride in water at 298 K

We have made calorimetric measurements of the enthalpy of dissolving solid ferrous chloride in water at 298 K and have derived ΔH° = −19.82 kcal mol⁻¹ for the standard enthalpy of this process. This ΔH° is related by way of some thermodynamic calculations to other properties of iron.     

Charge transfer photochemistry of Ru(bpz)3 with carboxylic acids and carboxylate ions

Ru(bpz)₃²⁺ (bpz = 2,2′-bipyrazine) has six peripheral uncoordinated nitrogen atoms potentially available for protonation in presence of acids. The emission from ^*Ru(bpz)₃²⁺ is efficiently quenched by organic acids and the observed quenching rate constants are explained in terms of proton transfer from acids to ^*Ru(bpz)₃²⁺. The absorption and emission intensity of Ru(bpz)₃²⁺ increases with increasing concentration of carboxylate ion suggesting the complex formation between the two reactants in the ground state. From these studies, the formation constant (K_f) have been evaluated by Benesi–Hildebrand method. The K_f values indicate that generally the ion pair association constants estimated from absorption and emission techniques are comparable and these values are sensitive to the structure of the carboxylate ions.     

Thermodynamic properties of solutions of benzoic and citric acids in formamide at different tempertures

The standard thermodynamic quantities, ΔG⁰, ΔH⁰, and ΔS⁰, associated with the ionization process of benzoic acid in formamide have been evaluated. The standard potentials of the Ag(s)/AgCNS(s)/CNS⁻ and the Ag(s)/Ag₃Ci(s)/Ci³⁻ electrode, and the standard thermodynamic quantities for the electrode processes have been calculated in formamide, at different temperatures.     

Étude thermochimique des méthylnaphtalènes

Calorimetric measurements of enthalpies of change of state (sublimation or vaporization) of methylnaphthalenes gave the following results:

1-methylnaphthalene: (ΔH√_vap)_m=(57.32±0.42) kJ mol⁻¹

2-methylnaphthalene: (ΔH√_sub)_m=(65.69±0.84) kJ mol⁻¹

Combination of these values with those obtained by Speros and Rossini¹ for enthalpies of combustion of these compounds makes it possible to determine their energy of isomerization more accurately. This energy is (2.97±2.41) kJ mol⁻¹ and should be attributed to steric hindrance in the 1-methylnaphthalene molecule.

The comparison of energies of conjugation, theoretical as well as experimental, which we have determine for both molecules studied, confirms the present result.     

Thermodynamics of hydrogen adsorption on the zeolite Li-ZSM-5

Thermodynamic characterisation of the adsorption process (at a low temperature) of dihydrogen on the zeolite Li-ZSM-5 was carried out by means of variable-temperature infrared spectroscopy, with the simultaneous measurement of temperature and equilibrium pressure. Adsorption renders the H–H stretching mode infrared active, at 4092 cm⁻¹. The standard adsorption enthalpy and entropy resulted to be ΔH⁰=−6.5(±0.5) kJmol⁻¹ and ΔS⁰=−90(±5) Jmol⁻¹ K⁻¹, respectively. The adsorption enthalpy is significantly larger than the liquefaction heat, and this fact renders Li-ZSM-5 a potential cryoadsorbent for hydrogen storage.     

Ligand deprotonation significance in the formation of the molybdate ion-malic acid complexes

Using the temperature jump technique, the study of the kinetics of the complexing of oxomolybdate anion with malic acid has been carried out in aqueous solutions of pH 7.15–8.5 at ionic strength 0.1 M (KNO₃) and 25°'C. A reaction scheme for the formation of 1 : 1 complexes is proposed which accounts for the observed relaxation rates.

The significance of the ligand deprotonation on the complexation reaction of MoO₄²⁻ by a single protonated ligand, i.e. MoO₄²⁻+LH^nk→MoO₃(OH)Lⁿ⁻², (where n = 1 -, 2 -, etc), is analysed on the basis of a simple model. A linear correlation between the log k and the pK of the monoprotonated ligand (LH) is found for this reaction when the global process is controlled by the proton transfer from the ligand to an oxogroup, i.e. log k = a - 0.5xpK. It is found that this correlation is satisfied by MoO₄²⁻ and WO₄²⁻. The experimental slopes for these oxyanions are −0.503 and −0.543 respectively, in agreement with the predictions.