Rotational barriers of disilane,hexafluorodisilane, and hexamethyldisilane: Ab initio,density functional,and molecular mechanics (MM3) studies

We investigated structures, vibrational frequencies, and rotational barriers of disilane (Si₂H₆), hexafluorodisilane (Si₂F₆), and hexamethyldisilane (Si₂Me₆) by using ab initio molecular orbital and density functional theories. We employed four different levels of theories (i.e., HF/6–31G*, MP2/6–31G*, BLYP/6–31G*, and B3LYP/6–31G*) to optimize the structures and to calculate the vibrational frequencies (except for Si₂Me₆ at MP2/6–31G*). MP2/6–31G* calculations reproduce experimental bond lengths well, while BLYP/6–31G* calculations largely overestimate some bond lengths. Vibrational frequencies from density functional theories (BLYP/6–31G* and B3LYP/6–31G*) were in reasonably good agreement with experimental values without employing additional correction factors. We calculated the ΔG^‡(298 K) values of the internal rotation by correcting zero-point vibration energies, thermal vibration energies, and entropies. We performed CISD/6–31G*//MP2/6–31G* calculations and found the ΔG^‡(298 K) values for the internal rotation of Si₂H₆, Si₂F₆, and Si₂Me₆ to be 1.36, 2.06, and 2.69 kcal/mol, respectively. The performance of this level was verified by using G2 and G2(MP2) methods in Si₂H₆. According to our theoretical results, the ΔG^‡(298 K) values were marginally greater than the ΔE^‡(0 K) values in Si₂F₆ and Si₂Me₆ due to the contribution of the entropy. In Si₂H₆ the ΔE^‡(0 K) and ΔG^‡(298 K) values were coincidently similar due to a cancellation of two opposing contributions between zero-point and thermal vibrational energies, and entropies. Our calculated ΔG^‡(298 K) values were in good agreement with experimental values published recently. In addition, we also performed MM3 calculations on Si₂H₆ and Si₂Me₆. MM3 calculated rotational barriers and thermodynamic properties were compared with high level ab initio results. Based on this comparison, MM3 calculations reproduced high level ab initio results in rotational barriers and thermodynamic properties of Si₂H₆ derivatives including vibrational energies and entropies, although large errors exist in some vibrational frequencies. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1523–1533, 1997     

Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium

The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG^≠, ΔH^≠, ΔS^≠ parameters related to the corresponding hindered rotation have been determined by ¹³C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG^≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG^≠ values decreases from 19.1 kcal mol^?1 (79.9 kJ mol^?1) to 12.6 kcal mol^?1 (52.7 kJ mol^?1) according to the following sequence for the R-4 substituents: -C₆H₅, -CH₃, -OCH₃, -N(CH₃)₂.     

Protonenresonanzuntersuchungen an Aminoboranen—II: Einflüsse von para-Phenylsubstituenten auf die Rotationsbarriere um die N?B-Bindung

The electronic influence of substituents on the free enthalpy of rotation around the N? B bond in aminoboranes was investigated in two series of compounds: (a) (CH₃)₂N?BCl (phenyl-p-X), containing the para-phenyl substituent at the boron atom, and (b) (p-X-phenyl)CH₃N?B(CH₃)₂, containing the para-phenyl substituent at the nitrogen atom of the N? B linkage (X = ? NR₂, ? OCH₃, ? C(CH₃)₃, ? Si(CH₃)₃, ? H, ? F, ? Cl, ? Br, ? I, ? CF₃ and ? NO₂). By comparing the rotational barriers in corresponding compounds of both series, a reverse effect of the substituents could be observed. Electron-withdrawing substituents in the para position of the phenyl ring increase the ΔG_c^≠ if the phenyl group is attached to the boron atom; on the other hand, a lower ΔG_c^≠ is observed if the phenyl ring is bonded to the nitrogen atom of the N? B system. Substitution of the phenyl ring with electron-donating substituents in the paraposition exerts the opposite effect. Within each series of compounds, the differences of ΔG_c^≠ values [δ(ΔG_c^≠) = ΔG_c^≠ (X) ? ΔG_c^≠ (X = H)] between substituted and unsubstituted compounds can be explained in terms of inductive and mesomeric effects of the ring substituents and can be correlated with the Hammett σ constant of each substituent. A comparison of the slopes of the plotted lines shows that the influence of the ring substituents is more pronounced in compounds with N-phenyl-p-X than in those with B-phenyl-p-X.     

Bond dissociation energies and radical heats of formation in CH3Cl,CH2Cl2, CH3Br,CH2Br2, CH2FCl,and CHFCl2

An analysis of thermochemical and kinetic data on the bromination of the halomethanes CH_4–nX_n (X = F, Cl, Br; n = 1–3), the two chlorofluoromethanes, CH₂FCl and CHFCl₂, and CH₄, shows that the recently reported heats of formation of the radicals CH₂Cl, CHCl₂, CHBr₂, and CFCl₂, and the C? H bond dissociation energies in the matching halomethanes are not compatible with the activation energies for the corresponding reverse reactions. From the observed trends in CH₄ and the other halomethanes, the following revised ΔH°_f,298 (R) values have been derived: ΔH°_f(CH₂Cl) = 29.1 ± 1.0, ΔH°_f(CHCl₂) = 23.5 ± 1.2, ΔH_f(CH₂Br) = 40.4 ± 1.0, ΔH°_f(CHBr₂) = 45.0 ± 2.2, and ΔH°_f(CFCl₂) = ?21.3 ± 2.4 kcal mol^?1. The previously unavailable radical heat of formation, ΔH°_f(CHFCl) = ?14.5 ± 2.4 kcal mol^?1 has also been deduced. These values are used with the heats of formation of the parent compounds from the literature to evaluate C? H and C? X bond dissociation energies in CH₃Cl, CH₂Cl₂, CH₃Br, CH₂Br₂, CH₂FCl, and CHFCl₂.     

Density functional theory investigation of the binding interactions between phosphoryl,carbonyl, imino,and thiocarbonyl ligands and the pentaaqua nickel(II) complex: Coordination affinity and associated parameters

Density Functional Theory (UB3LYP/6‐311++G(d,p)) calculations of the affinity of the pentaaqua nickel(II) complex for a set of phosphoryl [O?P(H)(CH₃)(PhR)], imino [HN?C(CH₃)(PhR)], thiocarbonyl [S?C(CH₃)(PhR)] and carbonyl [O?C(CH₃)(PhR)] ligands were performed, where R?NH₂, OCH₃, OH, CH₃, H, Cl, CN, and NO₂ is a substituent at the para‐position of a phenyl ring.The affinity of the pentaaqua nickel(II) complex for these ligands was analized and quantified in terms of interaction enthalpy (ΔH), Gibbs free energy (ΔG²⁹⁸), geometric and electronic parameters of the resultant octahedral complexes. The ΔH and ΔG²⁹⁸ results show that the ligand coordination strength increases in the following order: carbonyl < thiocarbonyl < imino < phosphoryl. This coordination strength order is also observed in the analysis of the metal‐ligand distances and charges on the ligand atom that interacts with the Ni(II) cation. The electronic character of the substituent R is the main parameter that affects the strength of the metal‐ligand coordination. Ligands containing electron‐donating groups (NH₂, OCH₃, OH) have more exothermic ΔH and ΔG²⁹⁸ than ligands with electron‐withdrawing groups (Cl, CN, NO₂). The metal‐ligand interaction decomposed by means of the energy decomposition analysis (EDA) method shows that the electronic character of the ligand modulates all the components of the metal‐ligand interaction. The absolute softness of the free ligands is correlated with the covalent contribution to the instantaneous interaction energy calculated using the EDA method. © 2013 Wiley Periodicals, Inc.     

Dissociation Energy Computations for Saline Bonds Implied in Interactions Mediated by Peptidoglicans

Summary: Dissociation energy and hydration energy calculations, in water solution, are presented for saline bonds mediated by Ca²⁺ and Mg²⁺ ions with Brőnstedt type bases ( COO ,  OSO₃ ,  OH). A computationally intensive method, Polarisable Continuum Model (PCM) using 6-31G^* basis set, was applied. Hydration energies were computed by various methods, as well as dissociation energies of some L₂M complexes. L₂Ca complexes result as more stable against dissociation than L₂Mg complexes. Hydration energy calculation results, for some of the methods, here used, seem rather reliable as compared to experimental results.     

Electronic and steric substituent influences on the conformational equilibria of cyclohexyl esters: the anomeric effect is not anomalous!

The cyclohexyl esters of a series of carboxylic acids, RCO₂H, spanning a range of electronegativities and quotients of steric hindrance for the R substituent (R=Me, Et, iPr, tBu, CF₃, CH₂Cl, CHCl₂, CCl₃, CH₂Br, CHBr₂, and CBr₃) were prepared. Their conformational equilibria in CD₂Cl₂ were examined by low‐temperature ¹H NMR spectroscopy to study the axial or equatorial orientation of the ester functionality with respect to the adopted chair conformation of the cyclohexane ring. The ab initio and DFT geometry‐optimized structures and relative free energies of the axial and equatorial conformers were also calculated at the HF/6‐311G**, MP2/6‐311G**, and B3LYP/6‐31G** levels of theory, both in the gas phase and in solution. In the latter case, a self‐consistent isodensity polarized continuum model was employed. Only by including electron correlation in the modeling calculations for the solvated molecules was it possible to obtain a reasonable correlation between ΔG°_calcd and ΔG°_exp. Both the structures and the free energy differences of the axial and equatorial conformers were evaluated with respect to the factors normally influencing conformational preference, namely, 1,3‐diaxial steric interactions in the axial conformer and hyperconjugation. It was assessed that hyperconjugative interactions, σ_{C? C}/σ_{C? H} and σ*_{C? O}, together with a steric effect—the destabilization of the equatorial conformer with increasing bulk of the R group—were the determinant factors for the position of the conformational equilibria. Thus, because hyperconjugation is held responsible as the mitigating factor for the anomeric effect in 2‐substituted, six‐membered saturated heterocyclic rings, and since it is also similarly responsible, at least partly, in these monosubstituted cyclohexanes for a preferential shift towards the axial conformer, the question is therefore raised: can the anomeric effect really be construed as anomalous?     

The torsional barrier in aromatic carbonyl compounds. VI. Carbon-13 and 1H DNMR study of protonated alkyl and haloalkyl p-tolyl ketones

The free energy of activation of the torsional barrier around the phenyl–carbonyl bond in a series of stable protonated α-substituted p-methylacetophenones (p-tolyl- where R = CH₃, C₂H₅, i-C₃H₇, t-C₄H₉, CH₂Cl, CHCl₂, CCl₃, CH₂F, CHF₂, CF₃ and CH₂Br) has been evaluated by complete lineshape analysis of ¹³C or proton low temperature NMR spectra. α-Substitution by an alkyl group leads to a decrease of the barrier height due to both steric and electronic effects. For the halo compounds, steric and electronic effects of the substituents are in competition, leading to a very high ΔG^≠ value in protonated α,α,α-trifluoro-p-methylacetophenone (63.2 kJ mol^?1). In addition, the twist angle of the protonated carbonyl group has been tentatively evaluated by the use of the (modified) Stothers's method.     

Benchmarking of DFT functionals for the kinetics and mechanisms of atmospheric addition reactions of OH radicals with phenyl and substituted phenyl‐based organic pollutants

^•OH addition reactions play a pivotal role in the atmospheric transformation of a number of phenyl and substituted phenyl‐based persistent and toxic organic pollutants. Here, we screened appropriate DFT functionals to predict reaction mechanisms and rate constants (k_OH) of the ^•OH additions by taking benzene and substituted benzenes (C₆H₅F, C₆H₅Cl, C₆H₅Br, C₆H₅CH₃, C₆H₅OH) as model compounds. By comparing the k_OH values calculated with DFT methods to experimental values, we found that the ωB97 functional is the best among the 18 functionals considered (using the basis sets 6‐31 + G(d,p) for optimizations and 6‐311++G(3df,2pd) for single point energy calculations) in the temperature range of 230‐330 K. In addition, we found that some other functionals performed well in specific conditions, e.g., BMKD3 is good for benzene, halogenated benzenes and C₆H₅CH₃, and CAM‐B3LYP is good for the reaction of C₆H₅OH at room temperature. Based on the diversity of the electronic structures of the selected model compounds and the frequent occurrence of certain substituents ( CH₃,  OH,  F,  Cl, and  Br) in the target compounds, the functionals recommended here can be used for future study of the reaction mechanisms and k_OH values for ^•OH addition to phenyl and substituted phenyl‐based persistent and toxic organic pollutants.     

Data on the thermochemistry of azoalkanes

The rate constant of the primary decomposition step was determined for four symmetrical and four unsymmetrical azoalkanes. From the experimental activation energies and some literature enthalpy data, the following enthalpies of formation of radicals and group contributions were calculated: ΔH_? (CH₃N₂) = 51.5 ± 1.8 kcal mol^?1, ΔH_? (C₂H₅N₂) = 44.8 ± 2.5 kcal mol^?1, ΔH_? (2?C₃H₇N₂) = 37.9 ± 2.2 kcal mol^?1, [N_A-(C)] = 27.6 ± 3.7 kcal mol^?1, [N_A-(?_A) (C)] = 61.2 ± 3.1 kcal mol^?1.     

Ab initio studies of three-membered ring formation through intramolecular nucleophilic substitution

Three-membered ring (3MR) forming processes of ⁻X(SINGLE BOND)CH₂(SINGLE BOND)CH₂(SINGLE BOND)F and ⁻CH₂(SINGLE BOND)C((SINGLE BOND)Y)(SINGLE BOND)CH₂(SINGLE BOND)F (X(DOUBLE BOND)CH₂, O, or S and Y(DOUBLE BOND)0 or S) through a gas phase neighboring group mechanism (S_Ni) are studied theoretically using the ab initio molecular orbital method with the 6–31+G* basis set. When electron correlation effects are considered, the activation (ΔG^≠) and reaction energies (ΔG⁰) are lowered by ca. 10 kcal mol⁻¹, indicating the importance of the electron correlation effect in these reactions. The contribution of entropy of activation (−TΔS^≠) at 298 K to ΔG^≠ is very small, and the reactions are enthalpy controlled. The ΔG^≠ and ΔG⁰ values for these ring closure processes largely depend on the stabilities of the reactants and the heteroatom acting as a nucleophilic center. The Bell–Evans–Polanyi principle applies well to all these reaction series. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1773–1784, 1997     

Determination of maximum loading capacity of polyamidoamine (PAMAM) dendrimers and evaluation of Cu55 dendrimer‐encapsulated nanoparticles for catalytic activity

Spectrophotometric titrations provide information about the interior of the polyamidoamine (PAMAM) dendrimers, and therefore how nanoparticles are encapsulated. In this work, binding studies were performed to determine maximum loading capacities (N) of hydroxyl terminated G4, G5, and G6 PAMAM dendrimers with Cu²⁺ ions. The values of N found via spectrophotometric titrations were 16.22, 31.86, and 57.36 for G4‐OH, G5‐OH, and G6‐OH, respectively. The determination of loading capacity was also done using Viva spin filtration, and the results were found to be in agreement with those found via spectrophotometric titrations. From the binding isotherm, the values of equilibrium constant (K′) were determined and found to be 0.0488 (G4‐OH), 0.0291 (G5‐OH), and 0.0158 (G6‐OH). Owing to instability of G4‐OH (Cu₁₆), G5‐OH (Cu₃₂), and G6‐OH (Cu₅₇) dendrimer‐encapsulated nanoparticles (DENs) synthesized, G6‐OH (Cu₅₅) DENs of average size 2.6 ± 0.3 nm were prepared and were found to be relatively stable. Thus G6‐OH (Cu₅₅) catalyst was evaluated for the reduction of 4‐nitrophenol and was found to be catalytically active toward reduction of 4‐nitrophenol. Reaction kinetics of 4NP reduction was thoroughly studied in light of the Langmuir‐Hinshelwood kinetic model, and surface rate k, and the adsorption rates K_4NP, and K_BH4 were determined. The reaction was performed at different temperatures, which further expanded the study into determination of thermodynamic (ΔH^‡, ΔS^‡, ΔG^‡, and E_A) parameters.     

Benchmarking of DFT methods using experimental free energies and volumes of activation for the cycloaddition of alkynes to cuboidal Mo3S4 clusters

Here, the kinetics of the concerted [3 + 2] cycloaddition reaction between the [Mo₃(μ₃-S)(μ-S)₃Cl₃(dmen)₃]⁺ (dmen = N,N′-dimethyl-ethylenediamine) ([ 1 ]⁺) cluster and various alkynes to form dithiolene derivatives is thoroughly studied, with measurements at different temperatures and pressures allowing the determination of the free energies and volumes of activation. These parameters, together with the available single-crystal X-ray diffraction structures, are used to test a number of commonly used density functional theory (DFT) methods from Jacob's ladder, as well as the effects associated with the size of the basis sets, the way in which solvent effects are taken into account, or the inclusion of dispersion effects. Overall, a protocol that leads to average deviations between experimental and computed ΔV^‡ and ΔG^‡ values similar to the uncertainty of the experimental measurements is obtained.     

Die Konformativen Umwandlungen des Ungesättigten Siebenringes in 4.5.6-Trithia-1.2-Benzocycloheptenen-(1) Konformative Beweglichkeit Flexibler Ringsysteme—XVI. Untersuchungen mit Hilfe Der Protonenresonanzspektroskopie

The conformational equilibria and conversions of 4.5.6-trithia-1.2-benzocycloheptene-(1) ( 1 ) and the 3′.6′-dimethoxy-, 3′.6′-dimethyl- and 3′.6′-diphenyl- derivatives ( 2, 3 and 4 ) were investigated by NMR spectroscopy. Solutions of these substances are equilibrium mixtures of two conformers, one presumably having a chair form and the other a boat form. The free enthalpy of the boat conformer ΔG_B is dependent on the size of the substituents (R) in the 3′ and 6′ positions. The ΔG_B values for R = H, OCH₃, C₆H₅ and CH₃ are 1,03, 0,82, 0,50 and ?0,19 kcal/moles, respectively. By slow crystallization one conformer of the substituted trithiabenzocycloheptenes may be obtained in a pure crystalline form. The dimethoxy derivative crystallizes in the chair form, whereas the dimethyl and the diphenyl derivatives crystallize in the boat form. After dissolving the crystals, the conformational equilibrium is restored; at 0°C the half-lifes range from 2 to 15 minutes. By means of the temperature dependence of the NMR spectra two different types of conformational changes may be distinguished experimentally: the slower one is assigned to the inversion of the seven membered ring and the faster one to its pseudorotation. The free enthalpy of activation ΔG_v^≠ of the inversion was determined for 4.5.6-trithia-1.2-benzocycloheptene-(1) by the ‘line-shape’ method and for the diphenyl derivative by the ‘equilibration’ method. Both methods were applied to the other derivatives. The ΔG_v^≠ values obtained by the two different methods agree well with one another. The free enthalpy of activation of the inversion ΔG_v^≠ and of the pseudorotation ΔG_p^≠ both depend on the nature of the substituents. The ΔG_v^≠ values range from 17,9 to 20,5 kcal/mole and the ΔG_p^≠ values are equal to or lower than 11,4 kcal/mole.     

Conformers of dehydrogenated glycine isomers

We report a comprehensive ab initio investigation of the conformers of dehydrogenated glycine radicals using the STO-3G, 3-21G, and aug-cc-pVDZ (aVDZ) basis sets and the UHF and UMP2 (H₂N-CH-COOH and HN-CH₂-COOH) as well as MCSCF and MRCI (H₂N-CH₂-COO) methods via two different conformational search strategies generating initial structures for optimizations by (a) removing H atoms from glycine conformers and (b) scanning torsional angles describing internal rotation along the C C, C N, and C O (except for H₂N-CH₂-COO) bonds of the radicals. We find four H₂N-CH-COOH {In_CH, IIn_CH, IIIn_CH, IVn_CH} and seven HN-CH₂-COOH {Ip_NH, IIp_NH, IIIn_NH, IVp_NH Vn_NH, VIp_NH, VIIp_NH} conformers with classical(adiabatic) relative energies of {0.00(0.00), 1.57(1.55), 5.25(5.03), 9.85(9.72)} and {0.00(0.00), 0.78(1.06), 1.93(2.08), 3.34(3.16), 3.39(3.29), 5.00(4.86), 9.27(8.87)} kcal/mol, respectively, obtained with UCCSD(T)-F12b/aug-cc-pVTZ(+UCCSD(T)-F12b/aVDZ ZPE correction) and four H₂N-CH₂-COO {Ip_COO, IIn_COO, IIIp_COO, IVn_COO} conformers with MRCI-F12+Q/aVDZ(+MRCI/aVDZ ZPE correction) energies of {0.00(0.00), 1.65(1.64), 1.78(1.75), 2.21(2.21)} kcal/mol, where n and p denote C₁ and C_s symmetry. The MRCI-F12+Q[UCCSD(T)-F12b] In_CH → Ip_NH and In_CH → Ip_COO classical(adiabatic) isomerization energies are 18.51(17.32)[21.20(20.01)] and 31.88(31.66) kcal/mol, respectively.     

Effects of Isomerization on the Measured Thermochemical Properties of Deprotonated Glycine/Protic‐Solvent Clusters

The thermochemical properties associated with the formation of an isomeric distribution of ROH???NH₂CH₂COO^? clusters (R=H, CH₃, C₂H₅) are measured by using high‐pressure mass spectrometry. A comparison of the measured properties with calculated values provides new insights into the thermochemical effects arising from the isomeric nature of this clustering system. When the distribution of isomers is correctly accounted for, the measured values of ΔH°, ΔS°, and ΔG°₂₉₈ consistently agree, to a very high degree of accuracy, with those predicted by MP2(full)/6‐311++G(d,p)//B3LYP/6‐311++G(d,p) calculations.     

Heterolysis of H2 Across a Classical Lewis Pair, 2,6‐Lutidine⋅BCl3: Synthesis,Characterization, and Mechanism

We report that 2,6‐lutidine?trichloroborane (Lut?BCl₃) reacts with H₂ in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut?BHCl₂. The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut?BCl₃ was calculated to react with H₂ and form the ion pair, [LutH⁺][HBCl₃^?], with a barrier of ΔH^≠=24.7 kcal mol^?1 (ΔG^≠=29.8 kcal mol^?1). Metathesis with a second molecule of Lut?BCl₃ produced Lut?BHCl₂ and [LutH⁺][BCl₄^?]. The overall reaction is exothermic by 6.0 kcal mol^?1 (Δ_rG°=?1.1). Alternate pathways were explored involving the borenium cation (LutBCl₂⁺) and the four‐membered boracycle [(CH₂{NC₅H₃Me})BCl₂]. Barriers for addition of H₂ across the Lut/LutBCl₂⁺ pair and the boracycle B?C bond are substantially higher (ΔG^≠=42.1 and 49.4 kcal mol^?1, respectively), such that these pathways are excluded. The barrier for addition of H₂ to the boracycle B?N bond is comparable (ΔH^≠=28.5 and ΔG^≠=32 kcal mol^?1). Conversion of the intermediate 2‐(BHCl₂CH₂)‐6‐Me(C₅H₃NH) to Lut?BHCl₂ may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl₃. Intramolecular protodeboronation, which could form Lut?BHCl₂ directly, is prohibited by a high barrier (ΔH^≠=52, ΔG^≠=51 kcal mol^?1).     

Thermal [1,5] sigmatropic rearrangements in (σ-7-cycloheptatrienyl)triphenyltin and (σ-5-cyclohepta-1,3-dienyl)triphenyltin: A 1H and 13C NMR reinvestigation

It has been confirmed by ¹H and ¹³C NMR spectroscopies that Sn(σ-C₇H₇)Ph₃ undergoes either 1,4- or 1,5-shifts of the SnPh₃ moiety around the cycloheptatrienyl ring with ΔH³ = 13.8 ± 0.4 kcal mol^?1, ΔS3 = ?5.6 ± 1.2 cal mol^?1 deg^?1, and ΔG³₃₀₀ = 15.44 ± 0.14 kcal mol^?1. Similarly, (σ-5-cyclohepta-1,3-dienyl)triphenyltin undergoes 1,5-shifts with ΔH³ = 12.4 ± 0.6 kcal mol^?1, ΔS³ = ?11.2 ± 1.8 cal mol^?1 deg^?1, and ΔG³₃₀₀ = 15.76 ± 0.13 kcal mol^?1. It is therefore probable that Sn(σ-5-C₅H₅)R₃ and Sn(σ-3-indenyl)R₃ do not undergo 1,2-shifts as previously suggested but really undergo 1,5-shifts.     

Polyesters containing sulfur. VI. Synthesis and characterization of polyesters from naphthalene-1,4- or naphthalene-1,5-bis(methylthioacetic acid) and aliphatic diols

Two series of new linear polyesters containing sulfur in the main chain were obtained by melt polycondensation of naphthalene-1,4-bis(methylthioacetic acid) (N-1,4-BMTAA) or naphthalene-1,5-bis(methylthioacetic acid) (N-1,5-BMTAA) with some aliphatic diols using a 0.05 molar excess of diol. Softening temperatures ranging from 55 to 130°C, reduced viscosities in the range of 0.15–0.39 dL/g, and low-molecular weights were their characteristic. The structure and thermal properties of all polyesters were examined by using elemental analysis, FT-IR and ¹H-NMR spectroscopy, X-ray diffraction analysis, differential thermal analysis (DTA), thermogravimetric analysis (TGA), and differential scanning calorymetry (DSC). The kinetics of polyester formation by uncatalyzed melt polycondensation was studied in a model system: N-1,4-BMTAA or N-1,5-BMTAA and 2,2′-oxydiethanol (ODE) at 150, 160, and 170°C. Reaction rate constants (k₃) and activation parameters (ΔG^‡, ΔH^‡, ΔS^‡) from carboxyl group loss were determined using classical kinetic methods. Hydroxyl-terminated polyesters derived from 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol were used for preparation of the polyurethanes by melt polyaddition with hexamethylene diisocyanate (HDI). They were characterized by reduced viscosity, FT-IR spectroscopy, X-ray diffraction analysis, TGA, DSC, polarizing microscope observation, and hardness and tensile properties. The resulting polyurethanes behave like high-elasticity thermoplastic elastomers, except the one derived from N-1,5-BMTAA and 1,6-hexanediol-based polyester. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2359–2369, 1998     

Calorimetric determination of the thermodynamic properties involved in the complex formation of silver(I) with some sulphur-containing pyridines

The enthalpy changes involved in the complex formation of Ag⁺ with several sulphur-containing pyridines of general formula

where R = Ch₃, C₂H₅ and CH₂CH₂OH and x=1,2 have been determined by direct calorimetric measurements at 25°C in 0.5 M KNO₃ solution.From the ΔH values obtained and from the free energy changes reported in an earlier publication, the corresponding entropy changes ΔS have been calculated. Changes in ΔH and ΔS are discussed in terms of inductive and steric effects. It is shown that enthalpy values are linearly correlated with the Taft σ^* parameters of the substituents on the ligand.