Relative thermodynamic stabilities of allyl vinyl and propenyl vinyl ethers

The relative thermodynamic stabilities of a number of isomeric allyl vinyl and propenyl vinyl ethers were determined by chemical equilibration in DMSO solution with KOBu-t as catalyst. From the temperature dependence of the values of the equilibrium constant the parameters G _m , H _m and S _m of isomerization at 298.15 K were evaluated. Propenyl vinyl ethers, owing to their low enthalpy contents, are much more stable than the isomeric allyl vinyl ethers. It appears that in the parent propenyl vinyl ether, the Me group attached to C- of the divinyl ether skeleton has a strong stabilizing effect, comparable to that of alkyl groups in ordinary olefins, on the unsaturated system. In more heavily alkyl-substituted divinyl ethers, however, the stabilizing effects of alkyl groups are less prominent, being comparable to the low stabilization energies of alkyl groups in vinyl ethers, and depend moreover, on the pattern of substitution.     

Relative Stabilities and Molecular Structures of the Geometrical Isomers of α,α′-Dioxasubstituted Cycloalkylidenecycloalkanes. A DFT Study

The relative stabilities and molecular structures of the geometrical isomers of seven ,-dioxa derivatives of cycloalkylidenecycloalkanes with 3to 6-membered rings have been studied by DFT calculations at the B3LYP/6-31G* level of theory. In each case, the E form was calculated to have the lower total energy. The relative energy of the Z isomer proved to increase regularly with increasing sizes of the two heterocyclic rings, ranging from 1.1 to 13.8 kJ mol^–1 on going from the 3,3to the 6,6-membered rings. Intermediate values of the relative energy were calculated for the Z forms of compounds containing two dissimilar rings. The relative energy of the Z isomer was found to be proportional to d ^–9.3, where d = the distance separating the two O atoms of the Z compound. The molecular structures, electric dipole moments, and atomic charges on the O atoms are also discussed.     

Relative thermodynamic stabilities of the isomeric dihydrofurans and isomeric dihydropyrans. An experimental and DFT study

The relative thermodynamic stabilities of 2,5-dihydrofuran (1) and 2,3-dihydrofuran (2), and of 3,4-dihydro-6H-pyran (3) and 3,4-dihydro-2H-pyran (4), were determined at several temperatures by base-catalyzed equilibration in DMSO solution. For 1 → 2, = –15.4±0.1 kJ mol⁻¹, =–12.6±0.5 kJ mol⁻¹, and =9.5±1.3 J K⁻¹ mol⁻¹ at 298.15 K. The second-law reaction enthalpy agrees with literature data based on calorimetric enthalpies of hydrogenation of the isomeric forms in hexane. For 3 → 4, =–19.3±0.2 kJ mol⁻¹, = –18.9±1.1 kJ mol⁻¹ and =1.1±3.0 J K⁻¹ mol⁻¹ at 298.15 K: the experimental reaction enthalpy is in marked disagreement with literature data based on estimation. On the other hand, both of the experimental reaction enthalpies of the present study are in good agreement with DFT calculations using the B3LYP functional and 6-311+G(2d,p) basis set.     

Relative thermodynamic stabilities of 2-substituted 4-methylene-1,3-dioxolanes and 4-methyl-1,3-dioxoles

The relative thermodynamic stabilities of 24 pairs of carbon-carbon double-bondexo-endo isomeric 2-substituted 4-methylene-1,3-dioxolanes (a) and 4-methyl-1,3-dioxoles (b) have been determined by base-catalyzed chemical equilibration in DMSO solution. In all cases, theendo isomer (b) is the favored species at thermodynamic equilibrium. A single alkyl substitutent on C-2 gives only a negligible contribution to the relative stability of the isomeric forms, but the presence of two alkyl groups on C-2 increases the relative stability of theendo isomer by 2–3 kL mol^–1. A still higher effect in favor of theendo isomer is produced by introduction of a single alkoxy group on C-2; this effect is further slightly accentuated by 2,2-dialkoxy substitution at C-2. The origin of the favorable effect of 2-alkoxy substitution on the relative stability of theendo isomer is not clear, but it seems to arise from an unexpected stability of theendo isomer rather than from an enhanced destabilization of theexo form.     

Relative Stabilities of the Geometrical and Conformational Isomers of 2-Alkylideneoxacycloalkanes. A DFT Study

The relative stabilities of the geometrical and rotational isomers of 2-alkylideneoxacycloalkanes (-oxiranes, -oxetanes, -tetrahydrofurans, and -tetrahydropyrans; alkyl = Et, Pr, i-Bu, 2,2-dimethylpropyl) have been studied by DFT calculations. Independent of the size of the alkyl group, the E and Z isomers of alkylideneoxiranes have almost comparable stabilities (the Z form, however, being slightly favored), but, with increasing size of the heterocyclic ring, the relative stability of the E isomer decreases. This is particularly prominent for the tetrahydropyran derivatives with alkyl = 2,2-dimethylpropyl, in which marked repulsive interactions between the t-Bu group and the 3-CH₂ group of the tetrahydropyran ring make the E form, ca. 13 kJ mol^–1 less stable than the Z isomer. On the other hand, for alkyl = Et, Pr, and i-Bu, the relative stabilities of the geometrical isomers are almost independent of the alkyl group. Besides the relative stabilities of the geometrical isomers, energetics of the rotational isomerism of the alkyl group about the C(sp ³)—C(sp ²) bond is also surveyed.     

Relative Thermodynamic Stabilities of Isomeric Vinyl Ethers Derived from 2-Acetyland 2-Isobutyryltetrahydrofurans. An Experimental and DFT Study

The relative thermodynamic stabilities of four isomeric vinyl ethers obtained by acid-catalyzed cleavage of MeOH from the dimethyl acetals of both 2-acetyltetrahydrofuran and 2-isobutyryltetrahydrofuran have been determined by chemical equilibration in cyclohexane solution. In addition, the structures and relative energies of many of these compounds have also been studied theoretically by DFT calculations. In each series of compounds, the endocyclic isomers, 5-(1-methoxyethyl)-2,3-dihydrofuran and 5-(1-methoxy-2-methylpropyl)-2,3-dihydrofuran, respectively, show the highest thermodynamic stabilities. This is particularly prominent for the vinyl ethers derived from 2-isobutyryltetrahydrofuran in which the equilibrium mixture consists of 90% of the endocyclic isomer at room temperature. The geometrical (exocyclic) isomers of the latter series of compounds show unexpected relative thermodynamic stabilities, but on the basis of the geometry-optimized structures provided by the DFT calculations, a reasonable explanation can be found.     

Relative thermodynamic stabilities of isomeric alkyl allyl and alkyl (Z)-propenyl ethers

The relative thermodynamic stabilities of ten allyl ethers (ROCH₂CH=CH₂) and the corresponding isomeric (Z)-propenyl ethers (where R is an alkyl group, or a methoxysubstituted alkyl group) have been determined by chemical equilibration in DMSO solution with t-BuOK as catalyst. From the variation of the equilibrium constant with temperature, the values of the thermodynamic parameters ΔGΘ, ΔHΘ and ΔSΘ of isomerization at 298.15 K were evaluated. The propenyl ethers are highly favored at equilibrium, the values of both ΔGΘ and ΔHΘ for the allyl → propenyl reaction being ca. −18 to −25 kJ mol⁻¹. The favor of the propenyl ethers is increased by bulky alkyl substituents, and decreased by methoxy-substituted alkyl groups. In most cases the entropy contribution is negligible; however, for R = (MeO)₂CH and R = (MeO)₃C the values of ΔSΘ are ca. −5 J K⁻¹ mol⁻¹.     

Theoretical studies of structures and stabilities of a new odd-numbered fullerene dimer: C141

The possible isomers of a newly synthesized C(141) molecule are calculated using MNDO, AM1, PM3, B3LYP/3-21G, and B3LYP/6-31G(d) methods. The geometry optimizations showed that the isomer 8-8 has the lowest total energy in all 64 possible structures of C(141). Unlike those of C(130), C(140), etc., the C(141) 8-8 shows a new structure: two C(70) side cages open [6.6] ring junctions located at the equator (instead of cap) area to create new chemical bonds for the bridge atom. Theoretical measurements of the average length of the long and short axes of C(70) side cages in the C(141) molecule reveal that when two C(70) cages are connected with each other at the equators, their geometric shapes become more spherical compared with the pristine C(70); this leads to a reduction of the molecular polarizability. Analysis of the local and global strain indicates that the global strain of C(70) monomer in the C(141) 8-8 is greatly reduced compared to the pristine C(70). The stable C(70) derivatives that are formed with reacted C-C bonds in the equator area may put new insights into fullerene chemistry, in particular, for C(70) to react with a large molecule. The results are discussed together with the experimental data.     

Relative Thermodynamic Stabilities of Isomeric Dimethyl Acetals Derived from 2-Acetyltetrahydrofuran. An Experimental and Computational Study

In the presence of an acid catalyst, the dimethyl acetal of 2-acetyltetrahydrofuran (1) is converted into a mixture of three isomeric acetals composed of the reactant and two diastereomers of 2-methoxy-2-(1-methoxyethyl)tetrahydrofuran (2). The relative thermodynamic stabilities of these acetals have now been determined by chemical equilibration. The least stable isomer is 1, in the liquid phase 4–6 kJ mol^–1 less stable than the two diastereomers. The geometry-optimized structures and relative energies of the title compounds were also studied by theoretical calculations (ab initio and DFT). Comparison of the theoretically determined relative stabilities of the diastereomers with the corresponding experimental data suggests the more volatile (and more stable) diastereomer to exist as a racemic mixture of the (R,S) and (S,R) configurations.     

Relative Stabilities and Molecular Structures of Isomeric Dihalobenzenes. A DFT Study

The molecular structures, total energies, and other computational data of benzene, and its monoand dihalogenated derivatives (halogen = F, Cl, Br) have been studied by DFT calculations. The main aim of the study was to estimate the relative stabilities (energies) of the ortho, meta, and para isomers of the six series of dihalobenzenes investigated. The computational data show that the ortho isomers always have the highest, and the meta isomers usually, but not always, the lowest total energies. Thus, 1,2-difluorobenzene is ca. 16.6 kJ mol^–1, and 1,4-difluorobenzene 2.5 kJ mol^–1 less stable than 1,3-difluorobenzene. Among the other isomeric dihalobenzenes, the differences in stability are less pronounced. For the dibromo-, dichloro-, and bromochlorobenzenes, the para compounds are calculated to be slightly (0.2–0.4 kJ mol^–1) more stable than their meta isomers. In addition to the thermochemical aspect of the study, the computational molecular structures of the halobenzenes are compared with available experimental data and discussed in terms of the substituent-induced deformation of the ideal geometry of the benzene ring. The computational electric dipole moments, especially for the fluorine-containing compounds, compare favorably with the respective experimental (gas-phase) values.     

Relative stabilities of weakly coordinating anions: a computational study

This article describes BP86/SV(P) (DFT) calculations on a representative set of weakly coordinating anions (WCAs) of type [M(L)n]-, their parent neutral Lewis acids M(L)(n-1) and their ate complexes with fluoride, that is, [FM(L)(n)](n-1) (M=B, L=F, OTeF5, C6H5, C6F5, C6H3(CF3)2, CF3; M=P, As, Sb, L=F, OTeF5; M=Al, L=OC(CF3)3). Compounds with fluoride bridges, that is, Sb(n)F(5n) and [Sb(n)F(5n+1)]- (n=2, 3, 4), Al2(L)5F and [(L)3Al-F-Al(L)3]- (L=OC(CF3)3), (F4C6[1,2-B(L)2]2, [F4C6[1,2-B(L)2]2F]-, [F4C6[1,2-B(L)2]2OMe]- (L=C6F5) were also calculated. Based on these BP86/SV(P) and auxiliary MP2/TZVPP, G2, and CBS-Q calculations the relative stabilities and coordinating abilities of these WCAs were established with regard to the fluoride ion affinities (FIA) of the parent Lewis acids, the ligand affinity (LA) of the WCAs, the decomposition of a given WCA in the presence of a hard (H+, proton decomposition PD) and a soft electrophile (Cu+, copper decomposition CuD), the position of the HOMO, the HOMO-LUMO gap, and population analyses of the anions providing partial charges for all atoms. To obtain data that is more reliable, the assessed quantities were calculated through isodesmic reactions. If parts of the calculations could not be done isodesmically, higher levels such as MP2/TZVPP, G2, and CBS-Q were used to obtain reliable values for these reactions. Although the obtained results can not be taken as absolute, the relative ordering of the stabilities of all WCAs will undoubtedly be correct, since a single methodology was chosen for the investigation. To include media effects the decomposition reactions of a subset of 14 WCAs with the SiMe3+ and [Cp2ZrMe]+ ions were also calculated in PhCl and 1,2-F2C6H4 (COSMO solvation model). We found that in most cases gas-phase calculations and solution calculations give comparable results for the stability of the anion. Applications of the LA and FIA that allow one to decide, on thermodynamic grounds, which WCA or Lewis acid is the most suitable for a given problem are sketched.     

Rotational isomerism of vinyl ethers of azines based on the data of IR spectroscopy and quantum-chemical AM1 calculations

The IR spectra of vinyl ethers of azines have two C=C stretching vibration bands of the vinyl group at 1620 and 1640 cm⁻¹, which correspond to thes-cis and nonplanars-trans conformers, respectively. According to the results of AM1 calculations, the hetaryl fragment and the vinyl group in thes-trans conformers of all the ethers under study are twisted out of the C−O−C plane by ∼30°. In thes-cis conformation, the angle of rotation of the heterocycle with respect to the vinyloxy group varies from 45 to 90°. The character of the change in the intensity of the v(C=C) absorption bands of the doublet under changes of the temperature indicates that thes-trans conformer is energetically favorable. The exception is 5-vinyloxyacridine for which thes-cis conformation is more favorable. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2439–2442, December, 1998.     

Effect of bond orbital interactions on the conformational stabilities of 1,2-difluoroethane and 2,3-difluorobutanes

Several hypotheses have been proposed to explain the origin of the conformational stabilities of 1,2-difluoroalkanes, for which bond orbital interactions are an important factor. However, there is a limit to the effectiveness of the traditional approach focusing on only the antiperiplanar interactions between bonding and antibonding orbitals such as σ_C–H/σ_C–F*, σ_C–C/σ_C–F*, and σ_C–F/σ_C–F*, which cannot actually explain the conformational stabilities of 2,3-difuluorobutanes. In this study, to elucidate the effect of bond orbital interactions on the conformational stabilities of 1,2-difluoroethane, erythro-2,3-difluorobutane, and threo-2,3-difluorobutane, we extended the range of interactions considered to beyond these conventional interactions. The results showed that for 1,2-difluoroethane, the conformational stability can be understood by considering all antiperiplanar bond orbital interactions around the C1–C2 bond, and for 2,3-difluorobutanes, by considering all antiperiplanar bond orbital interactions around the C2–C3 bond in addition to bond orbital interactions between the methyl groups.     

PEG-400促进的硝酸铋氧化二芳基乙醇酮合成二芳基乙二酮

PEG-400作为绿色反应溶剂能有效地促进硝酸铋氧化二芳基乙醇酮为二芳基乙二酮。产物结构用IR和1H NMR光谱进行了表征。该方法具有反应时间短,产率较高,操作简便,环境友好等优点。     

硝酸铁催化氧化二芳基乙醇酮制备二芳基乙二酮

用AcOH作溶剂,PEG400做相转移催化剂,硝酸铁能将二芳基乙醇酮催化氧化为二芳基乙二酮。产物结构用IR,MS和1H NMR光谱对其进行了表征。该方法有反应时间短,产率较高,操作简便等优点,是一种有效的由二芳基乙醇酮氧化制备二芳基乙二酮的方法。     

Calculation of thermodynamic properties and transport coefficients for SF6-N2 mixtures in the temperature range 1,000–30,000 K

We have computed the equilibrium composition, the transport coefficients (viscosity, electrical and thermal conductivities), the thermodynamic properties (Gibbs and Helmholtz potentials, entropy, enthalpy, specific heats), and the derived quantities (mass density, sound velocity) for SF₆-N₂ mixtures in conditions relevant to circuit-breaker arcs: temperatures between 1000 and 30,000 K, pressures in the range 1–10 atm. The validity of our computation has been checked by a detailed comparison of our results with those available in the literature concerning pure SF₆ and pure N₂. In SF₆-N₂ mixtures the chemical reactions (dissociation, ionization) have a strong influence on thermal conduction and heat capacities. The effect of SF₆ on the properties of such mixtures is elucidated: in a mixture containing 40% SF₆, the amplitude of the thermal conduction peak appearing around 7500 K is reduced by a factor of 4 relative to that of pure N₂. The influence of pressure on the properties of the plasma between 1 and 10 atm is relatively low.     

Thermodynamic study of 2-methyl-tetrahydrofuran with isomeric chlorobutanes

Excess molar volumes, V^E, isentropic compressibility deviations, Δκ_S, and excess molar enthalpies, H^E, for the binary mixtures 2-methyl-tetrahydrofuran with 1-chlorobutane, 2-chlorobutane, 2-methyl-1-chloropropane and 2-methyl-2-chloropropane have been determined at temperatures 298.15 and 313.15 K, excess molar enthalpies were only measured at 298.15 K. We have applied the Prigogine-Flory-Patterson (PFP) theory to these mixtures at 298.15 K.     

syn‐1,2‐Carboboration of Alkynes with Borenium Cations

The reaction of 8‐(trimethylsiloxy)quinoline ( QOTMS ) with BCl₃ and (aryl)BCl₂ forms QOBCl₂ and QOBCl(aryl). The subsequent addition of stoichiometric AlCl₃ follows one of two paths, dependent on the steric demands of the QO ligand and the electrophilicity of the resulting borenium cation. The phenyl‐ and 5‐hexylthienylborenium cations, QOBPh⁺ and QOBTh⁺ , are formed, whereas QOBCl⁺ is not. Instead, AlCl₃ preferentially binds with QOBCl₂ at oxygen, forming QOBCl₂?AlCl₃ , rather than abstracting chloride. A modest increase in the steric demands around oxygen, by installing a methyl group at the 7‐position of the quinolato ligand, switches the reactivity with AlCl₃ back to chloride abstraction, allowing formation of QOBCl⁺ . All the prepared borenium cations are highly chlorophilic and exhibit significant interaction with AlCl₄^? resulting in an equilibrium concentration of Lewis acidic “AlCl₃” species. The presence of “AlCl₃^” species limits the alkyne substrates compatible with these borenium systems, with reaction of [ QOBPh][AlCl₄] with 1‐pentyne exclusively yielding the cyclotrimerised product, 1,3,5‐tripropylbenzene. In contrast, QOBPh⁺ and QOBTh⁺ systems effect the syn‐1,2‐carboboration of 3‐hexyne. DFT calculations at the M06‐2X/6‐311G(d,p)/PCM(DCM) level confirm that the higher migratory aptitude of Ph versus Me leads to a lower barrier to 1,2‐carboboration relative to 1,1‐carboboration.     

Synthesis of ethyl 4-chloro-2-oxobut-3-ynoate and its addition to alkyl vinyl ethers

Ethyl 4-chloro-2-oxobut-3-ynoate has been synthesized for the first time and its addition to alkyl vinyl ethers of various structures has been studied. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1537–1539, July, 2008.