Thermochemistry of 2- and 3-acetylthiophenes: calorimetric and computational study

The relative stabilities of 2- and 3-acetylthiophenes have been evaluated by experimental thermochemistry and the results compared to high-level ab initio calculations. The enthalpies of combustion, vaporization, and sublimation were measured by rotating-bomb combustion calorimetry, Calvet microcalorimetry, correlation gas chromatography, and Knudsen effusion techniques and the gas-phase enthalpies of formation, at T = 298.15 K, were determined. Standard ab initio molecular orbital calculations at the G2 and G3 levels were performed, and a theoretical study on the molecular and electronic structures of the compounds studied has been conducted. Calculated enthalpies of formation using atomization and isodesmic reactions are compared with the experimental data. Experimental and theoretical results show that 2-acetylthiophene is thermodynamically more stable than the 3-isomer. A comparison of the substituent effect of the acetyl group in benzene and thiophene rings has been carried out.     

Organic Thermochemistry at High ab Initio Levels. 3. A G3 Study of Cyclic Saturated and Unsaturated Hydrocarbons (Including Aromatics)

With the purpose of exploring the reliability of the enthalpies of formation calculated using the G3 method, we have examined a series of saturated and unsaturated alicyclic hydrocarbons varying the size and the number of formal double bonds in the molecule. Heats of formation have been calculated at the G3 level through both atomization reactions and bond separation isodesmic reactions, and comparisons with experimental values and with values previously calculated at the G2(MP2) and G2 levels have been made. The quality of the G3-calculated enthalpies of formation using atomization reactions is comparable to that obtained at the G2 level using bond separation reactions, whereas G3 calculations are two to three times faster than G2 calculations.     

Energetics of some sulphur heterocycles

The present work is part of a broader research program on the energetics of formation of heterocycles, aiming the study of the enthalpic effects of the introduction of different substituents into heterocycles. In this work we present the results of the thermochemical research on sulphur heterocycles of the type substituted thiophenes with different kind of substituents, mainly alkyl, ester, acetyl, carboxamide, acetamide, carbonitrile and carboxaldehyde. The standard (p ^o=0.1 MPa) molar enthalpies of formation, in the condensed phase, at T=298.15 K, of a large number of substituted thiophenes, were derived from their standard massic energies of combustion, measured by rotating-bomb combustion calorimetry, while the standard molar enthalpies of vaporization or sublimation of those compounds were obtained either by high temperature Calvet Microcalorimetry, or by the temperature dependence of their vapour pressures determined by the Knudsen effusion technique. The standard molar enthalpies of formation, of the studied sulphur heterocycles in the gaseous phase, were then derived. The results are interpreted in terms of structural contributions to the energetics of the substituted thiophenes, the internal consistency of the results is discussed and, whenever appropriate and possible, empirical correlations are suggested for the estimation of standard molar enthalpies of formation, at T=298.15 K, of substituted thiophenes. A Table of enthalpic increments for different group substituents in positions 2 or 3 of the thiophene ring has been established.     

Thermochemistry of 2,5-thiophenedicarboxylic acid

The enthalpies of combustion and sublimation of 2,5-thiophenedicarboxylic acid [CASRN 4282-31-9] were measured by rotary-bomb combustion calorimetry and the method of transference in a saturated stream of nitrogen, and the gas-phase enthalpy of formation was determined, Delta(f)H(o)(m)(g) = -(632.6 +/- 2.2) kJ x mol(-1). Standard ab initio molecular orbital calculations at the G2(MP2) and G3(MP2) levels were performed, and a theoretical study on the molecular and electronic structure of the compound has been carried out. The three most stable conformers have been explicitly taken into account. The calculated enthalpy of formation averaged using three different isodesmic reactions, -631.1 kJ x mol(-1), is in very good agreement with the experimental value. A comparison of the substituent effect of the carboxylic groups in benzene and thiophene ring has been made. The relative stability obtained for the substitution of two H atoms by COOH in position 2,5- for thiophene and 1,4- for benzene involve the same energetic effects, DeltaDelta(f)H(o)(m)= -747.6 +/- 2.4 and -748.2 +/- 2.7 kJ x mol(-1), respectively.     

Computational study of the thermochemistry of organophosphorus(III) compounds

The enthalpies of formation of organophosphorus(III) compounds have been calculated at the G3X, G3X(MP2), and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels of theory using the atomization energy procedure and the method of isodesmic reactions. The Delta f H298 degree values for 50 relatively large molecules with up to 10 non-hydrogen atoms, such as P(CH3)3, P(C2H5)3, P(OCH3)3, n-C4H9OPCl2, [(CH3)2N]2PCl, (C2H5)2NPCl2, and [(CH3)2N]2PCN, have been calculated directly from the G3X atomization energies. A good agreement between the known experimental values and G3X results for 14 compounds provides support to our predictions for remaining species whose experimental enthalpies of formation are unknown or known with relatively large uncertainties. On the basis of our calculations and sometimes conflicting experimental data a set of internally consistent enthalpies of formation has been recommended for organophosphorus(III) compounds. Our computational results call into question the experimental enthalpies of formation of P(C2H5)3 and P(n-C4H9)3. From comparison with most reliable experimental data, the accuracy of the theoretical enthalpies of formation is estimated as ranging from 5 to 10 kJ/mol. The recommended Delta f H298 degree values were used to derive the group additivity values (GAVs) for 45 groups involving the phosphorus(III) atom. These GAVs significantly extend the applicability of Benson's group additivity method and may be used to estimate the enthalpies of formation of larger organophosphorus(III) compounds, where high level quantum chemical calculations are impracticable.     

Thermochemical study of 2- and 3-alkyl substituted thiophenes

The standard (p ⁰=0.1 MPa) molar enthalpies of formation, in the condensed phase, of nine linear-alkyl substituted thiophenes, six in position 2- and three in position 3-, at T=298.15 K, were derived from the standard massic energies of combustion, in oxygen, to yield CO₂(g) and H₂SO₄·115H₂O(aq), measured by rotating-bomb combustion calorimetry. The standard molar enthalpies of vaporization of these compounds were measured by high temperature Calvet Microcalorimetry, so their standard molar enthalpies of formation, in the gaseous phase, were derived. The results are discussed in terms of structural contributions to the energetics of the alkyl-substituted thiophenes, and empirical correlations are suggested for the estimation of the standard molar enthalpies of formation, at T=298.15 K, for 2- and 3-alkyl-substituted thiophenes, both in the condensed and in the gaseous phases.     

Sulfur and selenium ylide bond enthalpies

The bond dissociation enthalpies (BDEs) of sulfur and selenium ylides have been estimated by applying MP2/6-311++G(3df,2p)//MP2/6-31G(d,p), G3, and other computational methods. Computed sulfoxide bond enthalpies were compared to experimental results to ensure the reliability of the computational methods before extending to related compounds. The examined ylides include the following: sulfoxides, sulfilimines, S,C-sulfonium ylides, and selenoxides. Selenoxides have BDEs about 10 kcal/mol smaller than the corresponding sulfoxides. N-H sulfilimines and CH2-S,C-sulfonium ylides have low BDEs, unless the sulfilimine or S,C-sulfonium ylide is stabilized by an electronegative substituent on N or C, respectively. Incorporation of the S or Se into a thiophene or selenophene-type ring lowers the BDE for the ylide.     

Are calculated enthalpies of formation sometimes more reliable than experimental? A test on alkyl substituted benzoic acids

Energies of 20 alkyl-substituted benzoic acids were calculated at the levels B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p)//MP2/6-311+G(d,p); the pertinent enthalpies at 298 K were calculated at the same levels. Comparison with experimental enthalpies of formation Delta(f)H degrees (g)(298) was carried out in terms of isodesmic reactions, that is, in the relative values. Of the four calculated quantities, the DFT enthalpies yielded best correlation with the standard deviation of 2.1 kJ mol(-1), near to the experimental uncertainty; the DFT energies are only slightly worse and the MP2 enthalpies or energies much worse. However, the DFT method overestimated systematically the substituent effects and had to be calibrated. Comparison with the experimental gas-phase acidities was less telling and the fit was worse because both methods overestimated the substituent effects. Extending the base in selected examples did not give better results. Although the systematic deviations are evidently due to the imperfections of the theoretical models, individual big deviations should be attributed to experimental errors or to the abnormal behavior of certain compounds at the experimental conditions. From this point of view, three examples of the so-called long-range effect claimed in the case of different benzoic acid derivatives, always for substituents in the meta position, must be refused as unproven because the experimental energies were not confirmed by calculations.     

Stepwise Oxidation of Thiophene and Its Derivatives by Hydrogen Peroxide Catalyzed by Methyltrioxorhenium(VII)

The oxidation of thiophene derivatives by hydrogen peroxide is catalyzed by methyltrioxorhenium(VII) (CH(3)ReO(3)). This compound reacts with hydrogen peroxide to form 1:1 and 1:2 rhenium peroxides, each of which transfers an oxygen atom to the sulfur atom of thiophene and its derivatives. Complete oxidation to the sulfone occurs readily by way of its sulfoxide intermediate. The rates for each oxidation step of dibenzothiophenes, benzothiophenes, and substituted thiophenes were determined. The rate constants for the oxidation of the thiophenes are 2-4 orders of magnitude smaller than those for the oxidation of aliphatic sulfides, whereas the rate constants are generally the same for the oxidation of the thiophene oxides and aliphatic sulfoxides. The rate constant for conversion of a sulfide to a sulfoxide (thiophene oxide) increases when a more electron-donating substituent is introduced into the molecule, whereas the opposite trend was found for the reaction that converts a sulfoxide to a sulfone (thiophene dioxide). Mechanisms consistent with this are proposed. The first trend reflects the attack of the nucleophilic sulfur atom of a thiophene center on a peroxide that has been electrophilically activated by coordination to rhenium. The second, more subtle, trend arises when both sulfoxide and peroxide are coordinated to rhenium; the inherently greater nucleophilicity of peroxide then takes control.     

Assessment of Gaussian-3X theory for chlorinated organic molecules. Enthalpies of formation of chlorobenzenes and predictions for polychlorinated aromatic compounds

The enthalpies of formation of chlorinated methanes, ethanes, ethylenes, phenols, and benzenes have been calculated at the G3X level of theory using the atomization energy procedure and the method of isodesmic reactions. By comparing the most reliable experimental data on chlorinated hydrocarbons recommended by Manion [Manion JA (2002) J Phys Chem Ref Data 31:123] with the G3X results, the accuracy of theoretical enthalpies of formation is estimated as ranging from ±4 to ±10 kJ/mol. Only for hexachloroethane, the difference between the experimental value and G3X result was outside this range and the experimental enthalpy of formation of hexachloroethane was called into question by theory. The G3X enthalpies of formation of all chlorobenzenes agree well with experimental data which were partly reanalyzed using recent experimental data on enthalpies of sublimation. Based on the G3X results, a set of self-consistent experimental data for chlorobenzenes is recommended. The enthalpies of formation of some polychlorinated dibenzo-p-dioxins were estimated using improved enthalpies of formation for chlorobenzenes. The possible inaccuracy of previously estimated values for polychlorinated aromatic compounds is discussed.     

基于GC/Q-TOF MS技术鉴定S-zorb汽油中噻吩类化合物

结合气相色谱分离技术和MS/MS串联质谱筛查技术,通过选定目标母离子进行碰撞诱导解离,获取精确质量子离子信息,实现汽油馏分中噻吩类化合物的痕量筛查分析.选取5种不同碳数取代噻吩类化合物作为标准物绘制定量标准曲线,建立了基于气相色谱-四极杆串联飞行时间质谱(GC/Q-TOF MS)直接测定汽油中噻吩类化合物的方法.利用该...     

Substituent effects on enthalpies of formation of nitrogen heterocycles: 2-substituted benzimidazoles and related compounds

The enthalpies of combustion, heat capacities, enthalpies of sublimation and enthalpies of formation of 2-tert-butylbenzimidazole (2tBuBIM) and 2-phenylimidazole (2PhIM) are reported and the results compared with those of benzene derivatives and a series of azoles (imidazoles, pyrazoles, benzimidazoles and indazoles). Theoretical estimates of the enthalpies of formation were obtained through the use of atom equivalent schemes. The necessary energies were obtained in single-point calculations at the B3LYP/6-311++G(d,p) on B3LYP/6-31G optimized geometries. The comparison of experimental and calculated values of all studied compounds bearing H (unsubstituted), methyl (Me) ethyl (Et), propyl (Pr), isopropyl (iPr), tert-butyl (tBu), benzyl (Bn) and phenyl (Ph) groups show remarkable homogeneity. The remarkable consistency of both the calculated and experimental results allows us to predict with reasonable certainty the missing experimental values. The crystal and molecular structure of the 2-benzylbenzimidazole (2BnBIM) has been determined by X-ray analysis. The observed molecular conformation permits the crystal being built up through N-H...N hydrogen bonds and van der Waals contacts between the molecules. An attempt has been made to relate the crystal structure to the enthalpies of sublimation.     

Enthalpy of the gas-phase CO2 + Mg reaction from ab initio total energies

Various highly accurate ab initio composite methods of Gaussian-n (G1, G2, G3), their variations (G2(MP2), G3(MP2), G3//B3LYP, G3(MP2)//B3LYP), and complete basis set (CBS-Q, CBS-Q//B3LYP) series of models were applied to compute reaction enthalpies of the ground-state reaction of CO2 with Mg. All model chemistries predict highly endothermic reactions, with DeltaH(298) = 63.6-69.7 kcal x mol(-1). The difference between the calculated reaction enthalpies and the experimental value, evaluated with recommended experimental standard enthalpies of formation for products and reactants, is more than 20 kcal x mol(-1) for all methods. This difference originates in the incorrect experimental enthalpy of formation of gaseous MgO given in thermochemical databases. When the theoretical formation enthalpy for MgO calculated by a particular method is used, the deviation is reduced to 1.3 kcal x mol(-1). The performance of the methodologies used to calculate the heat of this particular reaction and the enthalpy of formation of MgO are discussed.     

On the syntheses and some reactions of bis- and tris(methylthio)thiophenes

Reaction between various thienyllithium derivatives and dimethyl disulfide has been used for the preparation of 2,5-, 2,3-, and 3,4-bis(methylthio)thiophenes, as well as 2,3,4- and 2,3,5-tris(methylthio)thiophenes. Bromination of (methylthio)thiophenes with N-bromosuccinimide was found to be most convenient for the preparation of brominated (methylthio)thiophenes such as 3-bromo-2,5-bis(methylthio)- and 5-bromo-2,3-bis(methylthio)thiophene, 3,4-dibromo-2,5-bis(methylthio)-, 2,5-dibromo-3,4-bis(methylthio)- and 2,3-dibromo-4,5-bis(methylthio)thiophene as well as 3-bromo-2,4,5-tris(methylthio)thiophene. The reaction of methylthio substituted thienyllithium derivatives with methyl chloroformate was used for the syntheses of methyl methylthio substituted thiophenecarboxylates and using 1/3 of an equivalent for the direct preparation of methylthio substituted 3-thienylcarbinols as tris[2,4,5-tris(methylthio)-3-thienyl]carbinol.     

Calorimetric and computational study of 3-buten-1-ol and 3-butyn-1-ol. Estimation of the enthalpies of formation of 1-alkenols and 1-alkynols

The enthalpies of combustion and vaporization of 3-buten-1-ol and 3-butyn-1-ol have been measured by static bomb combustion calorimetry and correlation gas chromatography techniques, respectively, and the gas-phase enthalpies of formation, Delta(f)H degrees (m)(g), have been determined, the values being -147.3 +/- 1.8 and 16.7 +/- 1.6 kJ mol(-1), for 3-buten-1-ol and 3-butyn-1-ol, respectively. High level calculations at the G2 and G3 levels have also been carried out. Relationships between the enthalpies of formation of 1-alkanols, 1-alkenols and 1-alkynols and with the corresponding hydrocarbons have been discussed. From the calculated contributions to Delta(f)H degrees (m)(g) for the substitutions of CH(3) by CH(2)OH, CH(3)CH(2) by CH(2)=CH and CH(3)CH(2) by CH triple bond C, we have estimated the Delta(f)H degrees (m)(g) values for 3-buten-1-ol and 3-butyn-1-ol, in excellent agreement with the experimental ones. Delta(f)H degrees (m)(g) values for 1-alkenols and 1-alkynols up to 10 carbon atoms have also been estimated.     

Dioxygenase-catalysed oxidation of monosubstituted thiophenes: sulfoxidation versus dihydrodiol formation

Toluene dioxygenase (TDO)-catalysed sulfoxidation, using Pseudomonas putida UV4, was observed for the thiophene substrates 1A-1N. The unstable thiophene oxide metabolites, 6A-6G, 6K-6N, spontaneously dimerised yielding the corresponding racemic disulfoxide cycloadducts 7A-7G, 7K-7N. Dimeric or crossed [4 + 2] cycloaddition products, derived from the thiophene oxide intermediates 6A and 6D or 6B and 6D, were found when mixtures of thiophene substrates 1A and 1D or 1B and 1D were biotransformed. The thiophene sulfoxide metabolite 6B was also trapped as cycloadducts 17 or 18 using stable dienophiles. Preferential dioxygenase-catalysed oxidation of the substituent on the thiophene ring, including exocyclic sulfoxidation (1H-1J) and cis-dihydroxylation of a phenyl substituent (1G and 1N), was also observed. An enzyme-catalysed deoxygenation of a sulfoxide in P. putida UV4 was noticed when racemic disulfoxide cyclo-adducts 7A, 7B and 7K were converted to the corresponding enantioenriched monosulfoxides 8A, 8B and 8K via a kinetic resolution process. The parent thiophene 1A and the 3-substituted thiophenes 1K-1N were also found to undergo ring dihydroxylation yielding the cis/trans-dihydrodiol metabolites 9A and 9K-9N. Evidence is provided for a dehydrogenase-catalysed desaturation of a heterocyclic dihydrodiol (9Kcis/9Ktrans) to yield the corresponding 2,3-dihydroxythiophene (24) as its preferred thiolactone tautomer (23). A simple model to allow prediction of the structure of metabolites, formed from TDO-catalysed bacterial oxidation of thiophene substrates 1, is presented.     

Theoretical studies on the thermochemistry of stable closed-shell C1 and C2 brominated hydrocarbons

The enthalpies of formation of stable closed shell C1 and C2 brominated hydrocarbons have been predicted using Gaussian-3X model chemistry. The entropy, heat capacity, and thermal corrections are calculated from B3LYP/6-31G(2df,p) geometries and vibrational frequencies using rigid-rotor-harmonic-oscillator approximation, except for the quantities of the internal rotations in ethanes, which are calculated using the quantum-mechanical energy levels. Enthalpies of formation have been obtained from G3X atomization and isodesmic reactions. Good agreement is observed on the well-established experimental enthalpies of formation of CH 3Br, CH 2Br 2, CH 2ClBr, and C 2H 3Br from the high-resolution threshold photoelectron photoionization coincidence study.     

Thiophenes as traps for benzyne. 3. Diaryl sulfides and the role of dipolar intermediates

The reactions of seven thiophenes with benzyne generated from diphenyliodonium-2-carboxylate (DPIC) under a standard set of conditions led among other products to the formation of alpha- and beta-naphthyl phenyl sulfides 2a and 2b from thiophene (1a) and of 2c and 2d from 2-methylthiophene (1b). Dithienyl sulfides 4a-f were produced from the halothiophenes 1c-g. The structures of the naphthyl sulfides were proven by comparison with authentic samples of 2a-f, thus eliminating one of two possible mechanisms of formation. The remaining mechanism involves [4+2]-cycloaddition of benzyne to thiophene or to an S-phenylthiophenium ylide 10 to give the dipolar 2:1 benzyne/thiophene adduct 8 followed by ring-opening. Stevens-like rearrangements of 11, formed from 10 by proton transfer, may also explain the origin of arylated thiophenes such as 12 and 3 found in some reactions of benzynes with thiophene.     

Interaction of two functional groups through the benzene ring: theory and experiment

Energies of 132 benzene para bis-derivatives calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) were used for correlations of two types. Correlation with the experimental enthalpies of formation clearly revealed that the published experimental data are generally not dependable and may be loaded with errors of more than 10 kJ mol(-1). On the other hand, the calculated relative energies are biased so that the interaction of the two substituents is systematically overestimated. This shortcoming was insignificant for our correlations of the second type, in which the interaction of substituents expressed in terms of isodesmic reactions was analyzed depending on the effects of inductive and resonance. The results depended strongly on the character of substituents. When one substituent is an electron donor and the other is an acceptor, the inductive-resonance model works and the classical resonance picture is adequate. With two acceptor substituents, this model is still acceptable with lower precision (as crossed conjugation), but with two donors it fails completely and may be acceptable only for a much restricted subclass of strong donors. Many correlations described in the literature must be viewed with great caution when they are based only on a relatively small number of data, in which substituents of different types are not represented in a comparable number.     

Computational study of the reactions of SiH3X (X=H, Cl, Br, I) with HCN

Ab initio calculations were carried out for the reactions of silane and halosilanes (SiH3X, X=H, Cl, Br, I) with HCN. Geometries of the reactants, transition states, intermediates and products were optimized at HF, MP2, and B3LYP levels of theory using the 6-31G(d) and 6-31G(d,p) basis sets. Energies were also obtained using G3MP2 and G3B3 levels of theory. Intrinsic reaction coordinate (IRC) calculations were performed to characterize the transition states on the potential energy surface. It was found that HCN can react with silane and halosilanes via three different mechanisms. One involves HX elimination by a one-step pathway producing SiH3CN. The second mechanism consists of H2 elimination, producing SiH2XCN via a one-step pathway or three multiple-step pathways. The third mechanism involves dissociation of SiH3X to various products, which can then react with HCN. Activation energies, enthalpies, and free energies of activation along with the thermodynamic properties (DeltaE, DeltaH, and DeltaG) of each reaction pathway were calculated. The reaction of SiH3X with HCN produce different products depending on substituent X. We have found that the standard 6-31G(d) bromine basis set gave results which were in better agreement with the G3MP2 results than for the Binning-Curtiss basis set. Computed heats of formation (DeltaHf) for SiH3CN, SiH3NC, SiH2ClCN, SiH2BrCN, SiH2ICN, SiHCl, SiHBr, and SiHI were found to be 133.5, 150.8, -34.4, 23.6, 102.4, 48.7, 127.1, and 179.8 kJ mol-1, respectively. From enthalpies calculated at G3MP2, we predict that the DeltaHf for SiH2 to be 262.8 kJ mol-1 compared to the experimental value of 273.8+/-4.2 kJ mol-1.