Thermochemical and theoretical study of some methyldiazines

The standard (p ⁰ = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, atT=298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G^* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for all the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.     

The standard thermodynamic functions of ethyl-, formyl-, and benzoylferrocene in the ideal gas state

The standard enthalpies of formation of gaseous ethylferrocene, formylferrocene, and benzoylferrocene were calculated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) level of density functional theory using the method of isodesmic reactions. The absolute entropy, heat capacity, and enthalpy were calculated for ethylferrocene and formylferrocene by statistical thermodynamics methods over the temperature range 100–1000 K. The molecular constants necessary for calculations (structural parameters, vibrational frequencies, and internal rotation potential) were estimated at the B3LYP/6-31G(d,p) level. Empirical estimates were used to check the reliability and mutual consistency of literature experimental data on the enthalpies of formation and entropies of several ferrocene derivatives. Unreliable data were revealed and more reliable estimates of their errors were obtained.     

合成聚甲醛二甲基醚反应热力学的理论计算

采用密度泛函理论(DFT)方法在B3LYP/6-31＋G(d,p)水平上对聚甲醛二甲基醚(PODE)系列化合物进行了全优化和振动分析计算, 获得了该系列化合物的最优构型及热力学函数值. 通过设计等键反应计算了PODE的生成热, 并进一步计算了生成PODE的反应平衡常数及转化率, 进而判断这些反应的热力学可行性.     

3,6-二叠氮基-1,2,4,5-四嗪的合成及理论研究

以3,6-双(3,5-二甲基吡唑基)-1,2,4,5-四嗪为原料, 经过肼解反应和重氮化反应, 制得了3,6-二叠氮基-1,2,4,5-四嗪(DAT). 在DFT-B3LYP/6-31G*水平下求得了DAT的分子几何、IR光谱和热力学性质. 计算模拟IR光谱和实测IR光谱的对比表明DAT在固态下不发生叠氮-四唑互变异构反应. 根据IR光谱计算了DAT的热容、焓、熵等热力学参数, 也给出了这些参数和温度T之间的函数关系. 在不破坏四嗪环和叠氮基的原则下通过构建等键反应求得了DAT的精确生成热为1088 kJ•mol^—1. 爆轰性能计算表明DAT爆速D＝8.45 km•s^－1, 爆压P＝31.3 GPa, 高于TNT和HMX.     

Enthalpies of formation and substituent effects of ortho-, meta-, and para-aminotoluenes from thermochemical measurements and from ab initio calculations

The molar enthalpies of vaporization of 2-amino-, 3-amino-, and 4-aminotoluenes were obtained from the temperature dependence of the vapor pressure measured by the transpiration method. The molar enthalpy of sublimation of 4-aminotoluene was measured in the same way. The standard (p(o) = 0.1 MPa) molar enthalpy of formation delta fH(o)m(cr) at the temperature T = 298.15 K of crystalline 4-aminotoluene was measured using combustion calorimetry. The thermochemical investigations of aminotoluenes available in the literature were collected and combined with our own experimental results to obtain our own reliable standard molar enthalpies of formation at T = 298.15 K in the gaseous state. Ab initio calculations of aminotoluenes have been performed using the MP2Full/6-31G(d) and G3(MP2) basis sets, and the results from the bond separation method are in excellent agreement with the experiment. These new results help to resolve the uncertainty in the available thermochemical data on aminotoluenes. Weak mutual interactions of substituents in aminotoluenes have been realized using an isodesmic reaction procedure.     

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.     

Molecular structure, barriers to internal rotation, and the enthalpy of formation of cumene hydroperoxide PhCMe2OOH: a DFT study

Conformational analysis of cumene hydroperoxide PhCMe₂OOH (1) has been carried out using the density functional methods B3LYP/6-31G(d,p) and B3LYP/6-311+G(3df,2p). Ignoring rotation of methyl groups, molecule 1 has seven conformers differing in orientation of the — CMe₂OOH fragment relative to the benzene ring and in mutual position of atoms in this fragment. The molecular structures, relative energies, and statistical distribution of the conformers were determined, and intramolecular rotational barriers were estimated. The enthalpies of formation of all conformers of molecule 1 were calculated using two approximations with inclusion of zero-point vibrational energy and temperature correction. Calculations using the isodesmic reaction (IDR) scheme made it possible to reduce the systematic error of the determination of the enthalpy of reactions. The total enthalpy of formation of compound 1 calculated with inclusion of statistical distribution of rotamers equals −19.7±3.6 kcal mol⁻¹. The combination of the B3LYP/6-31G(d,p) approximation and the IDR scheme gives fairly accurate results (relative error is ±0.4 kcal mol⁻¹) as compared to those obtained with the extended basis set 6-311+G(3df,2p). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1157–1164, June, 2008.     

反应类等键反应方法及类反应热动力学参数计算

本文提出了反应类等键反应方法,将通常用于热力学性质计算的等键反应方法推广用于类反应中反应势垒和反应焓变的计算. 对碳氢燃料低温燃烧反应机理中的一类重要反应类：?烷基自由基过氧化氢裂解生成烯烃和HO2自由基的反应势垒和反应焓变进行了计算. 通过对该类16个反应中的5个代表反应分别在不同计算水平HF、DFT、MP2、CCSD(T)的比较计算发现,采用等键反应方法可在较低从头算级别计算得到类反应较高精度的反应势垒,提高了计算的效率和精度. 本文采用反应类等键反应方法在B3LYP/6-311G(d,p)计算水平对该类16个反应进行了反应势垒和反应焓变的计算,并建立了反应势垒和反应能的线性自由能关系：delta V=71.02+0.41?delta E.     

CF3CH2CF2CH3（HFC-365mfc）与Cl原子反应的微观机理及动力学性质

采用密度泛函理论方法 M06-2X结合6-31+G(d,p)基组研究了CF3CH2CF2CH3与Cl原子反应的反应机理.计算获得了CF3CH2CF2CH3的两种可区分的稳定几何构象RC1和RC2以及与它们相对应的8条氢提取反应通道和2条取代反应通道.运用改进的正则变分过渡态理论(ICVT)并结合小曲率隧道效应校正(SCT),在M06-2X/6-31+G(d,p)水平上计算了各氢提取通道的速率常数,并由Boltzmann配分函数得到总包反应的速率常数kT(cm3.molecule-1.s-1).计算结果表明,体系的总反应速率常数与已有实验值相吻合,进而给出了该反应在200～1000 K温度区间内反应速率常数kT的三参数表达式kT=1.88×10-22T3.76.exp(-1780.69/T),并讨论了两种构象RC1和RC2对总反应的贡献及各构象中氢提取发生在—CH3或—CH2—基团上的位置选择性.此外,由于缺少相关反应物及产物自由基标准生成焓ΔHf,298 K的数据,利用等化学键法估算了在上述物种的标准生成焓.     

Ab initio molecular orbital studies for compounds of magnesium

Using a 6-311G** basis set with estimation of correlation energy at the MP2 level, structural and energetic data for 40 molecular species containing magnesium have been calculated. For about half the species studied, further energetic data were obtained using Pople's G2 method. Enthalpy changes at 298.15 K were obtained for isogyric reactions and standard enthalpies of formation were derived from these. Comparison of the standard enthalpies of formation with the sparse literature data suggests the MP2/6-311G** standard enthalpies of formation are accurate to ± 15 kJ mol^?1 and the corresponding G2 enthalpies accurate to ± 10 kJ mol^?1. The calculated ΔH [MgN, g] revealed a gross error in the currently accepted value for this function. It is intended that these results will be used to parameterize the semiempirical molecular orbital package, MOPAC, for the element magnesium. © John Wiley & Sons, Inc.     

Comparative studies on the structures,infrared spectrum,and thermodynamic properties of phthalocyanine using ab initio Hartree–Fock and density functional theory methods

The molecular structure, vibrational spectrum, standard thermodynamic functions, and enthalpy of formation of free base phthalocyanine (Pc) have been studied using the density functional theory B3LYP procedure, as well as the ab initio Hartree–Fock method. Various basis sets 3‐21G, 6‐31G*, and LANL2DZ have been employed. The results obtained at various levels are discussed and compared with each other and with the available experimental data. It is shown that calculations performed at the Hartree–Fock level cannot produce a reliable geometry and related properties such as the dipole moment of Pc and similar porphyrin‐based systems. Electron correlation must be included in the calculations. The basis set has comparatively less effect on the calculated results. The results derived at the B3LYP level using the smaller 3‐21G and LANL2DZ basis sets are very close to those produced using the medium 6‐31G* basis set. The geometry of Pc obtained at the B3LYP level has D_2h symmetry and the diameter of the central macrocycle is about 4 Å. The enthalpy of formation of Pc in the gas phase has been predicted to be 1518.50 kJ/mol at the B3LYP/6‐311G(2d,2p)//B3LYP/6‐31G* level via an isodesmic reaction. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Thermochemical properties for n‐propyl,iso‐propyl,and tert‐butyl nitroalkanes,alkyl nitrites,and their carbon‐centered radicals

Density functional theory (DFT) based calculations are performed on a series of alkyl nitrites and nitroalkanes representing large‐scale primary, secondary, and tertiary nitro compounds and their radicals resulting from the loss of their skeletal hydrogen atoms. Geometries, vibration frequencies, and thermochemical properties [S°(T) and C°_p(T) (10 K ? T ? 5000 K)] are calculated at the B3LYP/6‐31G(d,p) DFT level. Δ_fH°₂₉₈ values are from B3LYP/6‐31G(d,p), B3LYP/6‐31+G(2d,2p), and the composite CBS‐QB3 levels. Potential energy barriers for the internal rotations have been computed at the B3LYP/6‐31G(d,p) level of theory, and the lower barrier contributions are incorporated into entropy and heat capacity data. The standard enthalpies of formation at 298 K are evaluated using isodesmic reaction schemes with several work reactions for each species. Recommended values derived from the most stable conformers of respective nitro‐ and nitrite isomers include ?30.57 and ?28.44 kcal mol^?1 for n‐propane‐, ?33.89 and ?32.32 kcal mol^?1 for iso‐propane‐, ?42.78 and ?41.36 kcal mol^?1 for tert‐butane‐nitro compounds and nitrites, respectively. Entropy and heat capacity values are also reported for the lower homologues: nitromethane, nitroethane, and corresponding nitrites. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 181–199, 2010     

Theoretical investigation on the kinetics and thermochemisty of H-atom abstraction reactions of 2-chloroethyl methyl ether (CH<Subscript>3</Subscript>OCH<Subscript>2</Subscript>CH<Subscript>2</Subscript>Cl) with OH radical at 298 K

Kinetics and thermochemistry of the H-atom abstraction reaction of CH₃OCH₂CH₂Cl with OH radical have been carried out using dual level of methods. Initially, geometry optimization and frequency calculations are performed at M06-2X/6-31+G(d, p) level of theory, and energetic calculations are further refined using CCSD(T)/6-311++G(d, p) level of theory in order to characterized all stationary points on potential energy surface (PES). The result shows that H-atom abstraction from –OCH₂ site of CH₃OCH₂CH₂Cl is dominant path. The rate constants are calculated using canonical transition state theory at 298 K, which are found to be in good agreement with the experimental data. We have presented the standard enthalpies of formation for CH₃OCH₂CH₂Cl and the radicals generated during the H-atom abstraction using group-balanced isodesmic reactions scheme. The atmospheric lifetime of title molecule is also calculated.     

Thermodynamic and ab initio analysis of the controversial enthalpy of formation of formaldehyde.

There are two values, -26.0 and -27.7 kcal mol(-1), that are routinely reported in literature evaluations for the standard enthalpy of formation, Delta(f) H(o)(298), of formaldehyde (CH(2)=O), where error limits are less than the difference in values. In this study, we summarize the reported literature for formaldehyde enthalpy values based on evaluated measurements and on computational studies. Using experimental reaction enthalpies for a series of reactions involving formaldehyde, in conjunction with known enthalpies of formation, its enthalpy is determined to be -26.05+/-0.42 kcal mol(-1), which we believe is the most accurate enthalpy currently available. For the same reaction series, the reaction enthalpies are evaluated using six computational methods: CBS-Q, CBS-Q//B3, CBS-APNO, G2, G3, and G3B3 yield Delta(f) H(o)(298)=-25.90+/-1.17 kcal mol(-1), which is in good agreement to our experimentally derived result. Furthermore, the computational chemistry methods G3, G3MP2B3, CCSD/6-311+G(2df,p)//B3LYP/6-31G(d), CCSD(T)/6-311+G(2df,p)//B3LYP/6-31G(d), and CBS-APNO in conjunction with isodesmic and homodesmic reactions are used to determine Delta(f) H(o)(298). Results from a series of five work reactions at the higher levels of calculation are -26.30+/-0.39 kcal mol(-1) with G3, -26.45+/-0.38 kcal mol(-1) with G3MP2B3, -26.09+/-0.37 kcal mol(-1) with CBS-APNO, -26.19+/-0.48 kcal mol(-1) with CCSD, and -26.16+/-0.58 kcal mol(-1) with CCSD(T). Results from heat of atomization calculations using seven accurate ab initio methods yields an enthalpy value of -26.82+/-0.99 kcal mol(-1). The results using isodesmic reactions are found to give enthalpies more accurate than both other computational approaches and are of similar accuracy to atomization enthalpy calculations derived from computationally intensive W1 and CBS-APNO methods. Overall, our most accurate calculations provide an enthalpy of formation in the range of -26.2 to -26.7 kcal mol(-1), which is within computational error of the suggested experimental value. The relative merits of each of the three computational methods are discussed and depend upon the accuracy of experimental enthalpies of formation required in the calculations and the importance of systematic computational errors in the work reaction. Our results also calculate Delta(f) H(o)(298) for the formyl anion (HCO(-)) as 1.28+/-0.43 kcal mol(-1).     

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.     

烷烃与氢过氧自由基氢提取反应类反应能垒与速率常数的精确计算

氢过氧自由基从烷烃分子中提取氢的反应是碳氢燃料中低温燃烧化学中非常重要的一类反应。本文用等键反应方法计算了这一类反应的动力学参数。所有反应物、过渡态、产物的几何结构均在HF/6-31+G(d)水平下优化得到。以反应中的过渡态反应中心的几何结构守恒为判据,该反应类可用等键反应处理。本文选取了乙烷和氢过氧自由基的氢提取反应为参考反应,其它反应作为目标反应,用等键反应方法对目标反应在HF/6-31+G(d)水平的近似能垒和反应速率常数进行了校正。为了验证方法的可靠性,选取C5以下的烷烃分子体系,对等键反应方法校正结果和高精度CCSD(T)/CBS直接计算结果进行了比较,最大绝对误差为5.58k J?mol~(-1),因此,采用等键反应方法只需用低水平HF从头算方法就可以再现高精度CCSD(T)/CBS计算结果,从而解决了该反应类中大分子体系的能垒的精确计算。本文的研究为碳氢化合物中低温燃烧模拟中重要的烷烃与氢过氧自由基氢提取反应提供了准确的动力学参数。     

Structures and stabilities of diacetylene-expanded polyhedranes by quantum mechanics and molecular mechanics

The structures, heats of formation, and strain energies of diacetylene (buta-1,3-diynediyl) expanded molecules have been computed with ab initio and molecular mechanics calculations. Expanded cubane, prismane, tetrahedrane, and expanded monocyclics and bicyclics were optimized at the HF/6-31G(d) and B3LYP/6-31G(d) levels. The heats of formation of these systems were obtained from isodesmic equations at the HF/6-31G(d) level. Heats of formation were also calculated from Benson group equivalents. The strain energies of these expanded molecules were estimated by several independent methods. An adapted MM3 molecular mechanics force field, specifically parametrized to treat conjugated acetylene units, was employed for one measure of strain energy and as an additional method for structural analysis. Expanded dodecahedrane and icosahedrane were calculated by this method. Expanded molecules were considered structurally in the context of their potential material applications.     

Thermochemical properties and bond dissociation energies of C3-C5 cycloalkyl hydroperoxides and peroxy radicals: cycloalkyl radical + (3)O2 reaction thermochemistry

Cyclic aliphatic hydrocarbons are major components in modern fuels; they can be present in the reactants, and they can be formed during the gas-phase oxidation processes. In combustion and thermal oxidation processes, these cyclics will form radicals that react with (3)O(2) to form peroxy radicals. In this study, density functional theory and higher level ab initio calculations are used to calculate thermochemical properties and bond dissociation energies of 3-5-membered cycloalkanes, corresponding hydroperoxides, hydroperoxycycloalkyl radicals, and cycloalkyl radicals that occur in these reaction systems. Geometries, vibration frequencies, and thermochemical properties, ΔH(f 298)°, are calculated with the B3LYP/6-31 g(d,p), B3LYP/6-31 g(2d,2p), composite CBS-QB3, and G3MP2B3 methods. Standard enthalpies of formation at 298 K are evaluated using isodesmic reaction schemes with several work reactions for each species. Group additivity contributions are developed, and application of group additivity with comparison to calculated values is illustrated. Entropy and heat capacities, S°(T) and C(p)°(T) (5 K ≤ T ≤ 5000), are determined using geometric parameters and frequencies from the B3LYP/6-31 g(d,p) calculations.     

Structure, thermochemistry, and conformational analysis of peroxides ROOR and hydroperoxides ROOH (R = Me, But, CF3)

The equilibrium structures, total energies, and harmonic frequencies of peroxides ROOR and ROOH (R = Me, Bu^t, CF₃) were calculated using the perturbation theory (MP4//MP2 method) and density functional approach (B3LYP) in the 6-31G(d,p) basis set. The conformational flexibility of peroxides under rotation about the O-O bond was investigated. It was found that the stable conformation of a peroxide molecule is determined by superposition of the destabilizing effects (repulsion between the lone electron pairs, steric hindrances) and the interaction of the nonbonding orbitals of oxygen atoms with the antibonding orbitals of the adjacent polar bonds. The latter effect stabilizes the nonplanar structure of the peroxide molecule. The role of orbital interactions in manifestation of the d-effect (distortion of the tetrahedral configuration of the X₃CO fragment of peroxide molecule) was revealed. The vibrational spectra of peroxides were calculated and compared with the experimental data. The potential energy distribution over normal vibrations was analyzed. The enthalpies of formation and the bond strengths in the molecules of compounds examined were calculated in the framework of the isodesmic reaction approach.     

Thermochemical and kinetic analyses on oxidation of isobutenyl radical and 2-hydroperoxymethyl-2-propenyl radical

In recognition of the importance of the isobutene oxidation reaction in the preignition chemistry associated with engine knock, the thermochemistry, chemical reaction pathways, and reaction kinetics of the isobutenyl radical oxidation at low to intermediate temperature range were computationally studied, focusing on both the first and the second O2 addition to the isobutenyl radical. The geometries of reactants, important intermediates, transition states, and products in the isobutenyl radical oxidation system were optimized at the B3LYP/6-311G(d,p) and MP2(full)/6-31G(d) levels, and the thermochemical properties were determined on the basis of ab initio, density functional theory, and statistical mechanics. Enthalpies of formation for several important intermediates were calculated using isodesmic reactions at the DFT and the CBS-QB3 levels. The kinetic analysis of the first O2 addition to the isobutenyl radical was performed using enthalpies at the CBS-QB3 and G3(MP2) levels. The reaction forms a chemically activated isobutenyl peroxy adduct which can be stabilized, dissociate back to reactants, cyclize to cyclic peroxide-alkyl radicals, and isomerize to the 2-hydroperoxymethyl-2-propenyl radical that further undergoes another O2 addition. The reaction channels for isomerization and cyclization and further dissociation on this second O2 addition were analyzed using enthalpies at the DFT level with energy corrections based on similar reaction channels for the first O2 addition. The high-pressure limit rate constants for each reaction channel were determined as functions of temperature by the canonical transition state theory for further kinetic model development.